摘要:

FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY NO. 71 1981 High Resolution Spectroscopy THE FARADAY DIVISION THE ROYAL SOCIETY OF CHEMISTRY LONDONFARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY NO. 71 1981 High Resolution Spectroscopy THE FARADAY DIVISION THE ROYAL SOCIETY OF CHEMISTRY LONDONA GENERAL DISCUSSION ON High Resolution Spectroscopy 13th, 14th and 15th April, 1981 A GENERAL Drscussro~ on High Resolution Spectroscopy was held a t the University of Bristol on 13th, 14th and 15th April, 1981. The President of the Faraday Division, Professor J. S. Rowlinson, F.R.S., was in the chair: about 150 Fellows of the Faraday Division and visitors from overseas attended the meeting. Among the overseas visitors were : Dr. J. A. Beswick, France Dr. P. Brechignac, France Prof.H. Burger, West Germany Dr. G. Cazzilo, ItaZy Dr. C. Cossart, France Prof. J. A. Coxon, Canada Mrs. C. Demuynck, France Dr. J. L. Destombes, France Dr. H. A. Dykerman, The Netherlands Dr. F. Engelke, West Germany Dr. K. M. Evenson, U.S.A. Prof. P. Favero, Italy Prof. R. Field, U.S.A. Mr. S . C. Foster, Canada Dr. J. Galica, West Germany Dr. R. D. Gordon, Canada Dr. G. Gouedard, France Prof. T. E. Gough, Canada Dr. W. Hack, West Germany Dr. M. Heaven, U.S.A. Dr. G. Herzberg, Canada Prof. E. Hirota, Japan Dr. Ch. Jungen, France Dr. L. Klynning, Sweden Dr. S. Leach, France Dr. B. Lingren, Sweden Dr. A. R. W. McKellar, Canada Dr. J. P. Maier, Switzerland Dr. T. A. Miller, U.S.A. Dr. E. Miiller Horsche, West Germany Dr. M. Quack, West Germany Dr. D. A. Rarnsay, Canada Dr.Al. Roche, France Mrs. J. Rostas, France Dr. G. Schmidtke, West Germany Prof. W. Urban, West Germany Prof. J. S . Winn, U.S.A. Dr. B. P. Winnewisser, West Germany Prof. M. Winnewisser, West Germany Prof. R. C . Woods, U.S.A.Organising Committee Prof. A. Carrington (Chairman) Dr. R. F. Barrow Dr. J. M. Brown Prof. R. N. Dixon Dr. P. B. Davies Dr. G. Duxbury Mrs Y. A. Fish Dr. D. M. Hirst Prof. I. M. Mills Prof. J. N. Murrell Prof. B. A. Thrush Dr. D. A. Young ISBN : 0-851 8671 8-9 ISSN 0301-7249 0 The Royal Society of Chemistry and Contributors, 1981. Printed in Great Britain by Fletcher & Son Ltd., NorwichA GENERAL DISCUSSION ON High Resolution Spectroscopy 13th, 14th and 15th April, 1981 A GENERAL Drscussro~ on High Resolution Spectroscopy was held a t the University of Bristol on 13th, 14th and 15th April, 1981.The President of the Faraday Division, Professor J. S. Rowlinson, F.R.S., was in the chair: about 150 Fellows of the Faraday Division and visitors from overseas attended the meeting. Among the overseas visitors were : Dr. J. A. Beswick, France Dr. P. Brechignac, France Prof. H. Burger, West Germany Dr. G. Cazzilo, ItaZy Dr. C. Cossart, France Prof. J. A. Coxon, Canada Mrs. C. Demuynck, France Dr. J. L. Destombes, France Dr. H. A. Dykerman, The Netherlands Dr. F. Engelke, West Germany Dr. K. M. Evenson, U.S.A. Prof. P. Favero, Italy Prof. R. Field, U.S.A. Mr. S . C. Foster, Canada Dr. J. Galica, West Germany Dr. R. D. Gordon, Canada Dr. G. Gouedard, France Prof. T. E. Gough, Canada Dr.W. Hack, West Germany Dr. M. Heaven, U.S.A. Dr. G. Herzberg, Canada Prof. E. Hirota, Japan Dr. Ch. Jungen, France Dr. L. Klynning, Sweden Dr. S. Leach, France Dr. B. Lingren, Sweden Dr. A. R. W. McKellar, Canada Dr. J. P. Maier, Switzerland Dr. T. A. Miller, U.S.A. Dr. E. Miiller Horsche, West Germany Dr. M. Quack, West Germany Dr. D. A. Rarnsay, Canada Dr. Al. Roche, France Mrs. J. Rostas, France Dr. G. Schmidtke, West Germany Prof. W. Urban, West Germany Prof. J. S . Winn, U.S.A. Dr. B. P. Winnewisser, West Germany Prof. M. Winnewisser, West Germany Prof. R. C . Woods, U.S.A.CONTENTS 191 205 21 3 233 253 273 287 30 1 Laser-induced Fluorescence of Trapped Molecular Ions: The CH+ A' I3 t X ' C.+ System by F. J. Grieman, B. H. Mahan, A. O'Keefe and J. S. Winn The 4114 A Absorption System of the HCCS Radical by S . L. N. G. Krishnamachari and D. A. Ramsay Doppler-limited Laser Spectroscopy of Electronic Transitions in SnO by M. A. A. Clyne and M. C. Heaven Crossed Laser and Molecular Beam Studies of Mixed Alkali Dimer: Prepara- tion, Perturbation and Predissociation by E. J. Breford, F. Engelke, G. Ennen and K. H. Meiwes Spectroscopy in the Ionisation Continuum: Vibrational Preionisation in H2 Calculated by Multichannel Quantum-defect Theory by Ch. Jungen and M. Raoult Local and Normal Vibrational States: a Harmonically Coupled Anharmonic- oscillator Model by M. S . Child and R. T. Lawton Exact Calculation of the Rotational- Vibrational Energy Levels of Triatomic Species by I. F. Kidd, G. G. Baht-Kurti and M. Shapiro GENERAL DISCUSSION 369 ADDITIONAL REMARKS 370 INDEX OF NAMES

ISSN:0301-7249    

DOI:10.1039/DC9817100001

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Spiers Memorial Lecture Far-infrared Laser Magnetic Resonance* BY K. M. EVENSON National Bureau of Standards, Boulder, Colorado, U.S.A. Received 18th May, 1981 Far-infrared laser magnetic resonance (1.m.r.) is now a laboratory spectroscopic technique used in at least six laboratories throughout the world, and some 50 papers on 1.m.r. spectroscopy have been p~blished.l-~~ L.m.r. is an extremely sensitive technique for finding rotational Zeeman spectra in paramagnetic atoms and molecules. Some 31 species have been detected, some of which had never been discovered before. L.m.r. is now also used in a number of laboratories to study the reaction rates of these paramagnetic free radical^.^' The field of mid-infrared 1.m.r. using CO and CO, lasers has also expanded rapidly and is summarized in McKellar’s paper5’ at this meeting.A review of far-infrared 1.m.r. was presented a little over one year ago.37 The purpose of the present report is to bring that comprehensive paper up to date and to point out some recent results in the field of laser frequency measurements which are leading the way to a redefinition of the metre. In this publication, I will list all of the far-infrared 1.m.r. spectrosopic papers chronologically; give a list of all the species observed with references, describe the design of a new 1.m.r. spectrometer in the N.B.S. Boulder Laboratories; present some new ideas on the sensitivity of intracavity absorption; and finally show how recent laser frequency measurements are leading to a new definition of the metre in terms of the second, thus fixing the value of the speed of light.FAR-INFRARED PARAMAGNETIC FREE RADICALS The first 50 entries in the bibliography are a current chronological list of the 1.m.r. spectroscopic papers. In table 1 a list of all the far-infrared 1.m.r. molecules identi- fied up to now is presented. The far-infrared 1.m.r. spectra of 26 different species, 5 metastable electronic states and several vibrationally excited states have been observed. Some of the observed species are not “ well-behaved ”, that is, the spin decouples from the orbital angular momentum at low field, and the 1.m.r. spectrum is not observable at fields of more than a few hundredths of a tesla (a few hundred gauss). Species exhibiting this behaviour are: CCH, CH,F and CH,OH. These weakly coupled mole- cules contrast with strongly coupled molecules such as OH, where spectra have been observed to 5 T (50 kG).’ Generally most spectra are observed below 2 T.The rate of new entries may be slower in the future because the “ easier ” radicals have already been observed. * Work supported in part by NASA grant W-15,047.8 SPIERS MEMORIAL LECTURE TABLE 1 .-ATOMS AND MOLECULES DETECTED BY FAR-INFRARED LASER MAGNETIC RESONANCE species state reference species state reference atoms 0 C diatomics 0 2 NO CH PH NH c10 CF OH OD SeH SeD SH ions HBr + DBr + 3P 28,34 3P 42 - 1, 8, 10, 13 2n 7 2n 5,27 3c - 20 3c - 19 2n 52 2rI 50 2n 9, 32, 33, v = 0 , 1 , 2 , 3 ... 47,49 41, 49 v = 0, 1, 2, 3 . . . 2n 29 2n 44 2n 30 2n 35 2n 35 triatomics NO2 HO2 HCO PH2 NH2 CH2 CCH polyatomics CH30 CH2F CH2OH metas ta bles 0 2 PH H02 co NF 6 12, 15, 16 17 21,36 18 38 53 24,40 26 45 48 20 16 54 46 EXPERIMENTAL DETAILS A new far-i.r, 1.m.r.spectrometer has been constructed in our N.B.S. Boulder laboratory which should be more sensitive, allow more lines to lase, and which will be significantly easier to operate. The new spectrometer is shown in fig. 1. A new com- 1 91.0 cm -- .l-l___-___.I.-___ .. [Quartz Spacer \Quartz Spacer FIG. 1 .-Laser magnetic resonance spectrometer, 40-1000 pm.K . M. EVENSON 9 puter-controllable 15 in. electromagnet with ring-shimmed Hyperco pole caps pro- duces a 7.5 cm homogeneous field region 5 times longer than that of our previous spectr~meter;~’ hence, it should be five times more sensitive. The new f i r .cavity is 25 cm shorter and should oscillate to well over 1000 pm. Better overlap between pump and f i r . laser yields a lower threshold of oscillation and consequently a larger number of f i r . lines. The improved overlap was accomplished with a nearly con- focal mirror geometry, the insertion of the CO, pump at the beam waist and the use of a cylindrical copper tube as the C 0 2 reff ector. This cylindrical copper tube was found to double the output power in a f i r . laser compared with the use of flat side mirrors. A section of 4 in. I.D. copper tube is used as the first cylindrical COz beam focuser to begin a series of consecutive refocuses of the pump beam. Quartz is used as the spacers in the spectrometer to provide better thermal stability.For ease of operation, the micrometer, coupler, detector, COz laser grating, and gas-handling system are all available from one side of the magnet. The beam splitter is rotatable about the laser axis so the polarization can be rotated. INTRA-CAVITY SENSITIVITY Measurements of the sensitivity of our previous intracavity far-infrared 1.m.r. spectrometer indicated an enhancement of sensitivity by about a factor of 20 over what one would expect from a passive ~avity.~’ Recently, Radford and RohbeckS5 used a simple expression of rig rod'^^^ for the saturated C.W. gain of a homogeneously broadened laser. The expression may explain the enhanced sensitivity. Rigrod’s expression is where g is the low-signal gain per unit length, L is the length of the gain cell, Pi, is the power density in the laser cavity, and P, is the self-saturation power level.Then, letting gain = loss where the loss is given by t + a, with t the fractional coupling, a the round-trip cavity loss, P the power output of the laser, and one obtains: For an additional small absorption d, where cc is the absorption coefficient and I is absorption length, the signal is given by AP = P(a + 2 4 - P(a>; and substituting in eqn (l), one obtains: Now, we would like to find AP =f(P), so we will eliminate g between eqn (1) and (2); thus AP = ( P + 9) (”’> t + a ’ (3)10 SPIERS MEMORIAL LECTURE We have found that the maximum sensitivity occurs with the spectrometer oscil- lating well above threshold, and this expression seems to be in agreement with the experimental results: (i) the sensitivity is nearly independent of P, i.e.tPJ2 > P, (ii) the sensitivity is some 20 times more than one would expect from a passive cavity, i.e. tPJ2 z 20P, (iii) AP cc 2al (the spectrometer is linear), (iv) AP cc cavity Q [i.e. AP cc l/(t + a)], (v) AP cc pressure of lasing gas (i.e. AP cc P,). However, before a positive confirmation can be made, further experimental verification is needed. RADIO ASTRONOMY Radio-astronomical observations (i. e. with heterodyne techniques) have recently been extended well into the far-infrared r e g i ~ n . ~ ~ ~ ~ ~ These observations are made at high altitudes in the NASA Kuiper Airborne Observatory or from the top of Mauna Kea in Hawaii. Radio-astronomical searches require an accurate knowledge of the frequencies of the atom or molecule sought.This accuracy generally cannot be supplied with optical data, but can be supplied from 1.m.r. data. In fact, the dis- covery of atomic carbons7 in Orion was made possible from 1.m.r. measurements of atomic carbon which yielded the fine-structure separation^^^ good to better than 1 MHz and some 100 times more accurate than optical data. The measurements of the 1.m.r. spectrum of OH47 also yielded improved rota- tional frequencies of OH and hence more accurate Doppler shifts in the recent astro- nomical observations of OH.59 These astronomical measurements were made with interferometric techniques and hence did not require high-accuracy data for the astro- nomical detection. The first terrestrial detection of CCH was performed with far-infrared 1.m.r.53 following the radio-astronomical detection of the N = 0 -+ 1 transition.60 A precise fit of the 1.m.r. data has yielded separate values for B and D and thus accurate N = 2 -+ 3 transition freq~encies.'~ These transitions have just been observed with radio techniques.61 With these successes in the last two years it seems likely that the measurement of other free radicals should lead to their detection in interstellar space. In fact, NASA is now supporting our spectroscopy for exactly this purpose. ACCURATE DETERMINATION OF MOLECULAR CONSTANTS FROM L.M.R. DATA To determine more accurately structural constants of paramagnetic molecules, Brown et al.,47 have fitted all the 1.m.r. data of OH to an effective Zeeman Hamil- tonian in order to obtain a highly accurate set of rotational constants.With these constants, predictions of the zero-field term values to ca. 3 MHz was possible. Al- though this determination is ca. 3 times less accurate than the atomic carbon num- bers, it is quite comparable when one considers that the two atomic carbon transitions were determined with 10 different laser frequencies. Thus, we see that the Zeeman theory for diatomics is approximately as accurate as it is for atoms. That is, the uncertainties are mainly caused by uncertainties in the experimental data, notably from measurements of the magnetic field and the laser frequency. The Zeeman theory of polyatomics is, in general, not as accurate. One of the best examples of a " successful " Zeeman theory of a triatomic is with H02.The abundant and regular 1.m.r. spectrum of H02 was fitted very nicely to the theory of Ho~gen.'~ In contrast, CH2 exhibits sparse and irregular and the spectra have not yet been assigned. A crucial test of the Zeeman theory of triatomics willK. M. EVENSON 11 be made in fitting these data to find a set of rotational constants for this elusive molecule.38 We feel that more 1.m.r. data are needed than are now at hand. A few of the lower levels of CH, will certainly be of interest to radio astronomers, and work is continuing on this molecule. One of the recent outstanding achievements in far-infrared 1.m.r. spectroscopy and Zeeman theory is the observation of a previously speculated molecule CH30,24 and the subsequent fit of its 1.m.r. spectrum4' yielding the rotational constants of this polyatomic radical.The successful fits of 1.m.r. data have been to radicals in which the spin remains coupled to the rotation of the molecule. However, the success with fitting the CCH spectrum indicates that some of these too may be amenable to analysis. THE FUTURE OF L.M.R. Most of the ground states of the more common paramagnetic hydrides have been observed. A few of the more elusive ones such as SiH still elude the experimentalist. However, hundreds of metastables and polyatomic radicals and many paramagnetic ions remain to be discovered by 1.m.r.; therefore, I predict that this highly sensitive tool will be used to measure dozens of new and exciting free radicals in the next few years. FAR-INFRARED LASERS, FREQUENCY MEASUREMENT AND THE REDEFINITION OF THE METRE The number of c.w.far-infrared lasing lines reported in Knight's recent tabulation6, is 1350. Of these, about one-quarter oscillate in conventional 1.m.r. spectrometers, and about one-half of these have been accurately (frequency) measured. In the operation of the spectrometer, the f i r . laser is tuned to the centre frequency of the lasing molecule by maximizing the laser power in single-mode operation. This ad- justment is reproducible to ca. &5 x and does not drift outside this range during a spectral search. The frequencies of these f.i.r. lasers are measured in separate experiments by syn- thesizing a frequency within a radiofrequency difference either from harmonics of microwave radiation or from difference frequencies between two CO, laser lines and possibly an additional X-band oscillator. It is this latter technique we have used63 because it covers the entire far-infrared spectral region.A tungsten-nickel point- contact diode64 is used as the non-linear element. The radiation from the f.i.r. laser, the two CO, lasers and the microwave oscillator (when needed) all radiate the tungsten antenna and electrons tunnel across the nickel oxide layer to the nickel base generating a radiofrequency difference. The far-infrared frequency difference, vf.i.r.9 is Vf.i.r. = n(vl - v2> & mvp Z k vb where n is the harmonic of the CO, difference frequency, v1 is one C02 laser frequency, v2 is the other CO, laser frequency, m is the microwave harmonic, v, is the microwave frequency and vb is the r.f.difference frequency. The uncertainty in this measurement is due to the uncertainty in finding the centre of the gain curve of the lasing medium Other uncertainties in deter- mining vf.i,r. are significantly less in most cases. To find the appropriate CO, lines, the frequency is first determined to an accuracy of <1 GHz by a wavelength measurement, made by measuring a number of half-wavelengths with the calibrated micrometer on the f i r . laser. x12 SPIERS MEMORIAL LECTURE y, = 260 103 419 U Ho-Na (1.15pm) x PHOTODIODE @ . vs = 3 710 218 5 CH,OH (80.8pm) 1 MIM DIOOE 32 226 vz - 525 127.5 MIM DIODE v, = 75061 SILICON DIODE e7 Cr FREO. STANDARD YO = 10723 KLYSTRON FIG. 2.-Laser frequency synthesis chain (to the visible).All frequencies in MHz. The overall result is that the uncertainty in the f i r . laser frequency is ca. &7 x in setting the laser to the top of the gain curve is multiplied by 24 because the laser must be adjusted to the top of its gain curve two times: once when the frequency is measured, and once when the 1.m.r. spectrometer is adjusted. We have not yet found it necessary to stabilize the f.i.r. cavity to the top of the gain curve. While I am discussing laser frequency measurements, I would like to point out that The uncertainty of 5 xK. M . EVENSON 13 the frequency of visible light has recently been measured.65 Frequencies can be measured orders of magnitude more accurately than wavelengths ; therefore, the entire electromagnetic spectrum up to the visible is now accessible with ultra-high- resolution and extremely accurate frequency metrology.These events stimulated the Cornit6 Consultatif Pour La Definition du Metre in 1979 to propose a new definition of the metre: “ The metre is the length equal to the distance travelled in a time in- terval of 1/299 792 458th of a second by plane electromagnetic waves in vacuum.” This recommendation will be acted upon by the general conference of weights and measures in 1984. The metre will be realized from this definition uia a vacuum wavelength obtained from any stabilized laser whose frequency has been measured. The vacuum wave- length is, of course, the “fixed” value of c (299 792 458 ms-’) divided by the measured frequency of that laser. It is therefore important that accurate frequencies of visible lasers be made.The first measurement of the frequency of visible light65 was not of sufficient accuracy for realizing the “ new ” metre. Hence, more accurate measure- ments are now underway. To extend frequency measurements to lasers, a series of steps are used in which harmonics of the radiation of one laser are heterodyned with the radiation from a higher-frequency laser to obtain a radiofrequency difference. The previously men- tioned measurement of the frequency of visible light consisted of a “ chain ” of such harmonically related lasers connecting the caesium standard with the frequency of an iodine absorption in the visible. We are presently at work on a simpler and more accurate chain shown in fig. 2, connecting the caesium frequency with the same iodine frequency. The accuracy ultimately is limited by the accuracy of the caesium stand- ard itself, & l x K.M. Evenson, H. P. Broida, J. S. Wells, R. J. Mahler and M. Mizushima, Phys. Reu. Lett., 1968, 21, 1038. K. M. Evenson and J. S. Wells, IEEE J . Quantum Electron., 1970, QE6, 184. K. M. Evenson, J. W. Wells, and H. E. Radford, Phys. Rev. Lett., 1970, 25, 199. K. M. Evenson, H. E. Radford, and M. M. Moran Jr, Appl. Phys. Lett., 1971, 18, 426. R. F. Curl, Jr, K. M. Evenson and J. S. Wells, Chem. Phys., 1972,56, 5144. M. Mizushima, K. M. Evenson and J. S. Wells, Phys. Rev. A, 1972, 5, 2276. M. Mizushima, J. S. Wells, K. M. Evenson and W. M. Welch, Phys. Rev. Lett., 1972, 29, 831. T. Kasuya and K. Shimoda, Jpn J. Appl.Phys., 1972, 11, 1571. lo K. M. Evenson and M. Mizushima, Phys. Rev. A, 1972, 6, 2197. K. M. Evenson and C. J. Howard, Laser Spectroscopy, ed. R. G. Brewer and A. Moorodian (Plenum Press, New York, 1975), p. 535. H. E. Radford, K. M. Evenson and C . J. Howard, J. Chern. Phys., 1974, 60, 3178. ’ J. S. Wells and K. M. Evenson, Rev. Sci. Znstr., 1970, 41, 227. l3 L. Tomuta, M. Mizushima, C. J. Howard and K. M. Evenson, Phys. Reu. A, 1975, 12, 974. l4 P. B. Davies and K. M. Evenson, Proc. 2nd Int. Con5 Laser Spectrosc., ed. S. Haroche, J. C . Pebay-Peyroula, T. W. Hansch and S. E. Harris (Springer-Verlag, Berlin, 1975), p. 132. Is Jon T. Hougen, J. Mol. Spectrosc., 1975, 54, 447. I6 Jon T. Hougen, H. E. Radford, K. M. Evenson and C. J. Howard, J. Mol. Spectrosc., 1975, I7 J.M. Cook, K. M. Evenson, C. J. Howard and R. F. Curl Jr, J. Chem. Phys., 1976, 64, 1381. l8 P. B. Davies, D. K. Russell, B. A. Thrush and F. D. Wayne, J. Chem. Phys., 1975, 62, 3739. ’O P. B. Davies, D. K. Russell and B. A. Thrush, Chem. Phys. Lett., 1975, 36, 280. 21 P. B. Davies, D. K. Russell and B. A. Thrush, Chem. Phys. Lett., 1976, 37, 43. ’‘ F. D. Wayne and H. E. Radford, Mol. Phys., 1976, 32, 1407. 23 P. B. Davies, D. K. Russell, B. A. Thrush and H. E. Radford, Proc. R. SOC. London, Ser. A, 24 H. E. Radford and D. K. Russell, J. Chem. Phys., 1977, 66, 2223. ’’ K. M. Evenson, D. A. Jennings, F. R. Peterson, J. A. Mucha, J. J. Jimenez, R. M. Charlton and C. J. Howard, IEEE J. Quantum Electron., 1977, QE13, 442. 56, 210. H. E. Radford and M. M. Litvak, Chem.Phys. Lett., 1975, 34, 561. 1978, 353, 299.14 S P I E R S MEMORIAL LECTURE 26 J. A. Mucha, D. A. Jennings, K. M. Evenson and J. T. Hougen, J. Mol. Spectrosc., 1977, 68, 27 J. T. Hougen, J. A. Mucha, D. A. Jennings and K. M. Evenson, J. Mol. Spectrosc., 1978, 72, 28 P. B. Davies, B. J. Handy, E. K. Murray-Lloyd and D. R. Smith, J. Chem. Phys., 1978,68,1135. 29 P. B. Davies, B. J. Handy, E. K. Murray-Llayd and D. K. Russell, J. Chem. Phys., 1978, 68, 30 P. B. Davies, B. J. Handy, E. K. Murray-Lloyd and D. K. Russell, Mol. Phys., 1978,36, 1005. 31 P. B. Davies, D. K. Russell, D. R. Smith and B. A. Thrush, J . Can. Phys., 1979, 57, 522. 32 P. B. Davies, W. Hack, A. W. Preuss and F. Temps, Chem. Phys. Lett., 1979,64,94. 33 J. P. Burrows, D. J. Cliff, P. B.Davies, G. W. Harris, B. A. Thrush and J. P. T. Wilkinson, 34 R. J. Saykally and K. M. Evenson, J. Chem. Phys., 1979, 71, 1564. 35 R. J. Saykally and K. M. Evenson, Phys. Reo. Lett., 1979, 43, 515. 36 P. B. Davies, D. K. Russell, B. A. Thrush and H. E. Radford, Chem. Phys., 1979,44,421. 37 K. M. Evenson, R. J. Saykally, D. A. Jennings, R. F. Curl Jr and J. M. Brown, Proc. Yamada ConJ I11 on Free Radicals, ed. Y . Morina, I. Tanaka, E. Hirota, K. Obi and S. Saito (Associa- tion for Science Documents, 1979), p. 159. 38 J. A. Mucha, K. M. Evenson, D. A. Jennings, G. B. Ellison and C. J. Howard, Chem. Phys. Lett., 1979, 66, 244. 39 K. M. Evenson and R. J. Saykally, Interstellar Molecules, ed. B. H. Andrews (International Astronomical Union, 1980), p. 239. 40 D. K.Russell and H. E. Radford, J . Chem. Phys., 1980, 72, 2750. 41 J. S. Geiger, D. R. Smith and J. D. Bonnett, Chem. Phys. Lett., 1980, 70, 600. 42 R. J. Saykally and K. M. Evenson, Astrophys. J., 1980, 238, L107. 43 K. M. Evenson, R. J. Saykally, D. A. Jennings, R. F. Curl Jr and J. M. Brown, Chemical and Biochemical Applications of Lasers, ed. C. B. Moore (Academic Press, New York, 1980), p. 95. 44 D. I. Cliff, P. B. Davies, B. J. Handy, B. A. Thrush and E. K. M. Lloyd, Chem. Phys. Lett., 1980, 75, 9. 45 H. E. Radford, K. M. Evenson and D. A. Jennings, Chem. Phys. Lelt., 1981, 78, 589. 46 P. B. Davies and F. Temps, J. Chem. Phys., in press. 47 J. M. Brown, C. M. L. Kerr, F. D. Wayne, K. M. Evenson and H. E. Radford, J. Mol. Spec- 48 A. Scalabrin, R. J. Saykally, K.M. Evenson, H. E. Radford and M. Mizushima, J. Mol. 49 P. B. Davies, W. Hack and J. G. Wagner, Faraday Discuss. Chem. SOC., 1981, 71, 15. 51 R. W. McKellar, High Resolution Spectroscopy, Faraday Discuss. Chem. SOC., 1981, 71, 63. 52 Y. P. Lee, R. M. Stimpfle, R. A. Perry, J. A. Mucha, K. M. Evenson, D. A. Jennings and C . J. 53 R. J. Saykally, D. C. Reuter and K. M. Evenson, in preparation. 54 R. J. Saykally and K. M. Evenson, in preparation. 55 H. E. Radford and W. Rohbeck, personal communication. 56 W. W. Rigrod, J. Appl. Phys., 1963, 34, 2602. 57 T. G. Phillips, P. J. Huggins, T. B. H. Kuiper and R. E. Miller, Astrophys. J. Lett., 1980, 238, 58 H. R. Fetterman, G. Koepf, P. F. Goldsmith, B. J. Clifton, D. Buhl, N. R. Erickson, D. D. 59 J. W, V. Storey, C. H. Townes and D. M. Watson, Astrophys. J . Lett., 1981, 244, L27. 6o K. D. Tucker, M. L. Kutner and P. Thaddeus, Astrophys. J. Lett., 1974, 193, L115. 122. 463. 3377. Chem. Phys. Lett., 1979, 65, 197. trosc., 1981, 86, 544. Spectrosc., to be published. R. J. Saykally, K. G. Lubic, A. Scalabrin and K. M. Evenson, in preparation. Howard, in preparation. L103. Peck, N. McAroy, P. E. Tannenwald, Science, 1981, 211, 580. L. M. Zuirys, R. J. Saykally, R. N. Plambeck and N. Erickson, in preparation. D. J. E. Knight, National Physical Laboratory, Teddington, Middlesex, U.K., NPL Report 63 F. R. Petersen, K. M. Evenson, D. A. Jennings, J. S. Wells, K. Goto and J. J. Jiminez, IEEE 64 A. Sanchez, C. F. Davis Jr, K. C . Liu and A. Javan, J. Appl. Phys., 1978,49,5270. 65 K. M. Baird, K. M. Evenson, G . R. Haines, D. A. Jennings and F. R. Peterson, Opt. Lett., QU-45 (1981). J. Quantum Electron., 1975, QE11, 838. 1979, 4, 263.

ISSN:0301-7249    

DOI:10.1039/DC9817100007

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Far-infrared Laser Magnetic Resonance Spectra of Vibrationally Excited OH and OD BY P. B. DAVIES,* W. HACK AND H. GG. WAGNER Max-Planck-Institut fur Stromungsforschung, 6-8 Bottingerstrasse, 3400 Gottingen, West Germany Received 14th January, 198 1 Several new far-infrared laser magnetic resonance spectra of OH and OD have been detected in the reactions of H or D atoms with ozone. The strongest spectra have been assigned to rotational tran- sitions in u = 6 OH and u = 4 OD of the X state. The OH radical is one of the most widely studied of all transient gaseous species, and is of fundamental importance in spectroscopy and in reaction kinetics and dynamics. Spectroscopic studies have ranged from the radiofrequency to the ultraviolet yielding a large amount of structural information which has been used to test ab initio calcu- lations.Of particular interest in the present work are the rotational transitions of the X 'II electronic state both in the ground and excited vibrational levels. Rotational term values were first tabulated by Dieke and Crosswhite' for levels up to v = 3 based on the analysis of the A 2C -+ X 211 system. Data on the higher vibrational levels have come from studies of the vibration-rotation Fourier transform spectrum.' Recently, Coxon3 has determined optimum molecular constants for OH based on the wealth of data available from optical, vibration-rotation and microwave spectra. His tabulated term values for OH from u = 0 to 5 are much more accurate than the earlier data and provide an excellent data set for calculating rotational transition wave- numbers.However, these term values exclude data based on the measurement of the rotational transitions themselves. Work by Clyne et al.4" on the A-X system of OD is a useful starting point for predicting approximate frequencies in the lower vibra- tional levels. However, much higher quality data have recently been published 4b from a simultaneous fit to vibration-rotation, electronic and microwave spectra. The transitions within the X 211i manifold of OH and OD that are amenable to study by far-infrared laser magnetic resonance (1.m.r.) can be approximately classified as: (i) pure rotational transitions within the R = 1/2 or 3/2 states and (ii) transitions where AQ = & 1 , which are forbidden electric dipole transitions in the limit of a pure Hund's case (a) molecule.However, although relatively weak the latter were among the first 1.m.r. transitions discovered and as~igned.~ Early work on the ground vibra- tional state at 79.1 ,urn was later extended to u = 1 , 2 and 3 at 78.4 and 79.1 Based on precisely measured or calculated g-factors these studies yielded accurate values of the 2113,2, J = 3 ++ 2111,2, J = + fine-structure spacing with change in vibra- tional quantum state. In contrast the allowed rotational transitions (i) are much stronger and in particular the OH 'rI3,2 u = 0 J = 3 + 4 1.m.r. transition around 118 pm is one of the most common and widely used in kinetics.' However, because the * Permanent address : Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP.16 FAR-INFRARED L .M . R . OF OH A N D OD rotational interval changes markedly with u, for the pure rotational transitions, it is difficult to obtain 1.m.r. spectra arising from more than one vibrational state with a single laser line. Several examples of both types of transition have recently been des- cribed by Geiger et al. for u = 0 OD(X Fortunately there is a convenient and efficient source of vibrationally excited OH and OD radicals, namely the reaction of H or D atoms with ozone.9 At short reac- tion times it is relatively easy to generate considerable concentrations of vibrationally excited molecules at pressures of 1.3 mbar or less by this reaction. In the present work we describe an extension of earlier studies6 on vibrationally excited OH at 78.4 and 79.1 pm to pure rotational transitions in vibrationally excited OH and OD using other water-vapour laser lines.The 1.m.r. studies complement earlier electron paramagnetic resonance measure- m e n t ~ ~ - ' ~ of vibrationally excited OH and OD which provide useful tests of the theo- retical accuracy of the g-factors necessary to calculate the Zeeman parameters. EXPERIMENTAL The far-infrared laser magnetic resonance spectrometer has been described in detail else- where.13 The fast flow system used to generate OHt was similar to that employed in the earlier studies6 but modified to maximise the effective sample volume by displacing the en- trance and exit ports of the flow system in the magnetic-field region. The nominal pumping speed was 4000 dm3 min-'. Optimum signals of OH? were produced by discharging He (99.996 %), which had been passed through Teflon tubing slightly porous to air, in a 2450 MHz discharge and mixing the discharge products with ozone as close to the laser axis as possible.Ozone was prepared by passing pure O2 through a commercial ozoniser (Argentox mbH), storing on silica gel (-78 "C) and displacing it from the trap as required with a flow of pure He. The concentration of vibrationally excited OH was found to be critically dependent on the amount of H2 present in accord with the known rapid relaxation14 of OH? by Hz. The addition of H2 to the discharge was only of marginal value in increasing [OH?] and often yielded considerably smaller concentrations. Vibrationally excited OD was produced by discharging D2 + He mixtures and reacting the products with 03.For both OH and OD, concentrations of vibrationally excited species were relatively inert to pressure between 0.4 and 0.9 mbar. Laser lines used in this study were those from H20 and D20 gas discharge lasers, namely: HzO D20 78.4, 79.1 and 118.6 ,uni 84, 108 and 171 ,urn. The frequencies of these lines are accurately known ( f l MHz) with the exception of the D20 line at 171 pm. This line does not appear to have been used before in published 1.m.r. work and Benedict et a1.16 give its wavenumber as 58.25 cm-'. Interchange between laser lines and suppression of higher order modes was greatly facilitated by incorporating an intracavity iris. The magnetic field was calibrated by measuring the positions of other well- known spectra.Examples of the strongest spectra obtained, with a He cooled bolometer detector, field modulation at 90 Hz and phase-sensitive detection are shown in fig. 1, 2 and 3 and discussed below. RESULTS ASSIGNMENT AND ZERO-FIELD SPACINGS The assignment of the new spectra reported here to " pure " rotational transitions within the 2113,t manifold of OH and OD is relatively straightforward. Allowed rotational transitions for levels up to u = 5 were first derived from Coxon's3 termP . B . DAVIES, W . HACK A N D H . G g . WAGNER rs < + t t I 'L -lr a t a -. I t , IN I 1 1 N I nlN f- t c: 3 4 n 5 .t -? N 3 c 0 1718 FAR-INFRARED L.M.R. OF OH AND OD I U 1 1 .o 1.1 magnetic field intensity/T FIG, 2.-High-field portion of the 108 pm c spectrum showing two components (0) of thef-tf series assigned in fig.4 and the M,. = + 1 /2 component of the e + e series shown in fig. 1 . values. For a particular transition the rotational spacing varies linearly with vibra- tional quantum number. The v = 0 to 5 data were fitted with a linear regression for- mula (r2 = 0.999 98) and then used to predict transition wavenumbers for levels from u = 6 to 9, which can also be populated in the H + O3 reaction9 Results are given in table 1. Transitions in the lower vibrational levels of OD taken from the term values of Amiot et are also given in table 1. These data are sufficient to assign the strongest spectra and examples for OH and OD are given below. i AM, = * I 0 0.5 1 .o magnetic field intensity/T FIG.3.-L.m.r. spectrum assigned to v = 4 OD X 2r13,2 in perpendicular polarisation with the 171 pm D20 laser.P. B. DAVIES, W. HACK AND H. Gg. WAGNER 19 TABLE 1 .-ROTATIONAL TRANSITION ENERGIES IN THE LOWER VIBRATIONAL LEVELS OF OH AND OD (X 2n3,2) OH" parity e - e f-f e - e f - f e + + e f -f v = o 1 2 3 4 5 6 7 8 9 83.723 80.733 77.772 74.828 71.888 68.931 65.97 63.01 60.06 57.102 83.869 80.868 77.897 74.944 7 1.994 69.027 66.06 63.09 60.12 57.16 1 18.208 113.904 109.649 105.427 101.21 6 96.990 92.73 88.49 84.25 80.01 1 18.453 114.136 109.865 105.627 101.401 97.160 92.88 88.63 84.3 7 80.12 153.189 147.524 141.930 136.385 130.859 125.319 119.71 114.14 108.57 103.01 153.535 147.850 142.236 136.670 13 1.123 125.562 119.94 114.35 108.76 103.17 OD v = o 46.384 46.414 65.063 65.117 83.833 83.916 1 45.159 45.187 63.333 63.384 81.587 8 1.666 2 43.944 43.970 61 -61 8 61.666 79.363 78.437 3 42.737 42.761 59.91 5 59.960 77.154 77.224 4 41.534 41.557 58.219 58.262 74.958 75.024 5 40.333 40.354 56.526 56.566 72.765 72.827 a Data for u = 0-5 taken from ref.(3) estimated uncertainty ca. 0.002 cm-'. Data for D = 6-9 from a linear extrapolation of the u = 0-5 data with an estimated error < 0.05 cm-'. Data taken from ref. (46) with stated accuracy 0.005 cm-'. THE 108 ,urn SPECTRUM OF OH The spectrum recorded with this laser line in perpendicular polarisation is shown in fig. 1 and 2. The frequency of the 108 pm D20 laser l5 is 2 783 066.6 Ifi 1 MHz = 92.833 109 cm-', close to the J = 3 -+ transition in u = 6 OH (table 1).The g- factors required to calculate the Zeeman effect in these rotational levels have not been measured but can be calculated using Radford's formulae.'' Lee and Tam9 have shown that the theoretical g-factors calculated in this way are in excellent agreement TABLE 2.-MOLECULAR PARAMETERS FOR OH V = 6 AND J = 512 ++ 7/2 ROTATIONAL INTERVALS vL = 2 783 066.6 -i 1 MHz = 92.833 lo9 cm-' E = 27 627.0 cm-' Lee and Tam9 V (ITIALy + 2 BLylZ>o = -56.16 cm-' <nlBLyl z>o R = ($) - - 9.812 J = 512 J = 712 g; calc 0.466 30 0.310 05 g7 calc 0.467 85 0.311 91 f i e - fk f i r - f 1 r 92.71 5" 92.874" " Estimated error <0.002 cm-'.20 FAR-INFRARED L.M.R. OF OH A N D OD with experimental e.p.r. measurements of g $ for OH X2113,,, J = 4 for levels ZJ = 5 to 9.We have adopted the same procedures to calculate the g-factors for J = 3 and z, u = 6, and these are given in table 2. The parameters required in the calculation were taken from Lee and Tam9 and are also given in table 2, together with the zero- field spacings which are derived from the 1.m.r. spectra. The results are in excellent agreement with predictions from the extrapolation of Coxon's work. By calculating the A doubling frequencies for both rotational levels it was confirmed that the laser frequency falls between the rotational transition frequencies. An alternative assign- ment in which both transitions are higher or lower in frequency than the laser gives an unacceptable value for vL(+) - vA(3). THE 171 pm SPECTRUM OF OD The much smaller A doubling in the corresponding rotational levels of OD results in a more compact 1.m.r.spectrum. This can be seen in the 171 pm D20 laser spec- trum shown in fig. 3, where the two series arising from thef-fand e t) e transitions are correspondingly closer together than in OH. The wavenumber given by Benedict et aZ.,16 58.25 crn-l, is close to the J = 9 -+ $ transition in u = 4 OD (table 1) and this strong spectrum can be positively assigned to this transition. The g-factors re- quired to calculate the Zeeman pattern were derived in a similar manner to OH and based on the data of Rashid et a1.12 (table 3). TABLE 3,-cALCULATED &'-FACTORS FOR J = 5/2 AND 712 OD, x ' ~ J / z IN THE U = 4 LEVEL J = 512 J = 712 g : 0.4329 0.2807 gJ 0 . 4 3 3 3 0.2812 Assuming that the laser frequency is sufficiently accurate, i.e., that the laser fre- quency is greater than the e t) e spacing and less than thef-fspacing, this leads to the assignment in fig.3. This was confirmed by frequency-pulling experiments, in which the laser frequency was changed slightly by cavity length adjustment and the field shifts examined. Supporting evidence for this ordering of the transition fre- quencies and laser frequency comes from a qualitative calculation of the A doubling intervals.12 DISCUSSION Assignment of the more intense spectra whether of the rotational or fine-structure type is relatively straightforward. However, we have detected several weaker spectra which can be attributed to 160H but which are accompanied by spectra of similar intensity probably due to HO, (DO,), other isotopic forms of OH and trace impurities.This makes assignment of the weaker 160H and 160D spectra difficult with the rela- tively few lines available from H20 and D20 lasers. For example, one of the more prominent of the weaker spectra (5' : N z 10 : 1) appears with the 118.6 pm H20 laser and could be due to the J = 3 -+ 3 transition in u = 8 OH. It is clear from table 1 that by using the more versatile optically pumped spectrometers there are many more accessible 1.m.r. spectra in the higher vibrational levels. This should permit rotational, fine-structure and A-doubling parameters to be measured for levels higher than. u =5 in OH for which there are already high-qualityP. B. DAVIES, W . HACK AND H . G g . WAGNER 21 0.7 0.6 0.5 0.4 0.3 0.2 4 0.1 E J; =I 2 0 - 7 - 0.1 cd -0.2 cd -0.3 -0.4 2 I .X J; - 5 2 2 - 0 .3 * U - 0 . 5 -0.6 I 1 0.5 10 magnetic field intensity/T FIG. 4.-Zeeman components of the J = 5/2 --f 7/2 ( f + f ) transition in OH X 'ILi2, u = 6. The DzO laser frequency has been subtracted from the zero-field frequency: 0, perpendicular; A, parallel 1.m.r. transitions. G. H. Dieke and H. M. Crosswhite, J . Quatit. Spectrosc. Radiat. Tratisfer, 1962, 2, 97. J. P. Maillard, J. Chauville and A. W. Mantz, J . Mol. Spectrosc., 1976, 63, 120. J. A. Coxon, Can. f. Phys., 1980, 58, 933. ( a ) M. A. A. Clyne, J. A. Coxon and A. R . Woon Fat, f. Mol. Spectrosc., 1973, 46, 146. (6) C. Amiot, J-P. Maillard and J. Chauville, J . Mol. Spectrosc., 1981, 87, 196. K. M. Evenson, J. S . Wells and H. E. Radford, Phys. Rev. Lett., 1970, 25, 199. P. B. Davies, W. Hack, A. W. Preuss and F. Temps, Chetn. Phys. Lett., 1979, 64, 94. C. J. Howard and K. M. Evenson, J . Cherii. Phys., 1974, 61, 1943. K. P. Lee and W. G. Tam, Cheni. Phys., 1974,4,434. * J. S. Geiger, D. R. Smith and J. D. Bonnett, Chein. Phys. Lett., 1980, 70, 600. lo K. P. Lee, W. G. Tam, R. Larouche and G. A. Woonton, Cait. J . Phys., 1971, 49,2207. l1 P. N. Clough, A. H. Curran and B. A. Thrush, Proc. R. Suc. Lorzdun, Ser. A, 1971, 323, 541. l2 M. H. Rashid, K. P. Lee and K. V. L. N. Sastry, J . Mol. Spectrosc., 1977, 68, 299. l3 W. Hack, A. W. Preuss and H. Gg. Wagner, Ber. Butisetrges. Phys. Cheni., 1978, 82, 1167. l4 J. E. Spencer, H. Ender and G. P. Glass, Sixteenth Znt. Synip. Cotiibiistiorz (The Combustion l5 J. T. Hougen, H. E. Radford, K. M. Evenson and C. J. Howard, J . Mul. Spectrosc., 1975, 56, l6 W. S. Benedict, M. A. Pollack and W. J. Tomlinson, ZEEE J . Qirantiim Electron., 1969, QE-5, l7 H. E. Radford, Phys. Rev., 1961, 122, 114. Institute, Pittsburg, 1976), p. 829. 210. 108. J. A. Coxon, K. V. L. N. Sastry, J. A. Austin and D. H. Levy, Can. J . Phys., 1979, 57, 619.

ISSN:0301-7249    

DOI:10.1039/DC9817100015

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Molecular-beam Spectroscopic Studies of Intermolecular Interactions BY BRIAN J. HOWARD Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 342 Received 8th January, 198 1 The results of molecular-beam electric-resonance studies of weakly bound van der Waals com- plexes are discussed. The use of spectroscopic constants to investigate intermolecular interactions is critically examined. Potential surfaces for the interaction of rare-gas atoms and hydrogen halides are presented and their accuracy is assessed. There have been great advances in recent years in our understanding of the inter- action between two molecules. Although two molecules in close proximity each modify the mechanical and electrical properties of the other,l most studies have been directed towards a determination of the intermolecular forces and a description of the interaction potential surface.The combination of bulk gas properties such as second virial coefficients and the viscosity of gases with crossed molecular-beam differential scattering has led to reliable potentials for a number of atom-atom systems.2 How- ever, the situation is far more complicated for atom-molecule or molecule-molecule systems since the potential is now anisotropic in that the potential is no longer just a function of the intermolecular distance but also depends upon the relative orientation of the molecules. Most attempts to invert the data from bulk gas properties or molecular-beam scattering have led to eflective isotropic potential^,^ since all the observed effects are some angular average over the anisotropic potential.Information on the anisotropic interactions has been obtained by a number of spectroscopic techniques. Both pressure-broadening of rotational lines and rotational-energy transfer5 depend upon the anisotropic interaction during collisions. The magnitude of these effects can provide useful information on the strength of the anisotropy of the potential, but it is difficult to invert such experimental data to provide meaningful anisotropic potentials. A more recent and a potentially far more informative approach is the detailed spectroscopy of van der Waals molecules, those weakly bound complexes held together by intermolecular forces, not covalent bonds. Since the strength of such van der Waals bonds is usually < 5 kJ mol-', the dimers exist in very low concentrations at most temperatures and pressures.The equilibrium population of the complexes may be increased by lowering the temperature or raising the pressure but this is limited by the equilibrium vapour pressure of the separate components. Despite these difficul- ties, the infrared spectra of a number of van der Waals molecules have been observed under equilibrium conditions,6 but only the molecular complexes of hydrogen studied by Welsh and co-workers7 have been fully analysed.24 MOLE c u LA R -BE A M s T u D I E s OF I N TER MOLE c u LA R I N T ERA c TI o N s MOLECULAR-BEAM SPECTROSCOPY An efficient way of overcoming the low concentration of van der Waals molecules in equilibrium systems is to use the cold molecular beams formed by expanding a gas at high pressure through a supersonic nozzle into a vacuum.During the expansion almost the entire enthalpy of the gas is converted into directed motion of the gas, The resulting very narrow spread in the velocities of the molecules in the jet may be characterised by a very low effective translational temperature, often < 1 K. Under these dynamic conditions complexes can be formed in large concentrations. The almost complete absence of collisions in such a beam also makes it an ideal environ- ment for the study of the spectra of weakly bound complexes. Several spectroscopic techniques have been applied to the study of complexes in a beam. Smalley et al. have used the technique of laser-induced fluorescence.* The molecules in the beam are excited by a tunable laser and the resulting fluorescence is monitored. More recently Gough et al.have detected the infrared spectrum of complexes by bolometric spectrometry.’ Here the molecular beam is incident on a cryogenic bolometer and crossed by a tunable infrared laser. Radiation absorbed by molecules in the beam is carried to the bolometer where it is converted to heat. If photodissociation of the complex is rapid the spectrum is observed as a negative signal at the bolometer. Despite the many recent advances in these methods, the most detailed information to date on the structure and dynamics of van der Waals molecules has been obtained in the microwave and radiofrequency region of the spectrum by molecular-beam electric-resonance spectroscopy.The supersonic nozzle beam passes through two inhomogeneous electric field regions which focus molecules in a given quantum state onto a high-sensitivity mass-spectrometer detector. Microwave or radiofrequency radiation is applied to the molecular beam between the two focusing fields and transitions are detected as a change in the beam flux at the detector. The rotational spectra of the van der Waals molecules obtained by this method may be analysed to give spectroscopic constants similar to those for normal semi-rigid molecules. The most important are the rotational constants, the centrifugal distortion constants, the electric dipole moment and the electric quadrupole coupling constants for nuclei with I >, 1. In order to use these constants to provide information on the structure and dynamics of the molecule it is necessary to assume a model for the com- plex.Since the van der Waals interaction is much weaker than a normal chemical bond it is normally a good approximation to assume that each of the constituent molecules retains its electrical and mechanical properties on complex formation. The rotational constants are directly related to the inverse of the moments of inertia of the complex. If the structures of the individual molecules in the complex are known the van der Waals bond length may then be determined. This is a vibra- tional average of possibly large-amplitude vibrational motion associated with the weak bond, but even a simple analysis gives direct information on the equilibrium structure of the complex and hence the position of the minimum in the potential-energy surface.The centrifugal distortion constants reflect the changes in the rotational constants due to the centrifugal forces in a rotating complex. These are related to all the bond force constants within the complex but, since the van der Waals bond is at least a factor of 100 weaker than normal chemical bonds, the observed centrifugal distortion effects are dominated by the force constants associated with the van der Waals inter- action. These data provide information on the curvature of the potential around its minimum. In pseudo-linear complexes like AroHC1 there is little direct information on theB . J . HOWARD 25 bending potential surface. It is in principle possible to observe the bending vibration of the complex by far-infrared spectroscopy, but such spectra are generally very weak and have not been observed in a beam.The only work to date was conducted in a high-pressure static cell and provides only low-resolution data." However, because of the large-amplitude bending motion in rare-gas-hydrogen-halide complexes, the dipole moments and quadrupole coupling constants provide information on the effective angular structure and hence on the amplitude of the bending motion. The values of the constants measured are the components of the monomer along the a-inertial axis. If y is the angle between this inertial axis and the bond of the diatomic molecule, the measured dipole moment is pa = po (Pl(cos y ) ) + small corrections where p o is the dipole moment of the isolated diatomic molecule; the small corrections arising from polarisation and possible charge transfer have also been included.The polarisation correction is in principle calculable and Flygare et al. have provided evidence that charge-transfer effects are unimportant." However, there is usually a large uncertainty in the polarisation correction SO that the values of (P,(cos y ) ) = (cos y ) also possess a large residual uncertainty. An inherently more accurate measure of the angular geometry is provided by the quadrupole coupling constants. The measured value is again that along the a-inertial axis and is related to that in the free molecule via the second Legendre polynomial This is expected to be a far better approximation than that for the dipole moment.The field gradient at the nucleus is dominated by the electrons close to the nucleus and their charge distribution is almost certainly little affected by the formation of a van der Waals bond. Furthermore it is now often possible to check the consistency of this model by measuring the quadrupole coupling constant of a second nucleus within constituent molecules or by measuring the nuclear spin-spin dipolar coupling constant, d,. Both constants should be related to the monomer constants by the same value of (P,(cos 7)). The results for Ar-HBr and Kr*HCl are summarised in table 1. All TABLE 1 .-COMPARISON OF ANGULAR EXPECTATION VALUES FOR Ar-DBr AND Kr*HCl P2(COS Y) y p 2 = arc cos <c0s2y)+/" AI-.D'~B~ 0.520 556( 0.511 8(89)' 0.5 16( 108) 0.514 O(95)' 0.523( 104) 84Kr*H35C1 0.432 386(13) 0.436 5(52)d 84Kr*H37C1 0.432 2(7) * 0.419 (34)d 84Kr.D35C1 0.605 467( 14) 0.604 4(46) 0.51 (1 5) ArmD81Br 0.520 837(12)" 34.427 l(4) 34.80(36) 34.6(44) 34.4156(5) 34.69(39) 37.9628(5) 37.8(2) 3 8 3 1 3) 30.8543(6) 30.9(2) 34.3(43) 37.97(3) 35(6) a From (eQq)"'; from (eQq)"'; from (eQq)"; from d,.26 MOLECULAR-BEAM STUDIES OF INTERMOLECULAR INTERACTIONS the values agree within their respective experimental uncertainties ; it appears that the assumptions of the model are accurate to much better than 1%.INTERMOLECULAR POTENTIAL-ENERGY SURFACES Although there have been few high-resolution studies of the vibrational motion of van der Waals molecules there is usually sufficient information from rotational spectra to determine an intermolecular potential-energy surface.Such spectroscopic data on van der Waals molecules have a particular advantage over those of bulk gas properties and molecular-beam scattering in that spectroscopic constants are usually of extremely high precision and are each only sensitive to one particular feature of the potential surface. In many ways, the potential surface is similar to the force field of a normal polyatomic molecule. However, because of the larger amplitude of the motions associated with the van der Waals bond, the potential can often be determined over a larger range of configuration space. All of the currently available data are fairly insensitive to the coupling of the vibrations of the free molecule to those of the van der Waals bond; such effects have only been investigated in the hydrogen complexes." The principal reason for this lack of sensitivity is the much greater frequency of normal vibrations than those associated with the van der Waals bond. The two types of motion may then be separated in a manner similar to the Born-Oppenheimer separation of electronic and nuclear motions.As a consequence the intermolecular potential that is determined is just an average over the effects of the internal motions of the constituent molecules. In the particular case of an atom-diatom complex the Hamiltonian for the system is greatly simplified. The intermolecular potential energy is now just a function of the intermolecular distance R and the angle between R and the diatom bond.The Hamiltonian for the system can be written in the form (2) h2 x=--- h2 + b j z = - ( J - j)' + V(R,O) 2p aRz 2pR2 where p is the reduced mass of the complex, bmo, is the rotational constant of the diatomic molecule, j is the angular momentum of the molecule and J is the total angular momentum of the complex. The terms describe, respectively, the kinetic energy for the stretching of the van der Waals bond, the rotational energy of the molecule (which becomes a vibration in the presence of an anisotropic potential), the end-over-end rotational energy of the compIex and the intermolecular potential energy. A convenient expansion of the potential is in terms of the Legendre polynomials P,(cos 0) which form a complete set of angular functions. Thus where V,(R) is any suitable function for a one-dimensional well, such as a Lennard- Jones function.A more realistic functional form is given by the Maitland-Smith parameterisation l3 in which the Lennard-Jones exponent n is allowed to vary linearly with intermolecular distance. The radial potential is then written as where x = R/R,, R , is the equilibrium separation, E is the well depth and n is allowed to vary with R, 12 = rn + y(x - 1) ( 5 )B. J . HOWARD 27 where m and y are constants. The radial curvature at the minimum is independent of y, so that y is not determined by the rotational spectrum. y is typically 10 for rare-gas potentials13 so it has been fixed at this value. In order to match the leading term in the long-range P,(cos 0) anisotropy, the radial potential for k = 1 has n replaced by 7 everywhere in eqn (4).This leaves a large number of adjustable parameters to be determined so that the summation in eqn (3) is normally truncated at k = 2 or 3. Although this multipole type series converges rapidly at large intermolecular distances, there is no reason to suppose that it will do so in the region of the well. Another disadvantage of the multipole parameterisation is that the most readily determined features of the surface, such as the position of the minimum and the curvature at the minimum, are not simply related to the adjustable parameters. An alternative, preferred parameterisation specifies the potential as a function of R at each of several values of 8. The potential at 8 = 0, 60, 120 and 180" is written as a Maitland-Smith function [eqn (4)] and at a given R is interpolated between these angles using the Legendre polynomials P,(cos 8) to P,(cos 8).This is referred to below as the " 4-angle " parameterisation. The vibrational motion of the hydrogen halide within a complex samples the region from 8 = 0" to, typically, 8 = 90". There is little information on the potential at 120 and 180"; these potential parameters are therefore fixed at some " physically reasonable " value. Several methods have been applied to obtain the eigenvalues and eigenfunctions of 5 .O 4 . 5 5 g 4 . 0 3.5 FIG. 1 .-Multipole the dashed I I 0 30 60 90 120 150 910 potential for KrHCl. Contours are at 10 cm-' intervals above the minimum; curves outline the classically accessible regions for KroHC1 and Kr-DCl.eqn (2). The close coupling14 and the secular equation'' methods have been favoured by some, but the most efficient appears to be the angular-radial separation method of Holmgren et aZ.I5 The eigenfunctions are assumed to be the simple product q(0;R) x(R) of an angular and a radial function. Initially the angular problem is solved at each value of R using free-rotor spherical harmonics as basis functions. This yields eigenfunctions q b ( 8 ; A) and eigenvalues ub(R) which depend parametrically on R.28 MOLECULAR-BEAM STUDIES OF INTERMOLECULAR INTERACTIONS Ub(R) provides an effective potential for the radial motion from which x(R) may be determined. This procedure is a fairly good approximation providing q,,(O;R) is a slowly varying function of R .For accurate calculations, however, the neglected non- adiabatic coupling of different qb(8; R) must be included: a rapid procedure developed by Hutson has been used.16 A least-squares fit to the rotational spectra of the different isotopic forms of a number of atom-diatom complexes has been performed using both the multipole and " 4-angle " parameterisations. Fig. 1 and 2 show the results for Kr*HCl. The data 5.0 4 . 5 4 . 0 3 . 5 I - 0 30 60 90 120 150 01" the dashed curves outline the classically accessible regions for Kr*HCl and Kr-DCl. FIG. 2.--" 4-angle " potential for Kr*HCl. Contours are at 10 cm-' intervals above the minimum; are insensitive to the absolute well depth, so that the isotropic well depth was set at 155 cm" by scaling the molecular-beam scattering results of Farrar and Lee for AI-*HC~.~ The multipole potential expansion was truncated at P,(cos 8).The repulsive wall parameters rn for the anisotropic potential were constrained to the same value as the isotropic m ; the values of R, in V,(R) and V,(R) were constrained to be the same, but different from the value in Y,(R). This gives a potential with just 5 adjustable parameters. The " 4-angle " potential was also constrained to 5 para- meters. The 6 = 0 and 60" well depths were allowed to float, while those at 8 = 120 and 180" were fixed at the same value to one another. The wall parameter rn was constrained to be the same at all angles. Lastly, R, was allowed to take different values at 6 = 0 and 60" but the values of R, at 120 and 180" were forced to be the same as that at 60".A comparison of the two potentials is physically illuminating. The multipole potential possesses a large secondary minimum at 0 = 180" whereas this is precluded in the " 4-angle " potential. However, within the classically accessible region around 8 = 0" the two potentials have a very similar shape. Thus although the results should be reliable in this classically accessible region, potentials obtained from spectroscopic data should not be extrapolated far beyond the classical turning points.B . J . HOWARD 29 CONCLUSION Spectroscopic data can be used to determine the shape of the well in the region of the potential minimum. However, in order to determine the entire potential surface it is essential to include the data from many different experiments.We are currently combining the results of molecular-beam scattering, rotational line-broaden- ing and second virial coefficients with spectroscopic data of the van der Waals complex to determine the potential for Ar.HC1.” I am grateful to Dr. A. E. Barton, Dr. P. R. R. Langridge-Smith and Mr. J. M. Hutson for numerous discussions of the results presented here. H. L. Welsh, M.T.P. International Review of Science, Physical Chemistry, Series One, 1972, 3, 3 3 ; J. Van Kranendonk, Physica, 1958,24,347. For example H. J. Hanley, J. A. Barker, J. M. Parson, Y. T. Lee and M. Klein, Mol. Phys., 1972, 24, 11. ’ J. M. Farrar and Y. T. Lee, Chem. Phys. Lett., 1974,26,428; U. Buck and P. McGuire, Chern. Phys., 1976, 16, 101. J. G. Kircz, G. J. Q. van der Peyl, J. van der Elsken and D. Frenkel, J. Chem. Phys., 1978,69, 4606. T. Oka, Adv. Atom. MoI. Phys., 1973, 9, 127. For example M. Lavor, J-P. Houdeau and C. Haesler, Can. J. Phys., 1978,56,334; C . A. Long and G. E. Ewing, J. Chem. Phys., 1973,58,4824. A. R. W. McKellar and H. L. Welsh, J. Chem. Phys., 1971, 55, 595; Can. J . Phys., 1972, 50, 1458; 1974,52, 1082. T. E. Gough, R. E. Miller and G. Scoles, J. Chem. Phys., 1978, 68, 1588. * D. H. Levy, Adv. Chem. Phys., 1981, to be published. lo E. W. Boom and J. van der Elsken, J. Chem. Phys., 1980,73, 15. l1 M. R. Keenan, L. W. Buxton, E. J. Campbell, T. J. Balle and W. H. Flygare, J. Chem. Phys., l2 R. J. LeRoy and J. Van Kranendonk, J. Chem. Phys., 1974, 61,4570. l3 G. C. Maitland and E. B. Smith, Chem. Phys. Lett., 1973, 22,443. l4 A. M. Dunker and R. G. Gordon, J. Chem. Phys., 1976, 64, 354. l5 S. L. Holmgren, M. Waldman and W. Klemperer, J. Chem. Phys., 1977, 67, 4414; J. Chem. l6 J. M. Hutson and B. J. Howard, MoI. Phys., 1980, 41, 1123. 1980,73, 3523. Phys., 69, 1661. J. M. Hutson and B. J. Howard, Mol. Phys., 1981, to be published.

ISSN:0301-7249    

DOI:10.1039/DC9817100023

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

New Developments in the Gas-phase Rotational Spectroscopy of High-temperature Species and Unstable Molecules BY MANFRED WINNEWISSER Physikalisch-Chemisches Institut, Justus-Liebig-Universitat Giessen, Heinrich-Buff-Ring 58, D-6300 Giessen, W. Germany Received 10th December, 1 980 The detection of gaseous molecules of unstable and high-temperature species in the interstellar environment has prompted the development in many laboratories of methods of detecting such spe- cies in hostile environments such as flames, plasmas and pyrolysis reaction processes. Our laboratory has participated in the development of a method for producing high-temperature species at ca. 500 K instead of the 1200 to 3000 K to which the solid alkaline earth oxides or sulphides must be heated in order to produce sufficient vapour pressure for millimetre-wave detection.The method is applicable to many molecular systems including KCN and probably LiCN and NaCN whose molecular dynamics is expected to be anomalous and is not yet understood. Included in the discussion is our work on the plasma formation of long-chain cyanoacetylenes and the pyrolysis production of N-cyanoformimine, the millimetre-wave spectrum of which we have studied. In the field of molecular dynamics our results for the v7 manifold of C30S will be presented and the pronounced change of the bending potential functions between C302 and C30S will be shown. 1. INTRODUCTION Flames, plasmas and pyrolysis are well-established chemical processes for oxida- tion reactions, the decomposition of compounds, the generation of heat and/or the formation of new species.Today there are huge industrial operations built around combustion, electrical discharges and pyrolysis or cracking processes. All these pro- cesses, not forgetting stellar chemistry, involve high-temperature phenomena and atomic and/or molecular high-temperature species. I wish to describe in this paper the contribution made primarily in our laboratory towards the development of milli- metre-wave rotational spectroscopy of high-temperature species, unstable species, especially those occurring in plasmas and pyrolysis experiments, and molecular dyna- mics. These heterogeneous-sounding topics are intimately linked, as I hope to show in the course of the discussion, by overlapping aspects of high-temperature gas-phase chemistry, interstellar chemistry and molecular dynamics, and by a growing need for extensive high-precision data concerning the types of molecular species to be discussed here.2. MILLIMETRE WAVES A N D HIGH-TEMPERATURE SPECIES Being associated with the radioastronomical aspects of microwave spectroscopy one learns to appreciate the astronomical fact that at the dawn of our Galaxy all of matter was in the form of hydrogen, some deuterium and possibly helium. Since then, some 10" years ago, 90% of the galaxies' mass (1.1 x 10" M o where 1 M o - 2 x g) has been converted into stars. The remaining 10% of its total mass is in the form of interstellar matter composed of the fossil material of the " big bang " and the exhaust materials returned by the stars in the various stages of their evolution.'32 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES The distribution of interstellar matter within the galaxy thus follows that of the stars.This also means that most matter finds itself in a state of high temperature, despite the fact that most interstellar molecules so far observed are rather stable species, with the exception of a few radicals, molecular ions and unstable species such as HNC.' Stimulated by the consideration of the calcium-bearing molecules in stars and the interstellar medium, where an apparent depletion of atomic gas-phase calcium was observed [the solar abundance of calcium is n(Ca)/n(H) - 2.14 x 10-6,2 whereas in the interstellar medium it is 1.8 x 10-9-l.l x 10-9,3*4], G. Winnewisser suggested in 1975 the development in our laboratory of millimetre-wave spectroscopy of high- temperature species such as CaO.However, despite all the efforts which will be sketched in this paper, the radioastronomical search for CaO in various stars and molecular clouds and by various research groups was unsuccessful, thus strengthening the suspicion that interstellar Ca occurs in grains.' (1). DEVELOPMENT OF DATA BASE It was realized by that the classical microwave spectroscopy of high-tempera- ture gases as reviewed by Lovas and Lide9 and developed to a high level of sophistica- tion by several research groups runs into difficulties with the refractory oxides and sulphides of the alkaline earth metals Ba, Sr, Ca, Mg and Be. This fact can be appre- ciated by inspecting table 1.As one ascends the alkaline earth group, progressively higher temperatures are required to vaporize the solid oxide or sulphide. The in- creasing tendency towards decomposition proved insurmountable for conventional hot-cell experiments; the vapour above solid BaO at 2200 K consists largely of dia- tomic BaO, but the vapour above SrO at 2500 K is estimated to contain only 5% dia- tomic SrO.'O No data for CaO etc., are available. It was therefore necessary to cir- cumvent some of the high-temperature container problems by synthesizing the high- temperature species directly in the gaseous phase by non-equilibrium gas-phase reac- tions. It was furthermore realized that the high-resolution spectroscopic data base TABLE 1 .-SELECTED DIATOMIC OXIDES AND SULPHIDES PRODUCED BY OXIDATION OF METAL VAPOUR ~~ ~~ m.pt.of m.pt. of m.pt. of metal oxide su Ip hide 1st i.p. cosmic abundance 1°C 1°C 1°C /eV (H = 1, C = 3.3 x Mg 651 2800 d > 2000 7.64 2.6 x 10-5 Ca 843 2580 d - 6.11 2.0 x Sr 769 2430 > 2000 5.69 7.1 x 10-lo Ba 725 1920 1200 5.21 8.9 x A1 660 2045 1100 5.99 2.5 x Fe 1535 1420 1193 7.90 4.0 x 1 0 - 5 Li 181 > 1700 975 5.39 5.0 x (I Li abundance should be much greater in certain stars. for many " exotic " molecular species (closed and open shell molecules, radicals and ions) present in a high-temperature environment should be expanded and that the millimetre-wave region is particularly suitable for such experiments since quasioptical techniques can be employed for the propagation of the electromagnetic radiation.New classes of molecules could be investigated. This situation was recently emphas-M . WINNEWISSER 33 ized by the committee on High Temperature Science and Technology in a report on “ High Temperature Science : Future Needs and Anticipated Developments.” l1 Since the review article by Lovas and Lide9 considerable progress has been made in the field of rotational spectroscopy of high-temperature molecules whose outstand- ing developments were pointed out in 1978 by Torring and Tiemann.12 These authors also emphasized the importance of combining microwave techniques with the available tunable lasers in double-resonance experiments (MODR). (2). LOW-PRESSURE FLAME SPECTROMETERS The first observation of chemiluminescence spectra from the reactions of Ba and Ca with NO, and N20 were reported by Ottinger and Zare in 1970.13 The motivation was to learn about the unrelaxed internal-state distributions of products following a highly exothermic reaction.A plethora of papers has since been published on the spectral analysis of the chemiluminescence and the laser-induced chemiluminescence of metal-vapour plus oxidant reactions 4-24 and optical-optical double-resonance excitation spect r o ~ c o p y . ~ ~ - 27 In our laboratory6-s a modified version of the produc- tion method given by Broida and c o w o r k e r ~ ~ ~ ~ ~ ~ was adapted to the free-space cell type millimetre-wave spe~trometer.~**~~ In these methods diatomic metal oxides and sulphides are produced by gas-phase oxidation of metal vapour entrained in an inert carrier gas.In the most recent paper from this laboratory, the millimetre-wave spec- trum of Bas in a low-pressure flame was reported.8 The formation of gaseous Bas was performed in a flow system within a reaction sphere of 35 cm i.d. by the reaction Ba + COS -+ Bas + CO. A block diagram of the millimetre-wave spectrometer with reaction sphere and free- space absorption cell is reproduced in fig. 1. The low-pressure flame can spread throughout the entire volume of the sphere, allowing the oxidation reaction to take place under conditions minimizing the effects of wall collisions. The reaction zone was characterized by a diffuse but intense greenish-yellow chemiluminescence glow which filled the entire sphere. Therefore, the path of oxidation has been lengthened, producing an enhancement of the metal oxide and sulphide abundances and resulting therefore in a pronounced increase in the signal-to-noise ratio of the absorption lines.This can be appreciated by inspecting fig. 2, where in the upper part the video dis- play is given for the 138Ba160 millimetre-wave spectrum as obtained with our initial system7 which was built with a Pyrex cross-piece of 10 cm i.d. as the low-pressure flame compartment. The lower part of fig. 2 gives an oscilloscope display of a se- quence of rotational lines of 13*Ba3*S, obtained with the third to eighth harmonics of the klystron fundamental frequency, thus covering simultaneously portions of the millimetre-wave spectrum between 100 and 297 GHz. The Bas spectrum was re- corded with the new spectrometer shown in fig.1. From the various measurements carried out on the species BaO, SrO, CaO, BaS and lately SrS it is estimated that our present system is more sensitive by a factor of 50 than the system reported in ref. (7). This was achieved by improving our basic milli- metre-wave system 2 8 9 2 9 in three different respects: (a) Introduction of a reaction sphere with a diameter of 35 cm instead of the usual free-space cell [fig. 1, ref. (7)] for the low-pressure flame. (b) Introduction of a dynamic stabilization scheme for the coherent radiation source while maintaining a fast sweep capability. (c) Use of an InSb photoconducting detector operating at 1.7 K for detection of millinletre-wave power. It should be pointed out that our (1) The entire system is discussed in ref.(8).34 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES combination of video-detection, dynamic frequency-drift stabilization, He-cooled InSb detector and signal averaging is particularly useful for searching in a dynamic chemical system which requires real-time monitoring of the spectrum while adjusting parameters. It is efficient when only low power levels are available, as is the case if crystal harmonic generators are employed as millimetre radiation sources. CELL FOR IUNING HOLECULT5 NEEDLE VALVES '\ L I n w w w n u 20mr POWER SUPPLY - - - r - y 2 F r < < e l ? s m o u ERROR VOLTAGE KLYSTRON SWEEP STABILIZER 5---7-iL II - 2 UARKER DRIFT u u - I. I SIGNAL - I 1L 3m, START OF MrA ALOUISIlIOII n- STA81112AllON POSITION k 1 - ERROR SIGNAI TRIGGER YbRKER ANG SECOND MARKER 1- TRIGGER PULSE FMI 1 I !5cm-mIGGERI I C W L .] [ C W M L l I L A W R A r O W PERIPUERAL INTERFACE MULTlPLEXER FOR INPUT CHANNELS M E AN0 HOLD AUFiIFIER ANALOG TO OlGlTAL WNVERlER REAL 1lUE CLOCK I[ PDPBII DEDICATED COMPUTER AND PERIPHERALS I1 1) ZSODATA POINTS IN CHANNEL 0 AND I 2 8 ~ 1 SAMPLING RATE SOFlWARE DATA ACOUISlrlONf ~* REOUCTION L !'CENTER FIG. 1 .-Block diagram of the millimetre-wave spectrometer with reaction sphere and free-space absorption cell for the study of low-pressure flames at the Justus Liebig University, Giessen [from ref. (S)]. The logic flow chart of the stabilization is given in fig. 1 together with the data- acquisition system. Fig. 3 presents the sequence of the stabilizer timing signals.The klystron frequency sweep stabilizer produces a correction voltage for each individual sweep so as to keep one point in the klystron frequency sweep fixed. Our results for BaO, SrO, CaO and Bas are collected in ref. (7) and (8). The constants listed there are expansion coefficients of the Dunham series.30 A few significant features of this type of rotational spectroscopy should be men- ti oned.M. WINNEWISSER 35 37396 37403 MHz lx 3 J = 10--9 8-7 6+5 13*Ba 32S I _ - . . ,*- J = 18-17 11-11 36-35 30-29 2!-23 18-17 FIG. 2.-Video display of absorption lines of 138Ba l6O and 138Ba32S. The 13'Ba160 lines were observed with our old system described in ref, (7), while the I3*Baj2S lines were obtained with the spectrometer shown in fig. 1 . (a) The relatively narrow linewidths of the observed millimetre-wave transitlons for the alkaline earth metal oxides and sulphides indicate low concentrations, which are probably offset by the very large dipole moments of these species.(b) From comparison of the relative intensities of the u = 0 and Y = 1 transitions shown in fig. 4 it can be shown that BaO, Bas and also the other oxides' are produced with a rather low vibrational temperature, a fact which was also observed in the chemi- SEOUENCE OF STABILIZER TIMING SIGNALS I I I n I I I I U - I <P I I I I n u ; I U I /I ADJUSTABLE GATE FOR SUPPRESSION OF SECOND MARKER VARIABLE GATE FOR STABILIZATION POSlllON (sp) RESPONSE CONTROL MARK@ SIGNAL EQROR SIGNAL PULSE iNIEGRATtD VOtlAGE (SAMPLE AND HOLD) BUILD-UP OF CORRECTION SIGNAL FINAL ERROR VOllAGE FOR INDlVlOUAl SWEEPS MARKER AFTER 5P MARKLR BEFORE SP FIG.3.--Sequence of stabilizer timing signals in the dynamic klystron stabilisation system indicated in fig. 1 [from ref. (8)].36 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES 13' Ba'b J=8-7 4 . . . . . + . . .? ' . * . . . l3*B0 32S 148927.802 MHz V = I 149599.838(301 MHz v = o * . - . . . . . . . 15L096.577 v = 1 154570.323 MHz v = o T I I 191041.354 v = 1 I 191 626.900 MHz v = o FIG. 4.-Computer-averaged absorption lines of 138Ba160 and '''Ba3'S in the states u = 0, I and 2 obtained with the original flame arrangement [first row, ref. (7)] and with the improved spectrometer of fig. 1 (second to fourth rows).M . WINNEWISSER 37 I SUPERCOND. B W 0 - DETECTOR. - - BSORPTION CELL I R - I J 1 300 GHz BIAS PR E AM PL I F.1 I PLL If _I I- FREQUENCY t NORMAL LOCK IN MIXER 5 MHz luminescence Approximate relative intensity measurements on the observed millimetre-wave lines yield an effective vibrational temperature of 500 & 100 K for both BaO and Bas. This is to be contrasted with the temperatures needed for va- porising the oxides or sulphides (see table 1) and even the metals. (c) Rotational spectroscopy of low-pressure flames can be considered a real alter- native to hot-cell spectrometers which operate in the temperature ranges between 1300 and 2300 K. In such systems the high-temperature container problems pose severe technical difficulties. In addition to these problems a great reduction of line strength in the high-temperature regime is suffered as Torring and Tiemann12 have pointed out, since the peak absorption coefficient for a rotational transition of a diatomic molecule, neglecting all hyperfine interactions, is approximately given by = 1.5 x 10-9.~,ic1L21~03p73~v (2) where Av is the linewidth parameter and may be considered independent of tempera- ture T,fv is the fraction of molecules in the vibrational state 21, i, is the fractional iso- --- 30 GHz !I POLYETHYLENE LENGTH 1G3Gmm ~ --+ OVEN +-- FIG.5.-Block diagram of millimetre-wave spectrometer for the study of low-pressure flames at the Free University of Berlin and schematic drawing of the reaction vessel (courtesy of T. Torring). t DISPLAY BWO 1 1 - SIGNAL AVERAGER SATURATION I - EFFECT 11 I38 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES topic abundance, p is the electric dipole moment in Debye, and vo is the frequency of the absorption line in MHz.In high-temperature systems the absorption path is usually considerably less than in room-temperature systems. Typical cell lengths are 100-20 cm for systems designed for 1000-2000 K. Therefore the peak absorption drops even further. In our flame experiment (fig. 1) the absorption path length is ca. 35-40 cm if we take the full dia- meter of the sphere. This also holds approximately for the millimetre-wave low-pres- sure flame spectrometer built by Torring and Hoeft and their respective coworkers, the block diagram of which is given in fig. 5 . We can conclude from the above dis- cussion that the peak absorption coefficient for molecules in the ground vibrational state is reduced approximately by a factor of 150 if the temperature is raised from 300 to 1300 K.For more complex molecules such as polyatomic linear, symmetric and asymmetric rotors the situation is even worse, owing to the large number of vibrational and rotational states. Therefore, the pivotal requirements, which this type of spectrometer does fulfil for many species, are high spectrometer sensitivity in the milli- metre- and submillimetre-wave region and an efficient production scheme at a mini- mum temperature for the molecules to be investigated. (3). HIGH-TEMPERATURE SPECTROMETER Besides the flame spectrometers discussed above and shown in fig. 1 and 5 Torring and c o - ~ o r k e r s ~ ~ ’ ~ ~ have developed the conventional hot-cell spectrometer to a high degree of sophistication. The temperature range has been expanded to close to 2300 K by a heated absorption cell with a current of Z 21 1000 A.In order to reduce pick- up from stray magnetic fields this current is rectified and the lock-in amplifier is gated in such a way that during the period of no current flow the phase-sensitive detection of the absorption signal takes place. An elegant modulation scheme which was introduced by T o r ~ - i n g ~ ~ makes use of the available high power of 20-100 mW of the backward wave oscillator millimetre- wave radiation sources now commercially available. This is indicated in the block diagram of fig. 6. Torring’s “ saturation-effect ” modulation can be employed when the incident power levels are sufficient to saturate a transition and the absorption becomes non-linear.This non-linearity allows the line signals resulting from the gas absorptions to be separated from the background. The sensitivity of such a spectro- meter employing saturation modulation compares favourably with the sensitivity of a Stark spectrometer. Fig. 7 shows chart-recorder traces for barium and strontium monofluorides obtained with the spectrometer system shown in fig. 6. This type of modulation can be used for any type of absorption cell since no Stark septum is re- quired. Both rectangular and cylindrical cells as shown in the lower part of fig. 6 have been used. The cells were directly heated by a current of a few hundred A. The Berlin group has successfully proved that in the temperature range 1000-2300 K not only the evaporation of highly refractory compounds can be achieved but also many high-temperature reactions can be exploited.In fact all high-temperature, open-shell molecules observed so far in the millimetre-wave region were first observed using this m e t h ~ d . ~ ~ ’ ~ ~ Subsequently, however, Ryzlewicz and Torring have found that both BaF and SrF can be observed more easily in a flame spectrometer. The hot-cell method retains the advantage of populating higher vibrational states sufficiently for their meas~rement.~~M. WINNEWISSER 39 I KLYSTRON OPEN MOUNT -100GHz ABSORPTION CEL Go As ' I 1 DETECTOR , - _ - _ - ~7 MICROWAVE TRANSITION I AND ELECTRICAL GUIOE I s 1000A LOCK IN 50 kHz SATU RAT I0 N I EFFECT , MODULATION FIG.6.-Block diagram of high-temperature spectrometer with saturation modulation at the Free University of Berlin, and various absorption cell cross-sections and reaction arrangements (a, b and c) that have been tried (courtesy of T. Torring).40 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES 88Sr "F N =6+5 d L=O R C = 3 s , single sweep i n 50slMHz i I 138g,19~ N = 7-6 I 1 v = o I \ - 1MHz 0 -6 FIG. 7.-Absorption lines of s8Sr'9F and 138Ba19F observed with the spectrometer in fig. 6 (courtesy of T. Torring). (4). COMMENTS ON THE SYNTHESIS OF HIGH-TEMPERATURE SPECIES Torring and TiemannI2 have recently summarized the various methods of produc- ing high-temperature molecules, and the reader is referred to their work. However, a few comments should be made. In standard hot-cell experiments the following types of reaction have been employed for obtaining sufficient vapour pressure (0.01 to 0.1 Torr): (a) direct evaporation : T W s , 1) + AB(g); (3) (b) reduction reaction and evaporation : T AB,(s, 1) + A(s, 1) -+ AB(g) + - - - ; (4) (c) reduction reaction and evaporation when A(s, 1) is the most volatile partner: T AB,(s, 1) + C -+ AB + BC + .. . . Ryzlewicz and TOrring3, have successfully employed the third type of reaction in pro- ducing a sufficient concentration of the 2E-state radicals BaF and SrF by using the react ions T BaF, + A1 -+ BaF + A1F + Ba + . . . , T = 1450 K and T SrF, + A1 --f SrF + AlF + Sr + . . . , T = 1750 K. (7) No lines were found with arrangement (a) in fig. 6. However, with arrangement (b), where the cell walls were coated with A1 as a reducing agent, BaF lines were obtainedM. WINNEWISSER 41 with a signal-to-noise ratio of ca.50 in the ground vibrational state. The BaF, was evaporated directly into the cell from a separate sample boat which is part of the hot cell, so that no temperature gradient between cell and boat should be present. Thus, a non-equilibrium process cannot be excluded. The gas-phase oxidation reactions for producing the alkaline earth oxides and sul- phides have already been discussed in detail. The reaction takes place far from equilibrium conditions in the gaseous phase and is exothermic. Ryzlewicz and Tor- ring32 have employed this type of reaction in their low-pressure flame spectrometer (see fig. 5), Ba + IF6 --f BaF + .. . . (8) They mention that the IF6 concentration is critical since BaF + IF6 -+ BaF, can react further, and under stable conditions a signal-to-noise ratio of 50-100 was obtained for the rotational transitions in the ground state. Our experience agrees with that of Ryzlewicz and Torring that in flame experiments it is much easier to optimize the conditions of the flame reactions, especially since chemiluminescence can be used to monitor the reaction. Our laboratory has used flame reactions to explore a variety of chemical systems, including some which are potentially capable of producing polyatomic high-temperature species. The reaction K(g) -t BrCN 3 KBr + KCN (9) produces two gas-phase constituents, the spectra of both of which could be monitored.The KCN lines of Kuijpers et An attempt to find the polyatomic species CH3Li has failed so far. Only the lines of LiI as seen in fig. 8 and could be seen by video. relative frequency /MHz FIG. 8.-Absorption lines of the J = 5 t 4 transition of 'LiI2'I at 13 1 01 1.85 MHz, LiI observed by video in the reaction CH31 + 2Li (vapour) +LiI + CH3Li. Lower half shows calculated intensity and positions of quadrupole hyperfine components. solid (CH,Li), on the walls could be found. However, it is hoped that with the im- proved system described in this paper the detection of the gas-phase spectrum of CH3Li has become more feasible. As indicated by the above example, the spectroscopy of polyatomic high-tempera- ture species is considerably more difficult. In spite of that, progress has been made in the study of the alkali h y d r o ~ i d e s ~ ~ * ~ ~ with the investigation of the effects of large- amplitude motion.Perhaps the most interesting triatomic species so far studied in detail is KCN. Its rotational spectrum and structure have recently been reported by42 HIGH-TEMPERATURE SPECIES A N D UNSTABLE MOLECULES Torring et u Z . ~ ~ The molecule was found to have a non-linear, T-shaped molecular structure, N with a low-lying bending fundamental w2 157 cm-'. The authors combined mole- cular-beam electric-resonance and microwave data with the earlier millimetre-wave data23 and assigned 63 transitions to the ground vibrational state. The fit was made to a semi-rigid asymmetric rotor model with A = 58 265.62(7) MHz, B = 4 940.064(2) MHz, C = 4 536.208(2) MHz, five quartic centrifugal distortion constants and three sextic distortion coefficients. This result is undoubtedly one of the most interesting ones reported not only in the realm of high-temperature gas-phase spectroscopy but also for triatomic molecules in general.From the comparative ease with which we could obtain KCN lines with our low- pressure flame spectrometer I strongly believe that a wide variety of presently unknown high-temperature species such as LiCN, CH3Li and high-temperature radicals and ions can be investigated. Work on LiCN should be particularly rewarding since ab initio calculation^^^ indicate that the Li+ ion may move freely around the (CN)' anion at room temperature. All of the gas-phase species LiCN, NaCN, KCN, etc., must be characterized by large-amplitude motion of the lowest lying bending mode and should be further investigated by this type of method.3 . MILLIMETRE WAVES, PLASMA CHEMISTRY AND PYROLYSIS (1). INTERSTELLAR PLASMA CHEMISTRY In interstellar space hydrogen cyanide, HCN, and cyanoacetylene, HC3N, are distributed throughout a wide variety of different molecular clouds.' Winnewisser and W a l m ~ l e y ~ ~ have pointed out that the distribution of these molecules and the oc- currence of the cyanopoly-ynes (HC,N, HC7N and HC9N) in sources across the galaxy underline the trend that complex organic molecules are not limited in their occur- rence to the galactic-centre sources only, but appear in sources with very different physical conditions. These molecules are relatively abundant in cold (10 K) dark mole- cular clouds with densities in the order of lo4 ~ m - ~ , in some molecular clouds associ- ated with H I1 regions and in the molecule-rich circumstellar shells of late-type stars.As early as 1973 Buhl and Snyder4' and later Morris et al.41 noticed that the inter- stellar distribution of HNC and HCO+ is particularly highly correlated with that of HCN. Creswell et al.42 in their work on the millimetre-wave spectrum of HNC have summarized the interstellar emission features of HNC, HCN and HCO+. In fig. 9 the interstellar emission signals of OMC-2 as observed by Morris et aZ.41 are shown in the upper part for HCN and HNC. The appropriate laboratory measurements are shown in the lower trace with a summary of the relevant experimental data.It should be noted that in interstellar space the densities of HCN and HNC are roughly the same, while under laboratory conditions measurable HNC concentrations can only be produced by thermal isomerization at 1300 K of HCN43 or by the use of either d . ~ . ~ ~ or radiofrequency discharges with a variety of H, N and C containing molecule^.^^*^^*^^ In interstellar space the Taurus Molecular Cloud 1 (TMC 1) is the only source where HC3N,47 HC5N,48 HC7N49 and HC,N5' have been discovered to be present simul- taneously with HCN and CN.39 As can be seen from the comparision of the high-re- solution spectra (see fig. 10) taken with the Effelsberg radiotelescopeS1 the molecular concentrations drop only slowly as one goes up the series, which indicates that theM .WINNEWISSER 43 chemistry in this source must be special with regard to the production of long-chain molecules. Churchwell et aZ.47 suggested that one single reaction involving C2Hz+ or C2H3+ may transform the lower cyanoacetylenes into HC5N, HC7N and HC,N. Mitchell et aZ.52 showed that the observed abundances of cyanopoly-ynes can be ob- HCN HNC 14 a ysR/km s-' A LABORATORY 1 - 1 - 0 0-1 188633.954 MHz 2-1 188 631.871 MHz 1 1-1 I 88 630.431 MHz Sampling rate: 0 Sweep duration: 20 ms Sweep rate : 15.5 Hz Sweeps accumulated : 1 Pressure : (static) Ton 14 a v , / km 5-1 .n. . . . . . . '4 SMOOTHING I I I I I I I -1.0 *l.OMHz 90 663.602(50)MHz 28 .us 15 ms 15.5 Hz 2000 (flow) 3.5 x lo-' Torr (total) N4 + CHjI-tHNC + . . . F~G. 9.-Interstellar emission (upper) and laboratory absorption (lower) lines observed for HCN and HNC, showing relative intensity which reflects relative abundance.Data-acquisition parameters are given for laboratory lines [interstellar lines from ref. (41)]. tained by the gas-phase reaction pathways of H2CN+ with C2H2. Another possibi- lity, discussed and pursued by us in the l a b ~ r a t o r y , ~ ~ is that polyacetylene chains54 are built up first and then react with HCN or H,CN+ to give the cyanopoly-ynes. (2). PLASMA DIAGNOSTICS In order to shed more light upon these problems our laboratory started to investi- gate various gas-phase plasmas composed of hydrocarbons and nitrogen-containing molecules. In the production of HNC it was noticed that besides HNC and HCN large amounts of HCC-CN and other, not yet identified species were produced.The formation of cyanoacetylene by the reaction r.f. discharge HCCH + HCN - HCC-CN + 2H' (10)44 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECIJLES I was then used to produce enriched HCC-l3CNS5 in quantities adequate for rotational spectroscopy, as can be seen in fig. 1 1 . HC5N can be synthesized by a continuous or flash discharge in a 2: 1 mixture of HCCH + HCN as Winnewisser et have shown. Our results are summarized in fig. 12. Under continuous-flow conditions at a pressure of 10-2-10-1 Torr the 2: 1 mixture of HCCH + HCN is passed through a 25 cm long r.f. discharge tube powered by a 300-1000 W, 4.5 MHz, r.f. generator. HC3N is produced in preparative quantities and HC5N in The gas-phase production of higher cyanopoly-ynes is also possible.I I RESOLUTION +I+- 0.027 krn 5”’ TA T HC3 N I = l-O,F=2-1 / \ HCs N /=9---0 +k- 0.041 krn s-’ Ti +I+- HC7 N 0.042 krn 5’ I 21-20 I I I I I I I 4.0 5.0 6.0 7.0 U ‘ S R h s-’ FIG. 10.-Interstellar emission lines of HC3N, HCsN and HC7N from TMC 1, u = 04 h 38 m 38 s, 6 = 25” 36’ 00”, indicating relative intensity and thus abundance, since transition moments are similar [from ref. (51)].M . WINNEWISSER 45 J=20-19 SAMPLE HC 13CCN HCC 13CN CHEM I CAI LY NATURAL ABUNDANCE PREPARE 0 1.1% l3C DISCHARGE GENERATE0 H CC H* H13C N I , I , I 1 181 170 181 190 frequency/MHz FIG. 11 .-Laboratory absorption lines of cyanoacetylene prepared chemically (upper trace) and pre- pared in a discharge (lower trace) of acetylene with 13C enriched HCN [from ref.(55)l. DIRECT OBSERVATION (FREE-SPACE CELL) HCC-CCH} . . . . HC, N 1 . . . . HC5 N . . . . ? H ~ N 0-1 I N DI R E C T 0 BS E RVAT I 0 N (PRODUCTS TRAPPED AND FRACTIONALLY DlSTl l l E O 1 24-23 16-15 i 1x6 1x5 1 x 4 1x3 HARMONIC I I 36.387 36.391 GHz 42-41 I 1x3 HARMONIC I 37.274 G H t ROTATIONAL -TRANSITIONS J.1- J 65-64 39-38 I I 78-77 1 55154 1 26j25 I c . . . . ? 1 x 6 1.5 1 x 4 1x3 1 x 2 HARMONIC I I 34.605 34.613 GHz FJG. 12.-Detection of HCSN in the products of a discharge of HCN with acetylene (lower trace) and with diacetylene (upper pair of traces). In the former case HCSN was concentrated by fractional dis- tillation to obtain an observable amount, while in the latter case it could be seen video jn the dis- charge products directly.The HCsN line has roughly the same intensity as the HC3N line; owing to the large number of states which are populated in HCsN this means a larger concentration of HCsN, supporting the theory relying on p~lyacetylenes.~~46 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES TABLE 2.-ROTATIONAL FREQUENCIES OBSERVED IN THE LABORATORY TOGETHER WITH CAL- CULATED FREQUENCIES AND CONSTANTS OBTAINED FOR HCSN IN THE GROUND VIBRATIONAL STATE ~~ ~ ~~ ~ J" v(o bs)/MHz v(calc)/MHz v(0bs) - v(calc)/kHz 0" 1" 3" 6 7" 8" 9 10 11 12 13 14 18 25 30" 32 " 36" 38 39 43 51 54 64 65 77 10 650.657 18 638.615 21 301.262' 23 963.8968 26 426.543' 29 289.152 " 31 951.772' 34 614.387 " 37 276.994" 39 939.591 50 589.81 5 " 69 227.1824 103 836.8169 " 106 498.9104" 117 147.0163 " 138 441.6788 146 426.5550" 173 040.1865 " 175 701.2968 " 207 630.7692 2 662.6653 5 325.3299 10 650.6540 18 638.6167 21 301.2618 23 963.9011 26 626.5339 29 289.1595 31 951.7771 34 614.3860 37 276.9856 39 939.5751 50 589.8173 69 227.1 849 82 539.0412 87 863.6321 98 512.5219 103 836.8093 106 498.91 11 117 147.0221 138 441.6722 146 426.5659 173 040.1862 175 701.3019 207 630.7643 3 .O - 1.7 0.2 -4.3 9.1 - 7.5 -5.1 1 .o 8.4 15.9 - 2.3 - 2.5 7.6 - 0.7 - 5.8 6.6 - 10.9 0.3 -5.1 4.9 Bo = 1 331.332 714(46) MHz Do = 30.101 6(58) HZ " Lines observed in interstellar sources;' ref. (53); ref.(56); ref. (47); ref. (75). small amounts (ca. 10 mmol). CH,CN and H2C2HCN were not detected. HCSN was isolated by trapping the discharge products and then using a low-temperature distillation for separation.HC7N and higher poly-ynes have not yet been detected in these mixtures. In the lower part of fig. 12 the results of the flash discharge reaction is shown using diacetylene, HCC-CCH, and HCN as starting materials. HC5N is produced imme- diately with approximately the same yield as HC3N. The duration of the single dis- charge flash is 0.5-1 s. In table 2 our measurements of HCSN in the millimetre-wave region are combined with earlier published results making precision line positions available throughout the millimetre-wavelength range for HC5N.s6 HCC-CN, in particular, seems to be a chemical sink. We have observed it in the reactions just discussed, in all reactions producing HNC and in the following discharge reactions: We are no longer surprised at finding the longer cyanoacetylenes. r.f.discharge A HCC-CN + 2H' \ /" (4 // c = c H \ H' CN (b) HCCH + HCNO -H- HCC-CNO + 2H' (12) r.f. discharge HCC-CN + H20.M . WINNEWISSER 47 Apparently, the discharge of any combination of hydrocarbons and a source of nitro- gen will generate HCN and HCC-CN. The latter experiment (b) brought another aspect of the interstellar chemistry to light; in the reducing atmosphere of a hydrogen-containing cell (or cloud), hydrogen atoms will scavenge the oxygen atoms from such molecules as HCNO. The milli- metre-wave H,O line at 183 GHz appears immediately when the discharge is turned on in the mixture of acetylene and fulminic acid, HCNO. Therefore HCC-CNO was not found. It has long been noted that NNO is absent from the list of interstellar molecules.The NO bond in N,O, like that in HCNO, is not stable in such an atmo- sphere. A search was also initiated to obtain the millimetre-wave spectrum of isocyano- acetylene, HC=C--NC, since equal interstellar concentrations of HCN and HNC (see fig. 9) strongly indicate that isocyanopoly-ynes might be constituents of the inter- stellar gas, particularly in the molecular envelope of IRC-10216 or similar carbon stars.57 Therefore, experiments are under way in our laboratory to synthesize and detect this species for example uia the reactim H H \ / r.f. discharge + HC EE C - NC + 2H- (13) NC H F="\ in analogy to the formation of HC3N discussed above. (3). PYROLYSIS EXPERIMENTS The stability of HCN, HCC-CN and vinyl cyanide, H,CCHCN, in the inter- stellar atmosphere and in r.f.discharges leads us to believe that N-cyanoformimine H,CNCN, as an addition product of HCN, might be a species responsible for some TABLE 3.-ROTATIONAL AND CENTRIFUGAL DISTORTION CONSTANTS IN THE S-REDUCED ROTA- TIONAL HAMILTONIAN CHLNCN" CHzCHCN 63 373.235(58) 5 449.339 62(110) 5 009.563 26(98) 0.166 872 7 2.386 O(22) - 123.970(19) 5870( 17) -0.480 69(88) -0.032 14(30) -0.003 4(22) - 0.441 (25) -27.79(22) 7470( 1240) 0.001 45(70) 0.000 43( 19) - 0.000 54(46) -0.010 74(71) 119 0.054 49 850.6982(87) 4 971.164 06(70) 4 513.877 20(76) 0.161 219 6 2.182 l(17) - 85.076( 18) 2 717.1(11) -0.457 14(58) -0.030 53(20) - 0.343(26) - 7.09( 12) 0.004 4( 13) 433( 37) 0.002 91(71) 0.000 15(48) -0.000 12(38) - 129 0.044 ~~ " Ref.(61): * Ref. (64).P 00 I I I I I I I 9 7 1 8 ' , ' I 1 I A A FIG. 13.-Video mosaic of the J = 10 t- 9 a-type R-branch transitions of N-cyanoformimine, H,CNCN, and stick diagram of ground-state lines. Fortrat diagram follows the band head, where K represents K,. Note quadrupole hyperfine splitting of K, = 8 and 9 lines.61 r m mM. WINNEWISSER 49 of the still unassigned lines observed in the discharge experiments yielding HNC, HC3N, etc. The addition product N-cyanoformimine was first produced by Wentrup'' in 1971 and identified by mass spectrometry of the products of the pyrolysis of trimethylene- tetra-azole at 800 K and a pressure of 0.01 Torr (1.3 Pa). The first microwave detec- tion was carried out by Bak et aLS9 in 1978 using a different pyrolysis precursor. Bak and Svanholt6' realized that the interstellar emission line at 10 458.634 MHz, identi- fied as the J = 18 t 17 transition of HC9N," coincides closely with the lo, t Ooo transition of H,C=NCN at 10 458.49 MHz.We have extended the rotational tran- sition measurements into the millimetre-wave region where lines have been assigned in both the a-type and b-type spectrum, thus providing the data base for identifying the molecule in plasmas and in interstellar clouds.61 Fig. 13 shows the J = 10 t 9 rotational transitions with the characteristic K, band-head structure of the a-type R- branch spectrum.62 In table 3 the preliminary rotational and centrifugal distortion constants in the S-reduced rotational H a m i l t ~ n i a n ~ ~ are presented together with the corresponding constants obtained from the available data on vinyl cyanide.64 A re- fined rotational analysis must await completion of the analysis of the quadrupole N - CYANOFORMIM INE VINYLCYANIDE - + / d- / I I 1 I ' / i - + I I / I L, - + 2639 M H z 1320 MHz LLO MHz L572 MHz 27LL MHz 1372 MHz L57 MHz L l / / L - - - * FIG.14.-Rotational-energy-level diagram for H,CNCN and HzCCHCN showing lowest-lying states and probable interstellar transitions. The 211 -+ 212 and 10o,lo+- 90,9 (not shown) transitions of HzCCHCN have been observed in Sgr B2.'HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES HNCS LINEAR I 0 / I / / 3 l - 7 3 3 = r I I I , I + ' i ; I I I I I I I I I , I I I 1 I I I. / NEARLY BENT I I I I I I f I t I I I I I I I I I I I 4-?: I 1 f / X = O j ; 3 4 s I I I I I \ I \ I \ I I I I I ' < , ' I I 1 1 2 3 4 q / {'' hyperfine structure in the spectrum, in which we are now engaged.Fig. 14 shows a part of the rotational energy level scheme of N-cyanoformimine and vinyl cyanide showing the possible transitions opportune for a radioastronomical search. 4. MOLECULAR DYNAMICS The polytopic molecules KCN, NaCN and LiCN mentioned in Section 2 are examples of a special group of molecules from the point of view of molecular dynamics and structure. Because of a very low-lying, large-amplitude bending mode, theM . WINNEWISSER 51 analytical form of the vibrational potential surface is not self-evident, but is necessary for the understanding of their spectra and structure. Some aspects of these problems are touched on in the following brief discussion of the effects of quasilinearity on mole- cular energy levels.(I). POTENTIAL FUNCTION AND CORRELATION PARAMETER yo Quasilinearity is just one of the forms of large-amplitude motion which complicate molecular structure determination, and even structure definition. Yamada and Win- n e w i s ~ e r ~ ~ studied the correlation of the rotation-bending energy levels of linear and bent triatomic or longer chain-type and introduced a correlation para- meter yo to give a quantitative, empirical measure of quasilinearity, (14) E(1owest excited state with K or I = 0) E (lowest state with K or I = 1) p - y - 4 - ( where E represents the energy of a given state measured from the ground state. This parameter has the value of - 1 for an ideal linear molecule and a value of + 1 for an ideal bent molecule.Tt traces in fig. 15 from left to right the transition of a vibrational degree of freedom into a rotational degree of freedom. Although the molecules entered in fig. 15 are a real job-lot of chemical composition and properties they have the common physical property of one low-lying, large-amplitude bending mode. As 300 - 200 - d I E Y 4 100 - v7 1, Elcrn-' 5 1 191.1100 5 3 181.10" 5 5 164.55" 4 0 144.2985 4 2 136.9487 4 4 120.52' 3 1 97.1401 3 3 79.9312 0 60.7022 2 45.7977 1 18.1795 0 - P had FIG. 16.-Energy-level manifold of the v7 bending mode of GO2. The 1, 1 level was determined in ref. (67), the levels denoted by ' are from ref. (69) and the remaining levels are from ref. (68). can be seen from fig.15 carbon suboxide, C302, is the most quasilinear molecule with a low-lying bending mode of 18.1795 cm-' which has recently been observed in the sub-millimetre-wave region by Krupnov and The vibrational manifold of the quasilinear bending mode v, is reproduced in fig. 16 together with the energy levels as determined from sub-rnillimetre-wa~e,~~ infrared6* and Raman spectro~copy.~~52 HIGH-TEMPERATURE SPECIES AND UNSTABLE MOLECULES ---r- (2). V7 VIBRATIONAL MANIFOLD OF C3OS In our early work7* on the pure rotational spectrum of tricarbon oxide sulphide, C,OS, we thought we would be extending the study of molecules exhibiting quasilinea- rity. However, it was determined at a very early stage that C,OS showed no quasi- linear behaviour. Tn order to explore the vibrational manifold of v7, the lowest-lying bending mode, which is well-separated from the other two bending 71 Winnewisser et aZ.72 have carried out precision relative intensity measurements of the vibrational satellite lines in the microwave region from 8 to 40 GHz using a Hewlett- Packard bridge-type spectrometer.The results of our studies are summarized in fig. - -- -200 I "7 - 100 e 561,9 496,4 4 89 , 5;: 485,l 483,7 416 ,4 410,l 406,9 338,O 332,9 331,4 251,2 248 ,O 167,4 165,8 82,9 0 L - 600 - r- 400 Ho I V ( d / v; I ;f- 7ef i- J-"f I I 1 I I 3 1 I I I -3kf I I : ' i--l;f FIG. 17.-Energy-level manifold of the v7 bending mode of C30S, determined from relative intensity measurements in the pure rotation spectrum. The energy level 6$ indicated by * was derived via the /-resonance effect.12M .WINNEWISSER 53 17, showing a vibrational manifold belonging to a two-dimensional isotropic oscillator with only slight anharmonicity of the potential function, but without any flattening or barrier in the bottom of the potential. This is in sharp contrast to the C302 potential function and v7 manifold reproduced in fig. 16. It must be concluded that the substitution of one oxygen by sulphur in C,O, changes the delicate balance of the electron density on the central carbon atom notice- ably in such a way that a stiffening of the bonds occurs, despite the fact that the bend- ing force-constant corresponding to the central bend is still surprising small,71 indicat- ing a large-amplitude motion but one for which the Born-Oppenheimer separation of rotation and bending is still valid.(3). EXTENSION OF THE CORRELATION PARAMETER y o TO THE FREE INTERNAL ROTOR In a recent contribution to the investigation of quasilinearity Bunker and Howe 73 have extended the correlation parameter of eqn (14) to (15) le, 1) - E(OO, 0) + E(OO, 1) - E(0 ) E(2O, 0) - E(OO, 0) O , O ) , (E(l y o = l - 4 in order to include the free internal-rotor limit. The notation is that for the energy levels of a linear molecule, E(d, J ) . Free internal rotation in a triatomic molecule means one atom being free to move 360” around the line joining the other two more rigidly bound atoms from one linear configuration to the other. van der Waals mole- cules, for example, are in this category. The ideal limiting values of - 1 and + 1 for linear and bent molecules are unaffected but a new range of values is covered, namely between either +1 or -1 and -3, the value assumed by a free internal rotor.The general definition put forward by Bunker and Howe is Y u = 1 - 4 b ” + W C ” l (16) where a, is equal to the energy difference between the (3v + 1)th rotation-bending level having J = 1 and the uth rotation-bending level having J = 0, bu corresponds to the energy difference between the (3u + 2)th rotation-bending level having J = 1 and the 0th rotation-bending level having J = 0, and c, is equal to the difference between the ( u + 1)th and uth rotation-bending energy levels having J = 0. From the above definition it follows that a series of parameters y, with v = 1, 2, 3, .. . can be obtained as a function of the degree of bending excitation. Furthermore the y, parameter will vary with the degree of excitation of the stretching vibrations. The polytopic molecules LiCN and KCN were used as model systems by Bunker and Howe7j and it will be interesting to see, when their spectra are obtained and analysed, which y, values and thus which potential functions will describe the dyna- mics of these intriguing high-temperature species. I express my gratitude to Prof. T. Torring for making available preprints and re- prints of the recent work carried out in the Berlin High Temperature Microwave Laboratory. I also would like to thank him very much for providing some figures for this paper, I am grateful to Dr. Brenda P. Winnewisser for numerous discussions and comments on this paper and to Dr.Gisbert Winnewisser for permission to use some results of forthcoming papers. The experimental work which was carried out in our laboratory was in part sup- ported by funds from the Deutsche Forschungsgemeinschaft, the Max-Planck-Institut fur Radioastronomie in Bonn and the Fonds der Chemischen Industrie.54 HIGH - TEMPE RAT U R E S P E C I ES A N D UNSTABLE M.OLECUEES G. Winnewisser, E. Churchwell and C. M. Walmsley, Modern Aspects of Microwave Spectro- scopy, ed. G. W. Chantry (Academic Press, London, 1979), chap. 6, p. 314. G. L. Withbroe, The Menzel Symposium, NBS Spec. Pub. No. 353, ed. K. G. Gebbie (U.S. Government Printing Office, Washington, D.C., 1971), p. 127. L. M. Hobbs, Astrophys. J . , 1974, 191, 381.E. B. Jenkins and B. D. Savage, Astrophys. J . , 1974, 187, 243. W. H. Hocking, G. Winnewisser, E. Churchwell and J. Percival, Astron. Astrophys., 1979, 75, 268. R. A. Creswell, W. H. Hocking and E. F. Pearson, Chem. Phys. Lett., 1977, 48, 369. W. H. Hocking, E. F. Pearson, R. A. Creswell and G. Winnewisser, J. Chem. Phys., 1978, 68, 1128. D. A. 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Winnewisser, Topics Current Chem., in press. ” A. J. Alexander, H. W. Kroto and D. R. M. Walton, J. Mol. Spectrosc., 1976, 62, 175.

ISSN:0301-7249    

DOI:10.1039/DC9817100031

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Microwave Spectroscopy of Molecular Ions and other Transient Species in Electric Discharges BY R. CLAUDE WOODS Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A. Receiced 5th January, 1981 We have employed large d.c. glow discharges to produce a variety of unstable species, including molecular ions, in sufficient abundance for detection of their microwave spectra. For simple ions (HCO+, HN, +, CO+) we have studied several isotopic forms and determined accurate molecular structures and hyperfine parameters, and for HCO + we have measured the linewidth parameter for pressure broadening by several gases. To facilitate observation of additional species we have devoted considerable effort to improving our methods of generating and detecting microwave radiation, of modulating the molecular absorption and of computer processing the resultant spectral data.One of the principal difficulties in searching for new transient species IS that of optimizing its production before any microwave spectrum has been seen. To alleviate this problem we have attached instru- mentation for high-resolution optical spectroscopy and quadrupole mass spectrometry to the same glow discharge used for the microwave work. The mass spectrometer can sample neutral molecules and positive or negative ions escaping from the discharge; in conjunction with microwave interfero- metric measurements of electron density it can provide absolute ion densities for particular ions of sufficient precision to determine in advance whether or not the microwave spectrum can be detected.The plasma of an electric discharge has for many years been an extremely useful medium for production of various transient species (atoms, radicals, excited states or ions) for high-resolution spectroscopy. We have demonstrated that if a d.c. glow dis- charge at suitably low pressures (5-100 mTorr) in a tube of sufficiently large dimen- sions (lengths of several feet and diameters of several inches) is employed,' microwave absorption spectroscopy on components of the plasma becomes possible and practical, with sensitivity adequate for the detection of a number of radicals and molecular ions and at least one metastable excited electronic state (a3H CO) so far. Four molecular ions have been studied by our group to date: CO+,2 HCO+,3 HN: ,4 and HCS+ .5 We detected the lowest rotational transition of CO+ after a search through a spectral region of ca.300 MHz, determined by use of available optical data, and eventually were able to obtain precise spectra of the main isotopic form and the isotopic variants P O + and 13CO+, which yield 13C hyperfine parameters. In the cases of HCO+ and HN,+, detection by radioastr~nomy"~ had preceded our work, so that no search in microwave frequency was required. Eventually we were able to obtain spectra of six isotopic forms of each of these ions and to use these data to deduce complete sub- stitution (r,) structures for the two ions. We have observed HCS+ very recently, and again were fortunate to have available an astronomical measurement" of the fre- quency in advance, avoiding the necessity of a long frequency search on our part.Important studies of neutral transient species have also been possible by this technique. We were able to detect and study the CN radical" and the HNC12 molecule using frequencies first determined by radioastr~nomy.'~.'~ While we were eventually able to obtain more precise and extensive high-resolution data than the radioastronomers,58 MICROWAVE SPECTROSCOPY OF MOLECULAR IONS including information on excited vibrational states not populated in the interstellar medium, and in the case of HNC to establish with certainty the identity of the carrier of the spectrum for the first time, it must be admitted that the availability of the radio- astronomy data were of crucial importance in our initial detection of these molecules.Yet another type of transient that we have observed is the high-temperature molecule e.g., SiS and SiO. We have observed the latter in silane-oxygen discharges with near LN2 cooling, even though it normally has adequate vapour pressure for spectroscopy only at very elevated temperatures. The preceding summary indicates that interesting transient species of several types are accessible to microwave spectroscopy in a glow discharge. The sensitivity of microwave spectroscopy in relation to the abundance of transient molecules that can be produced in the plasma, however, is such that the spectra are normally fairly weak, so that long time constants or averaging times are required for adequate signal-to- noise ratios.When the region that must be searched for a new spectrum is of the order of hundreds of MHz, as it typically is when optical data or the best ab initio calculations are employed in making the prediction of a microwave transition, the time for a search at high sensitivity is measured in days or even weeks. If one also does not know the best way to make the species of interest and has no independent way of checking to see that it is being made in adequate quantities, then the required dose of patience is usually too high. This so-called blind search problem has been common to essentially all microwave spectroscopic work on previously unobserved transient molecules, regardless of the production scheme. We believe that the blind search problem can be overcome, at least in the case of molecular ions (and to some extent in the case of metastable electronic states), and doing so has been a major goal of our research in the last few years.Essentially one must understand all the factors that enter into the formula for the absorption coefficient ( y ) , and develop methods to mea- sure, calculate or estimate them all in advance with sufficient precision and reliability that one may decide with confidence whether or not the resultant y exceeds the sensiti- vity threshold of the microwave spectrometer. Given a positive decision one would then presumably tolerate the necessary long frequency search for an interesting new ion. This estimation and optimization of the absorption coefficient and our progress in implementing it, which we hope will make it possible to obtain spectra for new molecular ions that (like CO+) have not previously been seen by radioastronomy, is the main subject of this paper.The peak absorption coefficient can be written'' 8n2Nf (pi l2v; I) = 3ckTAv where N is the number density of ions of the desired type, f is the fraction of those ions in the lower state of the transition, lpijI2 is a squared combination of dipole moment and direction cosine matrix element, vo is the centre frequency of the transition, Av the half-width at half maximum, and the other symbols have their conventional meanings. The value of y that is detectable depends on the sensitivity of the detector used, the frequency range, the stability of the discharge and the length of the averag- ing time, but a typical value might be lo-* cm-' with current equipment and ordinary imperfect conditions.The fractionfcan be factored into a vibrational part and rota- tional part, which can be expressed in terms of the partition function (at equilibrium) asR. CLAUDE WOODS 59 for a linear molecule. Thus, y could be rewritten as 8n2NA.B(2J + 1)J,uij12v02 Y = 3 ~ ( k T ) ~ Av (3) B and vo are normally well-known in advance to sufficient accuracy for the purposes of this formula, but we must consider each of the quantities: N , f,, I,u1J[2, T and Av. Calculation of the direction cosine matrix element factor in lpiil2 is a technical matter that can always be done and need not be discussed here. The dipole moment is that in the centre-of-mass coordinate system and is not known experimentally for ions.Fortunately, theoretical calculations of this quantity, if done at the C1 level of approxi- mation, are accurate to a few percent (except for near-zero moments), more than ade- quate for computing spectral intensities. We have the capability now to carry out these calculations and have reported CI dipole moments for several important mole- cular ions.16-18 The temperature, T, would ordinarily be a trivial variable, measurable with a ther- mometer, but in an electric discharge quite the contrary is true. The plasma is a medium far from thermal equilibrium, so that we normally distinguish an electron kinetic temperature (Te), a vibrational temperature (T,), a rotational temperature (Trot) and a translational temperature (T,,,,,).The values of T,, Trot and Ttrans can vary from one species to another (there is no guarantee that they exist at all), and in particular might.be expected to be different for ions and neutrals. The electron tem- perature in a glow discharge is typically 20 000-30 000 K, and can be measured using Langmuir probe technique^.'^ Fortunately, all the other temperatures are much lower. The vibrational temperature, T,, which has essentially the same information content as the fraction,.f,, can be determined from the relative intensities of inicrowave spectra in different v states, and so far appears to behave quite differently for ions and neutral molecules. For the latter we have noticed a high vibrational temperature (1000-4000 K) in almost all cases, including transients like CN, HNC or S O , stable molecules like HCN or HC3N, or the metastable a3n state of CO.This has proved very convenient for obtaining high-resolution spectra in higher vibrational states. For ions, however, we have so far been unable to even observe any vibrational satellites, indicating a rather low T,. There seems to be a relaxation mechanism for vibrational excitation that is specific to ions. Thus, we can generally assume safely thatfv = 1 for the ground vibrational state of an ion when we calculate y. Careful consideration of eqn (3) re- veals that both factors of T i n the denominator are actually Trot, one arising from the rotational partition function and the other from the partial balance between absorp- tion and stimulated emission. This quantity Trot is often strongly coupled to Ttrans, and in the case of neutrals we feel that both are near the temperature at which the walls of the tube are maintained. We have tested this assumption with O2 in an oxygen discharge.In the low-pressure regime where Doppler broadening totally predominates we have measured the linewidth as a function of discharge current from 0 to 400 mA and found the width to be constant within our errors (273, indicating al- most no translational warming (<20 K) of O2 in the discharge. We have also deter- mined Trot directly from relative intensity data in and out of the discharge for 02, which is especially convenient since transitions for many different rotational quantum numbers appear in the same frequency region (near 60 GHz). The preliminary result is Trot = 350-400 K in the discharge, indicating only very modest rotational warming.The situation with respect to T,,,,, and Trot for ions is very much less clear, with some reason to worry that they may be substantially higher than the wall temperature. The electric field of the discharge is constantly accelerating the ions to energies of 2-3 eV, most, but not all, of which is lost on each collision with a thermal (near ambient)60 MICRO W A V E S P E C TROS C OP Y OF MOLECULAR I ON S neutral molecule. This mechanism, which does not exist for neutrals, could lead to Ttrans (and also Trot) for ions substantially above the wall temperature. (Remember that T, for ions is probably lower than for neutrals, just the opposite behaviour.) For ions, Trot is exceedingly difficult to measure by microwave spectroscopy, but can be measured (for diatomics at least) by high-resolution optical spectroscopy.We now have a monochromator with sufficiently high resolving power (a Jobin-Yvon 1.5 m double-pass instrument) to readily resolve the rotational structure in the U.V. for simple ions like CO+ and N,+. We have looked at the first negative bands of NZ+ in emission from our discharge tube and readily determined Trot from a logarithmic inten- sity plot to be ca. 350 K. Regrettably the value of Trot obtained from an emission spectrum like this applies to the distribution in the excited electronic state, and this in turn is probably more representative of the distribution in the neutral gas than in the desired ground state of the ion. What must really be done is a study of the high- resolution absorption spectrum, and we hope to be able to obtain one for the first negative bands.Such an experiment would also provide a reliable value for .fv (of CO+ or N2+) from the relative intensities of the vibrational bands and the well-known Franck-Condon factors. For metastable states the optical spectrometer can play yet another role, that of a concentration monitor. The forbidden triplet-singlet emission to the ground state provides a convenient indication of relative metastable concentration, so that conditions (current, flow rate, pressure, gas mixture, etc.) can be optimized for production of the metastable state. We used this method on the Cameron bands of CO prior to our microwave detection of the a311 state.An absolute concentration measurement, i.e., a value of N in the formula for y is, on the other hand, very difficult to obtain from an emission spectrum, requiring very elaborate cali- bration procedures. Absorption spectroscopy, e.g., of an allowed transition from a metastable state to a higher triplet state, in principle offers a better way of determining N. In this case we may write For molecular ions there is an even better way for determining N. where Nion is the total concentration of (positive) ions and /3 is the fraction of the total that are of the desired chemical form. For discharges that do not contain significant quantities of negative ions Nion = N,, the plasma (electron) density. The quantity N, can be measured by microwave interferometer phase shift, are directly proportional to it,20 118.4 v(Hz) Ayl(rad) N , ( c ~ - ~ ) = L(cm) We have set up a 12 GHz interferometer around our discharge tube and used it to measure N, for a variety of gases.For our newest discharge tube, which is 4 in. in diameter and has a microwave path length of 9 ft we obtain N, = 101o-lO1l cm-3 in various gases with maximum current (near 1.1 A). Thus, Nion is readily available to 5% accuracy, quite good enough for the y calculation. Any presence of negative ions can only make Nion > N,. The fraction /? can be determined by mass spectro- m e t ~ y . ~ l - ~ ~ We now have an Extranuclear Laboratories quadrupole mass spectro- meter attached to our newest discharge tube, and are using it to determine the relative ion densities of discharges in various combinations of gases.The ion-molecule chemistry is complex and difficult to predict in advance, so the mass spectrum often provides some surprises. The actual mass spectrometer chamber has been separated by an intervening region that is differentially pumped and through which the ions areR. CLAUDE WOODS 61 guided by electrostatic lenses, so that very reactive and messy discharges may be stu- died without undue contamination of the mass spectrometer. The greatest experi- mental problem arises in making the d.c. voltages of the quadrupole and ion optics track the fluctuations, both slow and fast, of the plasma potential near the sampling hole, which is typically 600 V above ground. This voltage referencing is necessary so that changes in ion signal when conditions are varied reflect changes in ion density, rather than changes in focusing efficiency.We are making steady progress in achiev- ing the required precision of voltage tracking. The combination of the mass spectro- meter and the microwave interferometer then can yield an absolute ion density for a particular ion, of suitable precision for our intensity estimation, and this promises to be extremely helpful in taking much of the guesswork out of the search for spectra of new ions. It should be mentioned that the mass spectrometer should be capable of looking at negative ions too, and that this capability is especially important in that case, since so little is known about the chemistry of negative ions in discharges like ours.Microwave spectroscopy seems to be one of a limited number of experimental techniques suitable for obtaining high-resolution spectra of negative ions. We re- quire only a stable ground electronic state, and most negative ions do not have the dis- crete excited states required for high-resolution optical spectroscopy. Finally, we come to the linewidth parameter, Av, which must be determined directly from microwave spectra. Before our initial detection of CO+ this quantity for mole- cular ions was very much an unknown. Some felt that it might be very much larger than the linewidths found in neutrals, due to the long-range nature of the monopole- dipole force. We had felt that it would not be too much larger for ions than for neutrals, since the charge-dipole interaction does not result in a torque on the ion in a classical model, and therefore might be expected to be inefficient in changing its rota- tional We are now accumulating a base of data on Av for ions, which will make it possible to estimate Av for new ions adequately for the y formula.Generally Av seems to be 2-3 times larger for ions than for neutrals with similar dipole moments. We have already reported26 results for Av of HCO+ broadened by H, at liquid nitrogen temperature, and more recently have studied room-temperature broadening and broadening with argon as collision partner. Extension of the linewidth studies to low pressures, where Doppler broadening is dominant over pressure broadening can also yield a value for the quantity Ttrans discussed previously.In addition to the random motion of the ions, reflected in the Doppler broadening, there is also a system- atic drift in the d.c. electric field of the discharge, which should be reflected in a Doppler shift of the centre frequency. We have invested a great deal of effort into improving our techniques of microwave measurement and data analysis, with the goal (among others) of being able to measure both the Doppler broadening and Doppler shift simultaneously with good reliability. We are now using the tone-burst modulation scheme of Pickett and Boyd,27 and the digital phase detector of Pickett,28 which make it possible to source-modulate and phase-lock the microwave source at the same time. Thus our frequency sweep is now totally digital, and the frequency stability and accu- racy of each point is 1 kHz or better.All frequency measurements are done by a complete least-squares lineshape analysis of the accumulated point-by-point spectral data. Nevertheless, we have not yet been able to obtain an adequate characterization of the Doppler shifts. We believe this is due to the small frequency shifts that may be occasioned by admixture of a small component of dispersion into our absorption line- shape. We do have preliminary values for the limiting linewidth at low pressure (Dopp- ler widths) for HCO+ and these seem to correspond to Ttrans near 1000 K for a room- temperature discharge. If Trot were this high it would of course have a very significant effect on the computation of y. Clearly a direct measurement of Trot, e.g., by optical62 MICROWAVE SPECTROSCOPY OF MOLECULAR IONS absorption spectroscopy, will be of great interest.In conclusion we feel that it will soon be possible to combine all the types of information that have been discussed in this paper to obtain a reliable assessment of the feasibility of microwave spectroscopy for a particular ion and to intelligently optimize all the factors that bear on successful detection. This work was supported by National Science Foundation Grant CHE-7950005 (7915126) and by the Wisconsin Alumni Research Foundation. ’ R. C. Woods, Rev. Sci. Instrum., 1973, 44, 282. ’ T. A. Dixon and R. C. Woods, Phys. Rev. Lett., 1975,34,61. R. C. Woods, T. A. Dixon, R. J. Saykally and P. G. Szanto, Phys. Rev. Lett., 1975, 35, 1269.R. J. Saykally, T. A. Dixon, T. G. Anderson, P. G. Szanto and R. C. Woods, Astrophys. J. Lett., 1976, 205, 101. C. S. Gudeman, N. N. Haese, N. D. Piltch and R. C. Woods, Astrophys. J. Lett., 1981, 246, L47. D. Buhl and L. E. Synder, Nature (London), 1970, 228, 267. ’ W. Klemperer, Nature (London), 1970, 227, 1230. * B. E. Turner, Astrophys. J. Lett., 1974, 193, 83. lo P. Thaddeus, M. Guelin and R. A. Linke, Astrophys. J. Lett., 1981, 246, L43. l1 T. A. Dixon and R. C. Woods, J. Chem. Phys., 1977,67,3956. l2 R. J. Saykally, P. G. Szanto, T. G . Anderson and R. C. Woods, Astrophys. J. Lett., 1976,204, l3 K. B. Jefferts, A. A. Penzias and R. W. Wilson, Astrophys. J. Lett., 1970, 161, 87. l4 L. E. Snyder and D. Buhl, Bull. Am. Astron. Soc., 1971, 3, 388. l5 C. H. Townes and A. L. Schawlow, Microwave Spectroscopy (McGraw-Hill, New York, 1955, S. Green, J. A. Montgomery and P. Thaddeus, Astrophys. J. Lett., 1974, 193, 89. 143. p. 19. N. N. Haese and R. C. Woods, Chem. Phys. Lett., 1979, 61, 396. N. N. Haese and R. C. Woods, J. Chem. Phys., 1980,73, 4521. N. N. Haese and R. C. Woods, Astrophys. J. Lett., 1981, 246, L51. l9 A. von Engel, Ionized Gases (Oxford University Press, London, 1965), p. 296. 2o M. A. Heald and C. B. Wharton, Plasma Diagnostics with Microwaves (Wiley, New York. ” D. Smith and I. C. Plumb, J. Phys. D, 1973, 6, 1431. ” J. P. Gaur and L. M. Chanin, J. Appl. Phys., 1970,41,106. 23 P. F. Knewstubb and A. W. Tickner, J. Chem. Phys., 1962, 36, 674. 24 M. J. Vasile and G. Smolinksy, Int. J. Mass. Spec. Ion Phys., 1973, 12, 133. ’’ R. C. Woods, P. R. Certain and R. B. Bernstein, Theoretical Chemistry Institute Report, 26 T. G. Anderson, C. S. Gudeman, T. A. Dixon and R. C. Woods, J. Chem. Phys., 1980,72,1332. 27 H. M. Pickett and T. L. Boyd, Chem. Phys. Lett., 1978, 58, 446. ’* H. M. Pickett, Rev. Sci. Instrum., 1977, 48, 706. 1965), p. 118. WIS-TCI-503 (Madison, WI, 1974).

ISSN:0301-7249    

DOI:10.1039/DC9817100057

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Mid-infrared Laser Magnetic Resonance Spectroscopy BY A. ROBERT W. MCKELLAR Herzberg Institute of Astrophysics, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR6 Received 10th December, 1980 The development of laser magnetic resonance (1.m.r.) spectroscopy with CO (h = 5-7 pm) and C02 (A = 9-1 1 pm) laser sources originally lagged behind the application of this technique in the far- infrared region. However, during the past five years, mid-infrared 1.m.r. has been extensively used to study vibration-rotation and electronic spectra of a large number of atoms, diatomic molecules and triatomic molecules. It has allowed the study of many unstable free radicals that could not be detected by more conventional infrared techniques. This paper presents a brief review of these developments with an emphasis on recent work.Some new analyses of already published mid-in- frared 1.m.r. data are also presented. These are made possible by newly available measurements of isotopic C02 laser frequencies and improved ground vibrational state data, and they result in improved molecular parameters for the following molecules: HCO(v2), H02(v3), D02(v2), PH2(v2) and ND2(v3. During the last ten years much of the increase in our understanding of the structure and properties of short-lived molecules has come from the application of laser magnetic resonance (1.m.r.) spectroscopy to the study of rotational spectra in the far-infrared. The technique was developed in 1968 by Evenson et al.,' and first used to study a transient species, OH, in 1970.2 In one sense, the effect of far-infrared 1.m.r.was to extend to higher frequencies earlier gas-phase electron paramagnetic resonance (e.p.r.) studies of free radicals. The extension to still higher frequencies may be ac- complished using COz (9-1 1 pm) and CO (5-7 pm) lasers in an 1.m.r. apparatus, and in this case molecular vibration-rotation transitions become accessible. The exploita- tion of the 1.m.r. technique in the mid-infrared region has lagged behind the far-infra- red, but in the last five years it too has been applied to an increasing number of atoms and molecules. Reviews of far-infrared 1.m.r. have been given by Davies and Evenson3 and by Evenson et aL4 A general comparision of the technique as applied in the far- and mid-infrared regions is given in table 1, which also lists the species studied to date.The stable and nearly stable paramagnetic molecules 02, NO, NO2, NF2 and C10, have all been studied, but the real value of 1.m.r. clearly lies in studying unstable free radicals which are generally difficult to detect by other means, especially in the infrared region. The essential features of an 1.m.r. spectrometer are illustrated schematically in fig. 1 (a), which shows an experimental configuration with the absorption cell located out- side the laser cavity. Fixed-frequency laser radiation is directed through a free- radical absorption cell located in a magnetic field and then monitored by a detector. The field is swept, and an absorption line of the sample gas that happens to be Zeeman- tuned through the laser frequency results in a decrease in detected laser power.In practice, a small modulation at 0.1-100 kHz is added to the magnetic field using coils (not shown) near the cell, and the change in transmitted laser power is recorded as a first derivative signal by processing the detector output in a phase-sensitive amplifier. A detailed analysis of the observed spectrum requires, of course, a detailed knowledge of the Zeeman effect in the atom or molecule studied.64 M I D - I . R . LASER MAGNETIC RESONANCE SPECTROSCOPY TABLE 1 .-COMPARISON OF FAR- AND MID-INFRARED LASER MAGNETIC RESONANCE far-infrared 1. m.r. mid-infrared 1.m.r. sensit iu it y Probably higher. Evenson et 0 1 . ~ Evenson et aL4 estimate 3 x 108 estimate 5 x lo7 molecule ~ r n - ~ .molecule crnP3. These estimates, however, are very dependent on the particular molecule. The sensitivity achieved in practice also depends on the refinement of the apparatus. resolution Typical Doppler widths: 0.5-20 Typical Doppler widths: 40-1 50 MHz. Pressure broadening: 2- MHz. Pressure broadening: 2- 20 MHz. Lamb dips can be 20 MHz. Lamb dips are gener- seen in some cases where pres- ally easier to detect, though this sure broadening is less than is very dependent on the strength Doppler broadening. of absorption. practical wavelength limits 10-250 cni-'. Can be extended CO, laser: 875-11 10 cm-I (lower beyond these limits. frequencies available from 14C1602). CO laser: 1450-2000 cm- (can be extended). Higher frequencies are available using HF, DF, etc.lasers, but exten- sions are more likely to come using tunable lasers and Zeeman modulation. experimental Probably more difficult. For Probably easier, especially in CO, dficulty optically pumped system, both laser region. Detector is less pump (CO,) and far4.r. lasers expensive. Magnet can be smal- are required. ler thanks to shorter wavelength. information in Ground vibrational state para- Ground and excited vibrational spectrum meters. state parameters. dificulty of Depends very much on the na- Analysis is similar but probably anaLysis ture of the molecule and whether more difficult, because: (1) the previous information is available resolution is generally lower ow- on it. ing to greater Doppler width and (2) more parameters are required to describe the spectrum, in parti- cular the vibrational band origin.One advantage is that low-J transitions are more easily seen (they tend to be at too low a fre- quency in the far-Lr.). atom\.and 0, C, 02, OH, NO,, NO, CH, C1, Hg*, He*, NO, NO2, NF2, HCO, CH30, CH2F, SH, SD, DO,, SeH, SeD, FO, ClO, BrO, SeH, CH2, C2H, CF, CO*, 02*, SD, C102, SeO, NSe, SO, SO*, N2H, HBr+, DBr', CH20H, OD CH3, CH30( ?), FCO(?), NCO molecules studied HO2, DO2, NHI, PH2, NH, PH, HCO,DCO,NH2,ND2,PH2,HOz,A. R . W. MCKELLAR 65 Fig. l(b) illustrates the alternate arrangement of an 1.m.r. spectrometer with intra- cavity absorption cell. Placing the cell inside the laser cavity results in a gain in sensitivity due to multipassing of the laser radiation. A further sensitivity gain may also arise from the interaction between the absorbing medium and the non-linear-gain medium of the laser; this effect is especially important for optically pumped far- infrared lasers.Another benefit of the int-racavity configuration is that it favours the I DETECTOR LASER DJ %, >n \AB SOR PT I ON I I CELL ( b ) DETECTOR I I-. LASER GAI I ~ LASER CAVITY ! FIG. 1 .-Schematic representation of an 1.m.r. spectrometer with (a) absorption cell located outside the laser cavity, and (b) intracavity location of absorption cell. Both arrangements have been widely used for mid-infrared l.m,r., but (b) has generally been used in the far-infared region. observation of saturation dips because it ensures perfect alignment of counterpropaga- ting laser radiation and gives higher power densities in the absorption cell.This is especially important in the mid-infrared region, where Doppler widths are greater and the high resolution of saturation spectroscopy is needed more. A transverse magnetic field as given by an ordinary iron-core electromagnet is implied in fig. 1, but it is also possible to use a solenoid in an 1.m.r. spectrometer. However, the application of solenoids has been limited to date because of the difficulty of achieving high fields with conventional solenoids and the experimental complications of superconducting mag- nets. In the first part of the present paper, the development of 1.m.r. spectroscopy using CO, and CO laser sources is briefly reviewed with an emphasis on recent develop- ments. The second part of the paper is devoted to an examination and reanalysis of already published 1.m.r. results on the HCO, HO,, DO2, PH, and ND, radicals.T H E D E V E L O P M E N T OF M I D - I N F R A R E D L . M . R . The first molecule to be observed in the mid-infrared region by 1.m.r. spectroscopy was NO, which is the species most accessible to the technique because of its stability and the location of its fundamental band in a favourable region for CO laser operation (5. 3 pm). In 1972, Kaldor et aL5 published spectra which exploited a close coinci- dence (ca. 780 MHz) between the R(1.5) line of NO and a CO laser line at 1884.35 cm-'. The spectrum, which appeared in the 0.6-7 kG region, was proposed as a sen- sitive means of monitoring atmospheric NO concentration. A similar observation66 MID-I.R.LASER MAGNETIC RESONANCE SPECTROSCOPY was also published in 1972 by Bonczyk and Ultee,6 and subsequently Bonczyk mea- sured the pressure broadening of the NO transition7 and built a compact 1.m.r. appara- tus specifically for trace NO detection.' It appears, however, that the proposal to use 1.m.r. as a sensitive pollution detector has not been seriously adopted. NO spectra were also observed by Zeiger et aL9 using a superconducting solenoid with fields up to 50 kG, and by Bridges and Burkhardt,'O who used the optoacoustic tech- nique to detect the absorption of CO laser radiation by placing a spectrophone in the NO cell in the magnetic field. The above studies utilized the original 1884.35 cm-l laser line, except for that of Zeiger et al., who used a laser line at 1876.31 cm" to observe the Q(1.5) and Q(2.5) NO transitions. More spectroscopically oriented 1.m.r.studies of the NO fundamental band were later made by Hakuta and Uehara," who measured spectra using three other CO laser lines, and by Dale et a1.,12 who detected spectra of 14N1'0, 15N160 and I4NI7O in natural abundance, as well as the 2 t 1 hot band and 2111/2 t ,I-I3/2 satellite band of l4NI6O. Dale et al. also observed one transition within the ,Ill/, substate of l4NI6O; all the previous 1.m.r. spectra involved the more magnetically tunable , I I 3 / 2 state. This latter work', was the first in which the absorption cell was placed inside the laser cavity for mid-infrared l.m.r., though this is almost universal practice with far-infra- red 1.m.r. Thus Dale et al.were able to observe saturation (inverse Lamb) dips and resolve hyperfine and A-doubling splittings as small as 2.7 MHz. A second stable molecule, NOz, was also studied using the CO laser by Freund et al.13 in 1975. They obtained zero-field frequencies and spin splittings for two I4NO, and six lSNO, transitions in the v3 band, and derived some lSNO2 spectroscopic con- stants. A more detailed 1.m.r. study of the v3 band of NO, is certainly possible, but has not been performed, probably because, in contrast to unstable free radicals, NO, can be studied easily by more conventional techniques. The first use of 1.m.r. with a CO, laser was made around 1975 in the Soviet Union by Broude et aI.,I4 who studied the semi-stable radical NF, which occurs in the gaseous state in equilibrium with N2F4.They observed 1.m.r. signals on 20 laser lines in both the 9.4 and 10.4 pm CO, bands. However, they were limited by their solenoid electro- magnet to fields below 500 G, and did not give any assignments or analysis of their results. In a later paper (Gershenzon et al.") the kinetics of the N2F4 + 2NF, reac- tion were explored using 1.m.r. to measure the NF, concentrations, the first applica- tion of mid-infrared 1.m.r. to chemical kinetics. However, the Soviet group has not published any further spectroscopic results. In 1976, the first 1.m.r. spectroscopy of an atom was reported by Dagenais et a1.16 They observed the ground-state fine-structure transition (,P1,, +- 2P3,2) in atomic chlorine using a 13C1602 laser line at 882.287 cm-'.The next year, Johns et al.17 observed a similar transition in an excited atomic state, 6s6p 3P1 t 3P0 in the mercury atom. The C1 transition was relatively easy to detect, and was studiedI6 using a small (6 cm) electromagnet and an extracavity absorption cell; it has also recently been detected using a tunable diode laser.l8 However, the Hg transition was more difficult to detect because of the problem of producing sufficient excited metastable 3P0 atoms. In this case, the intracavity CO laser spectrometer, also used', for NO, was employed. Following the observations of the C1 and Hg spectra, analogous fine-structure spectra have been observed in the ~ x y g e n ' ~ * ~ ~ and carbon,' atoms using far-infrared 1.m.r. A rather different sort of atomic 1.m.r.spectroscopy with a CO, laser has been reported by Rosenbluh et al.,,-,' These authors have measured Rydberg transitions in excited helium atoms using a very high field (140 kG) 1.m.r. apparatus in which the absorption of 10 pm CO, photons was detected in the optical region by a change in However, they did not attempt a full-scale analysis.A . R. W. MCKELLAR 67 atomic luminescence. This double-resonance technique was more sensitive than the direct detection of the absorbed infrared photons. They also observed a novel line- shape which was the result of a motional Stark effect experienced by the atoms moving in the intense magnetic field, and which in principle provided sub-Doppler resolution. The application of mid-infrared 1.m.r. to the study of transient molecular species began in 1977 with the report by Brown et of the detection of NH, and HCO in the 5 pm CO laser region.They observed three transitions of the v, (bending) fundamen- tal band of NHz and three in the v3 (C-0 stretch) fundamental band of HCO, but did not give detailed assignments. Complete analyses of these 1.m.r. spectra have recently been published by Kawaguchi et al.27 for NH, and Brown et a1.28 for HCO. The first detailed analysis of the rotation-vibration 1.m.r. spectrum of an unstable molecule was made by Riggin et al.29 for the v, (bending) fundamental band of HCO around 9.25 pm. They observed and assigned ca. 90 discrete HCO resonances on 17 different CO, laser lines between 1069 and 1101 cm-', and fitted these to obtain the band origin, rotational constants, centrifugal distortion constants and spin-rotation constants.The analysis was greatly aided by the slightly earlier work of Landsberg et al.30 who had studied the same band using the technique of laser Stark spectroscopy. This technique is very similar to l.m.r., with an electric field and the Stark effect taking the place of the magnetic field and Zeeman effect. However, laser Stark spectroscopy is generally a much less appropriate technique for short-lived free radicals. There are two reasons for this: first, it is necessary to use quite low gas pressures ( 5 20 mTorr) to avoid electrical breakdown, and this limits the practical sensitivity which may be obtained; secondly, the Stark technique does not automatically discriminate against non-paramagnetic molecules (as does l.m.r.), and this may result in serious spectral interference by unwanted species. The work of Landsberg et aL30 on HCO was all the more notable because of these difficulties in applying laser Stark spectroscopy to free radicals.In addition to the 5 pm CO laser spectra already menti~ned,,~.,~ some weak transi- tions in the v, band of NH, have also been detected3' in the 9 pm CO, laser region. A number of other triatomic free radicals have now been studied by mid-infrared l.m.r., including HO, (v3, ca. 1098 ~ m - ' ) , ~ , DO, (v2, ca. 1020 cm-1),33 PH, (v,, ca. 1102 ~ m - ' ) , ~ ~ ND, (v,, ca. 1109 ~ m - 9 , ~ ~ and DCO (vl, ca. 1910 cm-'; and v3, ca. 1795 ~ m - 9 . ~ ~ For two of these, DO2 and DCO, the analysis involved a Coriolis interaction between two nearby vibrational states.In the case of DO,, only one of the states could be ob- served by 1.m.r. and the properties of the other were inferred from their effect on the observed spectrum, whereas in the case of DCO, both states were observed in the 1.m.r. spectrum. Another triatomic radical, the semi-stable species ClO,, has exhibited a new type of 1.m.r. spectrum in studies by Hakuta and Uehara.37-40 It has usually been con- sidered that molecules with small spin-rotation interactions will give 1.m.r. spectra only at relatively low magnetic fields, because high fields decouple the electron spin from the molecular rotation, with a resulting loss of electric dipole transition intensity for fast-tuning transitions. However, high-field spectra were observed3' in the v1 band of C10, (ca. 946 cm-') in spite of its small spin-rotation interaction, and Uehara and Hakuta assigned these spectra to transitions induced by avoided crossings be- tween Zeeman levels having the same value of MJ but differing by one in N .This type of transition offers a new means to study molecules which might otherwise be inac- cessible to the 1.m.r. techniques, though it may be difficult to apply to new molecules about which little is known since the assignment of observed spectra is not easy, even for the relatively well-known C10,. A considerable number of short-lived diatomic free radicals have also been studied68 MID-I.R. LASER MAGNETIC RESONANCE SPECTROSCOPY by mid-infrared 1.m.r. The 1-0 band (ca. 11 15 cm-l) of the metastable a 'A excited electronic state of SO was detected by Yamada et aL4' The same authors also studied4, the 1-0 and 2-1 bands of the X3C- ground state of SO, which occur in the same spectral region. They produced SO by reacting OCS with 0 atoms, and found that the addition of CF, to the oxygen discharge enhanced the production of the SO ground state and diminished production of the a*A state.The closely related mole- cule SeO has recently been studied by Hakuta and U e h a ~ a , ~ ~ who observed and analysed the 1-0 bands of 4 of the Se isotopic species of SeO in the 11 pm CO, laser region. Among diatomic radicals with ,l-I ground electronic states, vibration-rotation transitions have been observed by 1.m.r. for FO (1-0 band, ca 1033 ~ m - ' ) , ~ ~ C10 (2-0 band, ca.1670 ~ m - ' ) , ~ ~ SD (1-0 band, ca. 1885 cm-1),46-47 and NSe (1-0 band, ca. 945 ~ m - ' ) . ~ ' Especially notable here was the detection of FO, which had not previously been observed by spectroscopic means in the gas phase. It was pro- duced simply by a microwave discharge in CF4, the oxygen atoms coming from the erosion of the quartz discharge tube, and gave remarkably strong spectra with pro- minent saturation dips and 19F hyperfine s t r ~ c t u r e . ~ ~ The above diatomics were observed by means of rotation-vibration transitions within the '113/, component. The electronic transition between the 21t3/2 and 2111,2 components may also happen to lie in the mid-infrared, and such a spectrum was in fact observed in SeH by Brown et aZ.49 using 1.m.r.in the 5.7 prn CO laser region. The transitions they observed were of magnetic dipole origin with the exception of one weak electric dipole line, and their analysis yielded a direct determination of the spin-orbit splitting, A = -1764.019 cm-l. Very recently, a similar study of SeD has been c~mpleted,'~ and a combination of the SeH and SeD results determines some further fundamental parameters for this molecule. A similar 2111/2 t 'II3/, spectrum has recently been observed by McKellar" for BrO in the 10.3 pm region. In this case, the spectrum consisted entirely of mag- netic dipole transitions, and extensive hyperfine, A-doubling and isotopic structure was observed. The spin-orbit splitting was determined to be A = -967.983 cm-l for 79Br0 and -967.998 cm-' for "BrO, and a number of rotational, hyperfine, A-doubling and Zeeman parameters were also determined.Some recent and as yet unpublished work using mid-infrared 1.m.r. also deserves to be mentioned. The group at the Institute for Molecular Science in Okazaki (Amano et al.)', has been studying infrared-optical double resonance in NH,. Their spectrometer resembles a conventional CO, 1.m.r. apparatus, but visible radiation from a tunable C.W. dye laser is introduced into the absorption cell as well as the 10 pm CO, laser radiation. The change in intensity of the visible fluorescence of NH, due to the presence of the infrared radiation is monitored while the magnetic field is swept. The resulting signals effectively give 1.m.r. spectra of vibration-rotation transitions within the 2, 2A1 excited electronic state of NH,; for example, the 2,' t 2, transition of the (0, 10,O) t (0,9,0) vibrational band has been observed.Jn other unpublished work, the group at the University of Southampton has been studying the 1-0 vibrational transition of Sen in the ,113/, state (v, z 1677 cm-') and the v3 (antisymmetric stretch) band of the linear radical NCO. This latter molecule is interesting because it exhibits strong vibronic interactions, but these also complicate the observed spectrum. Around 1977 the same Southampton group also observed the 1.m.r. spectrum of an unknown species in the 5.4 pm region.53 This " mystery molecule " could be pro- duced, for example, by discharging mixtures of CF, + CO or SF, + CO, and possible candidates were ground-state FCO, or electronically excited CF, or F2C0.How- ever, very little was known about these molecules, and there were various arguments against each one. Very recently, Nagai et aZ.54 have measured the v, (ca. 1862 crn-')A . R. W. MCKELLAR 69 and v, (ca. 1026 cm-') bands of FCO using a tunable diode laser, and using their molecular parameters it is now possible to calculate the expected v1 band 1.m.r. spec- trum. Preliminary indications are that FCO may indeed be the 5.4 pm mystery mole- cule. Unassigned spectra have also been observed in the 9.5 pm region by G. W. Hills using a discharge in CF,. Though the conditions required for this spectrum seemed somewhat different from those for the 5.4 pm mystery spectrum, there were also many similarities, and it was considered possible that both were due to the same species (0 atoms were available for the 9.5 pm spectrum from the erosion of the quartz discharge tube). However, it now seems that the parallel (a-type) component of v2 of FCO measured by Nagai et al.54 is not the source of the 9.5 pm 1.m.r.spectrum unless very high values of N and K are involved. It is still possible that the perpendicular (b-type) component of this band, which was too weak to be observed by diode laser,54 could be responsible, but perhaps more likely that the spectrum is due to another species, such as excited triplet state CF,. The symmetric rotor free-radical CH30 is one of the most interesting molecules to be studied and analysed by far-infrared l.m.r.55*56 Now that its rotational spectrum is understood thanks to the beautiful work of Russell and R a d f ~ r d , ~ ~ a search for rotation-vibration spectra is of some interest.Such a search has begun at N.R.C.,57 and some weak spectra have been observed in the 975 to 1020 cm-I region. The pro- duction scheme, mixing CH30H with the products of a discharge in CF, + He, was the same as used for CH,O by Radford. The spectra observed are not due to HO, or HCO, and are unlikely to be due to CH,OH, which Radford has found to be more I I magnetic field/kG FIG. 2.-L.m.r. spectrum, possibly owing to CH30, observed with perpendicular polarization using the 1zC1602 R(20) laser line at 978.930 cm-'. The free-radical source was methanol mixed with the products of a microwave discharge in CF4 + He.The two traces are simply repetitions of the same scan; on close examination, some of the structure that at first seems to be noise is actually due to real spectral features.70 MID-I.R. LASER MAGNETIC RESONANCE SPECTROSCOPY difficult to produce than CH30. However, there is no proof that the 10 pm spectra are really CH30 apart from the production scheme used. An example of an observed " CH30 " spectrum is shown in fig. 2, which was recorded with perpendicular polariz- ation using the 12C'60, R(20) laser line at 975.930 cm-'. There is considerable noise in the two traces of fig. 2, in spite of their having been recorded with a 3 s time constant and a very slow scan rate. The poor signal-to-noise ratio is a reflection of the weak- ness of the absorption, but on close examination of these and other traces of the same region, some of the " noise " is seen to be a dense succession of real overlapping fea- tures.Approximately similar spectra have been observed on a number of other laser lines between 975 and 1020 cm-l, though there are relatively few features as sharp and isolated as the prominent line near 3 kG in fig. 2. There is no evidence for any par- ticularly strong or simple 1.m.r. spectra in the region of 1015 cm-', which is thought from visible laser excited fluorescence meas~rements~~ to be the v3 (C-0 stretch) fre- quency of CH30. None of the spectra observed to date invite even an attempt at analysis; clearly it is necessary either to increase the signal-to-noise ratio considerably, or to find more spectra with isolated and easily measurable lines. In any event, analysis of CH30 spectra would not be easy, because the rotational structure is itself very complicateds6 and there may be a number of vibrational bands (some degenerate) in the 10 ,urn region.NEW FITS O F L . M . R . SPECTRA OF HCO, H02, D02, PH2 A N D ND, The reanalysis of already published results in molecular spectroscopy may generally seem to be a sterile pastime. Nevertheless, such a re-fitting of data is reported here for 5 triatomic free radicals that have been studied at N.R.C. by mid-infrared 1.m.r. during the past 4 years: HCO ( u ~ ) , ~ ~ HO, ( v J , ~ , DO, ( v , ) ~ ~ PH, (u,),~~ and ND, (u,).~~ There are three recent developments that make this effort worthwhile: (1) Most im- portantly, new improved measurements of isotopic CO, laser frequencies have become available59 which differ from the best previous values by up to 70 MHz (this is most important for HO,, PH2, and ND,).(2) New information on the ground vibrational states of some of the molecules has become available (this is most important for HCO, DO2, and PH,). (3) A sign error has been discovered in the computer program ori- ginally used to fit these spectra. The error involved the relative sign of the isotropic and anisotropic parts of the Zeeman matrix element connecting the Fl and F, levels of a given ( N , K,, K,) state, and had only a small effect at worst, being most important for HO, and DO,. In the following paragraphs, the results of new analyses of these previously reported spectra are given; for more complete information, the reader is also referred to the original HCO The new measurements of Freed et ~ 1 .~ ~ change the frequencies of laser lines used to the v, band of HCO by only small amounts ((7 MHz), and this molecule was unaffected by the sign error, because it was originally analysed using a simpler com- puter program. However, much better parameters for the ground vibrational state of HCO are now available60 from analyses of e.p.r. and far-infrared 1.m.r. spectra.61 In particular, the A rotational constant and the quadratic centrifugal distortion con- stants are now well-known. The results of the new fit of the HCO v, band data of Johns et aZ.29 are shown in table 2. The ground-state parameters were all fixed at the values of Brown60v61 and 9 excited-state parameters were varied, as compared with 13 ground- and excited-state parameters varied in the original fit.29 The new Zeeman calculation included the effects of the anisotropic g-tensor and AN = &I mixingA .R . W. MCKELLAR 71 approximated in a way previously described32 (these effects are small for HCO because of its relatively small spin-rotation interaction). The r.m.s. deviation of the fit was 25 MHz, slightly worse than the original29 20 MHz, but still well within the expected experimental error. The parameters of the ( Z J ~ Z J ~ V ~ ) = (010) excited state given in TABLE 2.-MOLECULAR PARAMETERS FOR HCO (IN Cm-') parameter ground (000) state" excited (010) state VO A B C A K lo6 A N lo5 A N K 104 6 , 1 0 7 t j N &a, lo4 Ebb 103 &,, 103 AL 24.329 01 1.494 045 1.398 576 0.030 683 1.50 3.919 1.93 3.656 0.387 752 6.324 - 6.861 - 1.643 1080.762 l(3) 26.574 98( 15) 1.500 256(53) 1.392 562(79) 0.049 561 (1 7) 3.22( 106) - 0.20(46) C C 0.475 93(50) C C - 3.160(62) From a fit to microwave, e.p.r.and 1.m.r. data.60.61 Present results, from a fit to 1.m.r. data Uncertainties in parentheses are la from the least-squares fit, in units of the last Rotational g-factors were also fixed at of Johns et quoted digit. the following values6' for both states: g;a Parameters fixed at the ground-state value. -0.0039; gp'b = 0; g,'" = -0.000 07. table 2 are changed from the previous determinati~n,,~ and should be more reliable thanks to the improved laser frequencies, ground-state parameters and Zeeman cal- culation.HO, The best parameters for the ground vibrational state of HO, (Barnes et aZ.)62 remain the same as those assumed by Johns, McKellar and Riggin32 in their analysis of the v3 band, except for slightly refined values of I , , and cbb due to Brown and Sears.63 However, the new laser-frequency measurements have a large effect for this molecule. Lines of the R-branch of the 9.2 pm band of 12C180, above 1100 cm-' are the most changed of the news9 measurements, and more than one third of the H02 data was obtained with these laser lines. The most changed of the frequencies used is that of R(44) at 1109.880 crn-l, which has moved up by ca. 70 MHz. The sign error also has a significant effect in this case, since HO, has the largest spin-rotation interaction of the molecules considered here, and hence the largest anisotropic electron g-factor effects.The parameters resulting from the new fit to the HO, v3 band data of Johns et aL3' are given in table 3. The r.m.s. deviation of the new fit was 19 MHz, as com- pared in the 26 MHz in the original fit; there were 12 parameters varied in each case. One pleasing aspect of the new results is that the values obtained for SE, and E:, are much closer to their ground-state values. DO2 The v2 band 1.m.r. spectrum of DOz at ca. 1020 cm-' was studied by M ~ K e l l a r , ~ ~ who found it necessary to include in the analysis the effects of the Coriolis interaction72 M I D - I . R . LASER MAGNETIC RESONANCE SPECTROSCOPY between the observed (010) vibrational state and the (001) state which lies about 100 cm-I higher in energy.Thus the v 2 analysis yielded a few effective v3 parameters even though this band was not directly observed. Only one microwave transition had been measured for the ground state of DOz, so a number of ground-state parameters were also varied and determined from the mid-infrared results.33 The laser frequen- cies used in the original analysis are little changed, but now accurate ground-state constants are available from the study of the far-infrared 1.m.r. spectrum of DO2 made TABLE 3.---MOLECULAR PARAMETERS OF H 0 2 (IN Cm-') parameter ground (000) state" excited (001) stateh VO A B C 104 A~ 104 los 6, 107 an: 104 E,, 104 A; lo6 Ajv &a, lo2 Ebb 20.356 560 1.118 0356 1.056 3188 4.124 1.155 3.893 6.71 1 -2.33 -1.653 516 - 1.409 6 2.88 7.61 1097.625 8( 1) 20.309 080( 50) 1.105 532(37) 1.042 649(38) 4.152 2(69) I .306( 29) 2.3 5( 65) 8.1(84) C -1.711 65(26) - 1.4200( 98) 2.9( 12) 7.3 1 (33) a From Barnes ef a/.,62 except for Ebb from Brown and Sears.63 Converted to the A-reduced Uncertainties in parentheses Parameter fixed at the ground- Sextic centrifugal distortion parameters and rotational g-factors were also fixed to the form.65 Present results, from a fit to the 1.m.r.data of Johns et are la from the least-squares fit, in units of the last quoted digit. state value. values of Barnes et ~ 1 . ~ ' recently by Barnes et af.64 The ground-state constants that could be determined from the mid-infrared analysis33 are in remarkably good agreement with the new64 values, but they can now be held fixed at more accurate values.And less well-known para- meters (e.g., A" and A:) that had previously to be fixed are now known much better. The results of the new fit to the DO2 v2 band 1.m.r. data33 are shown in table 4, which also lists the ground-state parameters of Barnes er af.,64 converted from the S-reduced to the A-reduced form of the asymmetric rotor Hamiltonian.6s The r.m.s. deviation of the fit was improved to ca. 19 MHz from the value of 24 MHz. There are fairly large changes in A' and A; between the old33 and new (table 4) fits, but this is simply a reflection of the newly assumed ground-state parameters. Only the dif- ferences A' - A'' and A; - A: are well-determined since vz is an a-type band (A& = 0) and these are little changed.The altered A values do, however, have an important secondary effect on the calculations, since they affect the relative positions of the (OlO), K, and (OOl), K, - 1 levels which are in Coriolis interaction. This is probably the reason that the effective value of vj inferred from the new calculation is ca. 1 cm-l higher than the previous value. PH2 At the time of the study of the v2 band of PH, by Hills and M ~ K e l l a r , ~ ~ some far- infrared 1.m.r. measurements on one ground-state rotational transition were also available.66 These two sets of 1.m.r. data were combined in a fit in which all the neces-A . R. W. MCKELLAR 73 TABLE 4.-MOLECULAR PARAMETERS FOR DO1 (IN Cm-') parameter ground (000) statea excited (010) stateb 11.194 508 1.055 982 8 0.961 059 2 1.281 8.304 3.612 5.17 0.253 -0.905 486 - 1.3077 2.03 2.97 1020.161 5(2) 11.431 885(42) 1.056 796(68) 0.955 891(67) 1.639 4(21) 8.83(23) 4.43(73) 5.2( 15) C -0.946 74(21) - 1.399(30) 3.2(29) 2.7 1 ( 1 6) Coriolis parameters 1121.35(3) 0.680 29(96) -0.013 4(18)d +0.008 5(59)e a From Barnes et u I ., ~ ~ converted to the A-reduced form.65 Present results, from a fit to the 1.m.r. data of M ~ K e l l a r . ~ ~ Uncertainties in parentheses are lo from the least-squares fit, in units of the last quoted digit. Rotational g-factors were also fixed to the values of Barnes et ~ 7 1 . ~ ~ d A s = 8(001) - 8(000); 8 = (B t C)/2. AEaa = E,, (001) - ~ ~ ~ ( 0 0 0 ) . sary ground- and excited-state parameters (25 in all) were determined.Since then, further far-infrared 1.m.r. measurements have been reported by Davies et al. ; 67 these involve 8 more rotational transitions which may now be added to a combined fit. The newly CO, laser frequencies alter one line used for PH, by ca. 40 MHz Parameters fixed at the ground-state value. All other (001) state parameters were fixed at their (000) state values. TABLE 5.-MOLECULAR PARAMETERS FOR PH2 (IN Cm-') parameter ground (000) statea excited (010) state" VO A B C lo3 AK lo3 ANK 104 A~ 104 aK 104 6, Eaa lo2 Ebb 104 E , , 104 ~k 9.132 45(8) 8.084 37( 11) 4.214 40(8) 2.821 O(54) 5.477(25) 2.524(19) - 1.758 O(74) - 1.048(27) -0.281 41(37) - 8.1 20( 24) -7.5(18) 1.62(20) 1 101.908 6(2) 9.432 87( 16) 8.252 70(13) 4.154 51(6) 3.358(26) 6.3 33(49) 2.91 9(23) -2.071(17) - 0.44(70) -0.303 60(63) - 8.81 8(35) 0.6( 17) 3.07(70) a All parameters determined in the present fit to the data of Davies et a1.66*67 and Hills and Mc- Kellar.34 Uncertainties are la from the least-squares fit, in units of the last quoted digit.Rota- tional g-factors were fixed at the following estimated values (see text) for both states: g;" = -0.000 94; g,"b = -0.00027; g:' = 0.74 MID-I.R. LASER MAGNETIC RESONANCE SPECTROSCOPY and the rest by 2 MHz or less; the above-mentioned sign error has very little effect for this molecule. 1 . m .r. data on PH2 are shown in table 5. All the data were equally weighted in the fit, and rotational g- factors were fixed at values (see table 5) estimated using a simple approximate rela- tion6’ for the electronic contribution to g , .The r.m.s. deviations resulting from the fit were 21 MHz for the v, band data34 and 14 MHz for the rotational data.66*67 The uncertainties in 24 out of the 25 parameters (table 5 ) are less than those in the earlier fit,34 and for many ground-state parameters the improvement is quite large. This is, of course, mostly due to the newly included ground-state data.67 In their analysis of the PH2 ground state, Davies et ~ 1 . ~ ’ fitted their own measure- m e n t ~ ~ ~ ~ ~ ~ plus the calculated ground-state term values from the v, band The latter were given a reduced weight (0.01), but were still required because the far- infrared data were insufficient for a good determination of all the parameters. It might thus be thought that the present results (table 5) and would be very similar, since all the same data contributed to each fit.However, this is not the case: the present results are to be preferred since all the original measurements were directly fitted. Fitting a mixture of measured lines and calculated term values was convenient, but failed to make best use of the available data. (There remains, however, an area of legitimate argument as to the best relative weights for the mid- and far-infrared mea- surements.) The quoted parameter uncertainties (converted to la) are generally lower in Davies et ~ 1 . ~ ~ than here, but this is likely an artifact of their fit (i.e., by fitting cal- culated term values one could even achieve zero apparent uncertainties). The results of the new fit to the ND2 The v2 band measurements of Hills and M ~ K e l l a r ~ ~ remain the only available high- resolution data for ND2.The new C02 laser frequencies result in significant changes (25 to 50 MHz) for three of the laser lines used in the original study. The results of the new fit to the ND, v2 band data are shown in table 6; this fit differs from the pre- vious in the assumed laser frequencies and in that the rotational g-factors were TABLE 6.-MOLECULAR PARAMETERS FOR ND2 (IN Cm-’) parameter ground (000) statea excited (010) statea b’0 A B C 103 AK lo3 ANK lo4 AN 1 0 4 6, 1 0 5 H~ 104 8 N Eaa Ebb Ecc 1 0 4 A; 13.344 18(30) 6.490 62( 11) 4.287 03(14) 6.707(35) 2.339(41) 0.59(26) 1.878(26) 1.55(15) -0.979( 13) - 0.1 70 49(29) -0.021 65(26) 0.000 21 1.34( 12) 1108.749 3(6) 14.234 96(18) 6.552 23(9) 4.237 39(12) 9.544(30) 2.982(28) 5.60( 16) 1.261 (16) 2.9 1 (29) - 1.366 2(80) -0.203 14(56) -0.024 09(23) - 0.0005 2.44(39) a Determined from a fit to the data of Hills and M ~ K e l l a r .~ ~ Uncertainties in parentheses are la from the least-squares fit, in units of the last quoted digit. Rotational g-factors were fixed at the following estimated values: g:a = -0.0024; g:b = -0.000 30; giC = -0.0000. be cc was fixed at values estimated from NH2.”A . R. W. MCKELLAR 75 fixed at approximate calculated6’ values instead of at zero. The r.m.s. deviation of the fit improved to 34 MHz from the previous 40 MHz. There were corresponding improvements in the uncertainties of the 25 parameters varied, but all remained within la of their previous3s values.CONCLUSTON In the past 5 years, mid-infrared 1.m.r. has yielded a remarkable amount of infor- mation on vibration-rotation and spin-orbit spectra of unstable molecules. In most cases, these results could not have been obtained by other, more conventional, in- frared spectroscopic techniques. We can anticipate a continuation of new results in this field during the coming years. However, the pace of developments may be somewhat slowed, since many of the better-known free radicals expected to absorb in the accessible (5-7 and 9-1 1.3 pm) regions have now been observed. As for lesser- known free radicals, l.m.r., especially of vibration-rotation bands, is a difficult way to obtain the first knowledge (at least for polyatomics) because the Zeeman effect must be analysed as well as the zero-field spectrum.Of course, it may be the only available technique, and remarkable successes are possible as illustrated by the case of CH,OS6 in the far-infrared. Tunable lasers are another factor in the development of infrared spectroscopy of high sensitivity and resolution. A prominent example of this is the great progress made by Hirota68 and co-workers at the Institute for Molecular Science with the appli- cation of semiconductor diode lasers to free radical spectroscopy. Of course, diode lasers have their own unique difficulties, but they are free from inherent wavelength limitations over a wide infrared range, and apparently can at least approach the sen- sitivity of 1.m.r. for the detection of unstable molecules. K.M. Evenson, H. P. Broida, J. S. Wells, R. J. Mahler and M. Mizushima, Phys. Reu. Lett., 1968, 21, 1038. K. M. Evenson, J. S . Wells and H. E. Radford, Phys. Rev. Letf., 1970, 25, 199. P. B. Davies and K. M. Evenson, Proc. Second Int. Conf. Laser Spectroscopy (Springer-Verlag, Berlin, 1975), p. 132. K. M. Evenson, R. J. Saykally, D. A. Jennings, R. F. Curl and J. M. Brown, Chemical and Biochemical Applications of Lasers, ed. C . B. Moore (Academic Press, New York, 1980), vol. 5, p. 95. A. Kaldor, W. B. Olson and A. G . Maki, Science, 1972, 176, 508. P. A. Bonczyk and C. J. Ultee, o p t . Commun., 1972, 6, 196. P. A. Bonczyk, Chem. Phys. Lett., 1973, 18, 147. P. A. Bonczyk, Rev. Sci. Instr., 1975, 46, 456. H. J. Zeiger, F. A. Blum and K. W. Nill, J. Chem. Phys., 1973, 59, 3968.lo T. J. Bridges and E. G . Burkhardt, Opt. Commun., 1977, 22, 248. l1 K. Hakuta and H. Uehara, J. Mol. Spectrosc., 1975, 58, 316. l2 R. M. Dale, J. W. C. Johns, A. R. W. McKellar and M. Riggin, J. Mol. Spectrosc., 1977, 67, l3 S. M. Freund, J. T. Hougen and W. J. Lafferty, Can. J. Phys., 1975, 53, 1929. l4 S. V. Broude, Y. M. Gershenzon, S. D. Il’in, S. A. Kolesnikov and Y. S. Lebedev, Dokl. Phys. l5 Y. M. Gershenzon, S . D. Il’in, S. A. Kolesnikov, Y . S . Lebedev, R. T. Malkhasyan, A. B. Nal- l6 M. Dagenais, J. W. C . Johns and A. R. W. McKellar, Can. J. Phys., 1976,54, 1438. l7 J. W. C . Johns, A. R. W. McKellar and M. Riggin, J . Chem. Phys., 1977, 66, 3962. la P. B. Davies and D. K. Russell, Chem. Phys. Lett., 1979, 67, 440. l9 P. B. Davies, B.J. Handy, E. K. Murray-Lloyd and D. R. Smith, J. Chem. Phys., 1978, 68, 2o R. J. Saykally and K . M. Evenson, J . Chem. Phys., 1979, 71, 1564. 21 R. J. Saykally and K. M. Evenson, Astrophys. J. Lett., 1980, 238, L107. 440. Chem., 1975, 223, 706. bandyan and V. B. Rozenshtein, Kinetics and Catalysis, 1979, 19, 1137. 1135.76 MID-I.R. LASER MAGNETIC RESONANCE SPECTROSCOPY 22 M. Rosenbluh, T. A. Miller, D. M. Larsen and B. Lax, Phys. Rev. Lett., 1977, 39, 874. 23 M. Rosenbluh, R. Panock, B. Lax and T. A. Miller, Phys. Rev. A, 1978, 18, 1103. 24 R. Panock, M. Rosenbluh, B. Lax and T. A. Miller, Phys. Rev. Lett., 1979, 42, 172. 25 R. Panock, M. Rosenbluh and B. Lax, Phys. Rev. A , 1980, 22, 1050. 26 J. M. Brown, J. Buttenshaw, A. Carrington and C. R. Parent, Mol.Phys., 1977, 33, 589. 27 K. Kawaguchi, C. Yamada, E. Hirota, J . M. Brown, J . Buttenshaw, C. R. Parent and T. J. ” J. M. Brown, J. Buttenshaw, A. Carrington, K. Dumper and C. R. Parent, J. Mol. Spectrosc., 29 J. W. C. Johns, A. R. W. McKellar and M. Riggin, J. Chem. Phys., 1977, 67, 2427. 30 B. M. Landsberg, A. J. Merer and T. Oka, J. Mol. Spectrosc., 1977, 67, 459. 31 G. W. Hills and A. R. W. McKellar, J. Mol. Spectrosc., 1979, 74, 224. 32 J. W. C. Johns, A. R. W. McKellar and M. Riggin, J. Chem. Phys., 1978, 68, 3957. 33 A. R. W. McKellar, J. Chem. Phys., 1979, 71, 81. 34 G. W. Hills and A. R. W. McKellar, J . Chem. Phys., 1979, 71, 1141. 35 G. W. Hills and A. R. W. McKellar, J. Chem. Phys., 1979, 71, 3330. 36 R. S. Lowe and A. R. W. McKellar, J. Chem. Phys., 1981, 74, 2686. 37 H. Uehara and K. Hakuta, Chem. Phys. Lett., 1978, 58, 287. 38 K. Hakuta and H. Uehara, Chem. Phys. Lett., 1979, 63, 496. 39 K, Hakuta and H. Uehara, Chem. Phys. Lett., 1979, 63, 500. 40 H. Uehara and K. Hakuta, J . Chem Phys., 1981, 74, 969. 41 C. Yamada, K. Kawaguchi and E. Hirota, J . Chem. Phys., 1978,69, 1942. 42 K. Kawaguchi, C. Yamada and E. Hirota, J . Chern. Phys., 1979, 71, 3338. 43 K. Hakuta and H. Uehara, J . Mol. Spectrosc., 1981, 86, 43. 44 A. R. W. McKellar, Can. J . Phys., 1979, 57, 2106. 45 R. S. Lowe and A. R. W. McKellar, J. Mol. Spectrosc., 1980, 79, 424. 46 R. S. Lowe, Mol. Phys., 1980, 41, 929. 47 W. Rohrbeck, A. Hinz and W. Urban, Mol. Phys., 1980, 41, 925. 48 K. Hakuta and H. Uehara, to be published. 49 J. M. Brown, A. Carrington and T. J . Sears, Mol. Phys., 1979, 37, 1837. 50 J . M. Brown, A. Carrington and A. D. Fackerell, Chem. Phys. Lett., 1980, 75, 13. 51 A. R. W. McKellar, J. Mol. Spectrosc., 1981, to be published. 52 T. Amano, K. Kawaguchi, M. Kakimoto, S. Saito and E. Hirota, to be published. 53 J. M. Brown, J. Buttenshaw and A. Carrington, personal communication. 54 K. Nagai, C. Yamada, Y. Endo and E. Hirota, to be published. 55 H. E. Radford and D. K. Russell, J . Chem. Phys., 1977, 66, 2222. 56 D. K. Russell and H. E. Radford, J . Chem. Phys., 1980, 72, 2750. 57 A. R. W. McKellar, unpublished. 58 G. Inoue, H. Akimoto and M. Okuda, J. Chem. Phys., 1980, 72, 1769. 59 C. Freed, L. C. Bradley, and G . R. O’Donnell, ZEEEJ. Quantum Electron., 1980, QE-16, 1195. 6o J. M. Brown, personal communication. 61 J. M. Brown, H. E. Radford and T. J . Sears, to be published. 62 C. E. Barnes, J. M. Brown, A. Carrington, J. Pinkstone, T. J. Sears and P. J. Thistlethwaite, 63 J . M. Brown and T. J. Sears, J. Mol. Spectrosc., 1979, 75, 111. 64 C, E. Barnes, J. M. Brown and H. E. Radford, J. Mol. Spectrosc., 1980, 84, 179. 65 J. K. G. Watson, J. Mol. Spectvosc., 1977, 65, 123. 66 P. B. Davies, D. K. Russell and B. A. Thrush, Chern. Phys. Lett., 1976, 37, 43. 67 P. B. Davies, D. K. Russell, B. A. Thrush and H. E. Radford, Chem. Phys., 1979, 44, 421. c8 E. Hirota, Chemical and Biochetnical Applications of Lasers, ed. C. B. Moore (Academic Press, Sears, J . Mol. Spectrosc., 1980, 81, 60. 1980, 79, 47. J. Mol. Spectrosc., 1978, 72, 86. New York, 1980), vol. 5, p. 39.

ISSN:0301-7249    

DOI:10.1039/DC9817100063

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Sub-Doppler Resolution Infrared Molecular-beam Spectroscopy Stark Effect Measurement of the Dipole Moment of Hydrogen Fluoride and Hydrogen Cyanide in Excited Vibrational States BY T. E. GOUGH, R. E. MILLER * AND G. SCOLES~ Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received 19th January, 1981 Sub-Doppler beam-calorimetric infrared spectroscopy is used to determine the dipole moments of hydrogen fluoride and hydrogen cyanide in vibrationally excited states. For the former pi = 1.872 f 0.003 D while for the latter pool = 3.012 * 0.002 D. The results for hydrogen fluoride are used to revise the literature value of p e for this molecule to 1.803 * 0.002 D. In recent years there has been a considerable amount of interest, both experimental and theoretical, in obtaining the dipole moments of diatomic molecules expressed as Taylor series in their internuclear separations.In particular, hydrogen fluoride has received much attention because of its use in chemical lasers: these lasers operate on several vibrationally excited states so that the computation of expected gain co- efficients requires knowledge of the dipole-moment function at internuclear distances appreciably removed from equilibrium. The experimental determination of the dipole-moment function involves the measurement of dipole-moment matrix elements. Off-diagonal dipole-moment matrix elements may be obtained from the intensities of vibration-rotation transitions, while the diagonal elements obviously involve measurements of the dipole moments of vibrationally excited molecules.The latter results have traditionally been obtained from Stark-split radiofrequency and microwave spectroscopy. To date the most precise measurements have been made using the molecular-beam electric resonance technique. Using such techniques Muenter and Klemperer,' and later Muenter,' have measured the dipole moment of hydrogen fluoride in its ground vibrational state as po = 1.826 18 D. In these experiments excited vibrational states were not sufficiently populated to allow measurement of the vibrational dependence of the dipole moment. A direct extrapolation to determine p e , the dipole moment at the potential-energy minimum was, therefore, not possible. Accordingly, Muenter and Klemperer' measur.ed puo for deuterium fluoride as 1.8881 D, and after assuming that the isotopic substitution does not modify the electronic structure evaluated pe, for both species, as 1.7965 D.In a related paper, Kaiser3 has cautioned against such an application of the Born-Oppenheimer approximation, showing that p e for hydrogen chloride is The uncertainty in p e is important because Sileo and Cool4 have made extensive * Physics Department, University of Waterloo, Waterloo, Ontario, Canada N2L 3G I . Present address : Research School of Physics, Australian National University, Canberra, ATC, Australia. t Also Physics Department, University of Waterloo, Waterloo, Ontario, Canada N2L 3G 1. D greater than p, for deuterium chloride.78 SUB-DOPPLER RESOLUTION SPECTROSCOPY F- CENTRE LASER DETECTORS measurements of the intensities of infrared emission from several vibrationally excited states of hydrogen fluoride. These results allow evaluation of the first derivatives of the dipole moment with respect to internuclear distance; however, in order to complete characterisation of the dipole moment function a value of ,ue must be supplied.In the present study we use sub-Doppler beam-calorimetric spectroscopy (the laser bolometric method) to study the infrared spectrum of hydrogen fluoride. In par- ticular, the measurement of Stark splittings of the R l component of the funda- mental vibrational transition of hydrogen fluoride is used to obtain a value for the dipole moment of this molecule in its u = 1 state, with an accuracy comparable to conventional microwave techniques.From this value of ,ul and the literature value of ,uo an improved value of pe is obtained. Similar experiments performed on the v3 transition of hydrogen cyanide will be reported but not analysed in detail. ----+ EXPERIMENTAL APPARATUS A N D RESULTS The apparatus used in the present study has been previously discussed in some detaiL5-’ Therefore, only significant modifications implemented since those reports are presented here. The infrared spectrum of the molecular beam is obtained using a 2 K doped silicon bolometer to measure Fig. 1 shows a schematic diagram of the present set-up. I Kr’ LASER FIG. 1 .-Schematic diagram of the apparatus used for sub-Doppler beam-calorimetric spectroscopy.T .E. GOUGH, R. E. MILLER AND G . SCOLES 79 energy imparted to the beam by an appropriately tuned infrared laser. The N.E.P. of the bolometer is W Hz-+ so that the apparatus is sufficiently sensitive to detect a flux of lo7 excited molecules per second impinging upon the bolometer. Further- more, the amount of scattered laser radiation reaching the bolometer is negligible, so that the laser and bolometric detector are only coupled through the excited molecules of the beam. As a result, the background problems which plague most traditional absorption experiments are eliminated. The sensitivity of the experiment allows the use of a highly collimated molecular beam which, when crossed orthogonally by laser radiation, provides a spectral resolution far superior to that obtainable in bulk gas experiments. Thus, the observed linewidth in the present experiments was 1.5 MHz, whereas the Doppler width for the corresponding transitions in a gas sample at room temperature is 330 MHz.The 1.5 MHz linewidth arises, with comparable contribu- tions, from the finite divergence of both molecular and laser beams, from the finite time spent by the molecules in the laser radiation, and from laser instabilities. The laser used in the present work was an F-centre laser (Burleigh FCL-20) pumped by a krypton-ion laser (Spectra Physics 171). The details of the operation and performance of F-centre lasers have been well-documented in the literature8e9 and will not be discussed here. A gas cell containing the gas of interest was used for preliminary location of the desired transition and for alignment of the scanning controls of the FCL-20.An hermetically sealed, temperature stabilised confocal etalon (Burleigh CF-500 P) having a free spectral range of 150 & 0.5 MHz was used to provide frequency markers for the calibration of Stark splittings. Because of the corrosive nature of hydrogen fluoride, the molecular-beam source, gas-handling system and gas cell were all constructed from monel. Sapphire windows were used in the gas cell. The molecular beam was formed by expanding a 1.6% hydrogen fluoride in helium mixture through a 35 pm monel nozzle at room tempera- ture. The stagnation pressure in the nozzle was adjusted so as to maximise the observed strength of the R l component of the fundamental vibrational transition of hydrogen fluoride, and was ca.5 atm. The intersection point between laser and molecular beams was surrounded by FIG. 2.-Experimental spectrum showing the Stark splitting of the R1 transition of hydrogen fluoride induced by an applied field of 46 kV cm-'. The upper trace provides frequency markers at 150 MHz intervals. polished stainless-steel Stark electrodes, 4 mm apart and 5 cm in diameter. The laser radiation was polarised along the direction of the molecular beam and perpendicular to the direction of the applied electric field. In such a configuration allowed transi- tions must satisfy the selection rule AM = & l . The temporal and spatial homo- geneity of electric field were such that the components of the Stark-split spectrum were not measurably broader than was the zero-field spectrum. Fig.2 shows the Stark splitting of the R1 transition of hydrogen fluoride produced80 SUB-DOPPLER RESOLUTION SPECTROSCOPY by an applied electric field of 46 kV cm-', together with the frequency derivative of the calibrating etalon's transmission function. The FCL-20 was scanned through the spectrum by simultaneously ramping, uia piezoelectric transducers, the lengths of the laser cavity and of the intracavity mode-selecting etalon. No attempt was made to stabilise the laser cavity thermally so that the frequency scan rate varied between and during runs. However, as will be shown below, the experimental quantity of most interest is the ratio of line splittings ( y -. z)/(x - y ) , where x, y and z are as defined in fig.2. Since this quantity is ca. 2 the applied electric field was adjusted until y - z 21 300 MHz (x - y E 150 MHz) corresponding to two (and one) free spectral ranges of the calibrating etalon. The bias on this etalon was then adjusted to bring spectral peaks and calibration markers into approximate coincidence. Because of pen offsets this coincidence is not immediately apparent in fig. 2. Spectral line positions were then measured relative to the closest calibration markers. Such an approach greatly improved the precision to which measurements could be made. Experimentally it was found for the R l component of the fundamental of hydrogen fluoride that 01 - z)/(x - y ) = 2.136 & 0.009 while for the R1 component of the v3 fundamental of hydrogen cyanide ( y - z)/(x - y ) = 2.305 0.006.The errors quoted are the standard deviations obtained from an analysis of twenty separate scans of each spectrum. These errors are generated by scanning errors and drifts in the applied Stark field. ANALYSIS OF D A T A The present experiments investigate the effects of an applied electric field upon the R1 component of parallel vibrations of linear molecules. To the precision of these experiments, their analysis must consider the second and fourth order interactions between the electric dipole moment of the molecule and the electric field, and the second-order interaction between the anistropy of the polarisability of the molecule and the electric field. The dominant interaction is the second-order dipole-applied- field interaction, the two remaining interactions providing corrections comparable to the standard deviation of the experiments.Thus, in an applied electric field, the electrical energies of the J , M levels are primarily determined by eqn (1)'O where p u and B, are, respectively, the dipole moment and rotational constant of the vibrational state u. Fig. 3 shows the situation for an R1 transition with the selection rules appropriate to the present experiments. The transitions x, y and z of fig. 2 are identified in fig. 3. Eqn (1) was used, neglecting the vibrational dependence of the dipole moment, to calculate, in terms of E2, the splitting x - y . The measured value of this splitting could then be used to calculate approximate values of the applied electric field to be used when making fourth-order corrections.Since these corrections are small, 10% accuracy in the electric field is more than sufficient. For hydrogen fluoride a field of 46 kV cm-' was used while for hydrogen cyanide the field was 7.8 kV cm-', these fields being such as to make x - y ca. 150 MHz for each molecule.T. E . GOUGH, R . E . MILLER AND G . SCOLES 81 The energy levels calculated from eqn (1) must be modified by the effects of the anistropic polarisability of the molecule according to eqn (2) 2J2 + 2J - 1 - 2M’ (2J + 3) (2J - 1) 2 &Jp,Jf = - where (al1 - axl) is the anisotropy in the polarisability of the molecule. It is not necessary to consider the vibrational dependence of this quantity because E ~ ~ , ~ is much less than ~$2.5,~.Because both E $ ~ : ~ , ~ and E : , ~ depend on E2 it is not necessary to know the applied electric field when computing the ratio (y - z)/(x - y ) . The only AM=? 1 ,- I I I I v = o I < J = l 0 -i1 - 2 2 -0 FIG. 3.-Energy levels and transitions corresponding to the experimental spectrum shown in fig. 2. unknown quantity appearing in such a calculated ratio is p1 which may thus be evalu- ated by equating the computed and experimental values for this ratio. The fourth-order contribution to the energies of the relevant states rJPM was calculated from eqn (3)” using the approximate value of E determined from the splittingx - y [(J + 1)2 - M2][(J + 212 - M’] (2J + 1) (2J + 5)(J + 1)2(2J + 3)3 [(J - 1)2 - M 2 ] [ J 2 - M2] [J’ - M2][(J + 1 - 144’1 4- (2J - 3)(2J + 1)J2(2J - 1)3 -I- (2J - 1) (2J 4- 3)(2J + 1)2J2(J 4- [J2 - M2I2 + ( 2 ~ + 3)2(2~ 3- i ) y ~ + 113 ( 2 ~ + 1 ) 2 ( 2 ~ - 1 ) ’ ~ ~ - [(J+ 1)2 - M2]’ Once again, the vibrational dependence of p and B may be neglected in making such corrections to the energy levels, In table 1 are listed the parameters taken from the literature used in the computa- tions, the uncorrected values of the excited-state dipole moments, the two corrections applied, and the final experimental values of ,ul for hydrogen fluoride and pool for hydrogen cyanide.The standard deviations on these dipole moments arise entirely from the experimental uncertainties in the ratio ( y - z)/(x - y ) . The precision of the present molecular-beam infrared measurements is comparable with that attainable from microwave spectroscopy of bulk gas samples.82 SUB-DOPPLER RESOLUTION SPECTROSCOPY We have performed experiments in which the FCL-20 was locked to the CF-5OOP etalon which was then ramped.This technique generated much more reproducible scans because of the high stability of the etalon ( < I MHz drift in 15 min). Un- fortunately a second etalon was not available to us so these scans could not be calibrated. Given such an etalon, we estimate that the precision of the present experiments could be improved by one order of magnitude. TABLE 1 .-EXPERIMENTAL DIPOLE MOMENTS OF VIBRATIONALLY EXCITED HYDROGEN FLUORIDE AND HYDROGEN CYANIDE AND THE MOLECULAR CONSTANTS USED IN THEIR EVALUATION hydrogen ff uoride hydrogen cyanide Bo B1 PO 0111 - 011 E Pl (uncorrected) correction for eqn (2) correction for eqn (3) P I 20.5602 cm-' l 9 19.7862 cm-' l 9 1.826 18 D2 0.220 i 0.006 A3 46 kV cm-' 1.8716 D +0.0006 D -0.0003 D 1.872 i 0.003 D Boo0 Boo 1 Po00 c( - E Po0 1 (uncorrected) correction for eqn (2) correction for eqn (3) Po0 1 1.4782 cm-l *O 1.4678 cm-' 2o 2.98459 D 7.8 kV cm-I 3.0199 D 1.0 A3 21 +0.0001 D -0.0078 D 3.012 & 0.002 D DISCUSSION (a) EXPERIMENTAL DIPOLE-MOMENT FUNCTION OF HYDROGEN FLUORIDE In the past decade, a considerable amount of effort has been expended in obtaining an accurate parameterization of the dipole-moment function of hydrogen fluoride.The most common expression of the dipole moment in terms of the internuclear separation has been that of a truncated Taylor series expansion about the equilibrium separation.Making use of such an expansion, the dipole moment in a vibrational state u can be written12*13 where pe is the equilibrium dipole moment and Ml and M2 are the first and second derivatives of ,u with respect to the deviation from the equilibrium separation. Eqn (4) is obtained by neglecting higher-order terms in BJco,. This ratio for hydrogen fluoride is 0.005 1. In fig. 4 are plotted the present value for p l and the molecular-beam electric resonance value for p0. Extrapolation to (u + +) = 0 gives a value of pe = 1.803 & 0.002 D, which should be compared with the literature value of pe = 1.7965 D.' This latter value lies well outside the range of the results of the present experiment, and was obtained by assuming that the difference in p o between hydrogen fluoride and deuterium fluoride is entirely due to their difference in zero-point energies, both species being assumed to share the same electronic potential well.Making this Born-Oppenheimer approxima- tion allows pe to be evaluated by an extrapolation through the two v = 0 levels to the Eqn (4) predicts that p" should plot linearly against (u + 3).T. E . GOUGH, R. E . MILLER AND G . SCOLES 83 minimum of the potential well. by using an effective (u + 4)eff which is found by solving This extrapolation may be cast in the form of fig. 4 The extrapolation is shown in fig. 4 as a dotted line. If the Born-Oppenheimer approximation holds, the dashed and full lines in fig. 4 should coincide. This is clearly not the case, as might be anticipated from the work of Kaiser3 on hydrogen and deuterium chlorides. Kaiser showed that the plot of pc, against (v + +)eff for deuterium chloride lay approximately D below the corresponding plot for hydro- gen chloride. From the solid line in fig.4 the anticipated dipole moment at FIG. 4.-Experimentally 1.79' I I I 0.5 1 .o 1.5 ( 0 + + > c r f . pfotted against (v + +)cff. measured dipole moments for (0) hydrogen and (0) deuterium fluorides (v + +)eff = 0.36, equivalent to the vibrational ground state of deuterium fluoride, is 1.820 D, D greater than the experimental value for deuterium fluoride of 1.8 18 8 1 D.l The implication is clear that the appropriate plot for deuterium fluoride should lie This is in accord with calculations by Schlier l4 which suggest that the largest perturbation resulting from the breakdown of the Born-Oppenheimer approximation is independent of vibrational quantum number.Sileo and Cool have made extensive measurements of infrared emission intensities for hydrogen fluoride and deuterium fluoride. However, because the infrared intensity measurements give no information on pe, these authors adopted the literature value, which we have now found to be inaccurate, in order to complete their character- isation of the dipole moment function. The quantity (po - pe) from infrared intensity measurements was found to be 0.023 D while the presumably more accurate molecular-beam electric resonance value for (po - pe) was 0.030 D. In table 2 we show predicted values for po and p1 of hydrogen fluoride obtained from the Sileo and Cool derivatives in conjunction with the literature and present values for pe.The excellent agreement between experiment and predictions based on the present value for pe strongly supports adoption of this new value, and shows that Sileo and Cool's intensity measurements are more reliable than previously appeared to be the case. below the plot for hydrogen fluoride.84 SUB-DOPPLER RESOLUTION SPECTROSCOPY TABLE 2.-EXPERIMENTAL DIPOLE MOMENTS OF HYDROGEN FLUORIDE, AND VALUES CALCULATED FROM INFRARED EMISSION INTENSITIES USING THE LITERATURE AND PRESENT VALUES OF ,!fe experiment calculated calculated P e P 1.796" 1.803 POID 1.826" 1.819 1.826 P I D 1.872 1.865 1.872 Ref. (1) and (2); present work. (b) COMPARISON WITH a priori CALCULATION The a priori calculation of molecular electric dipole moments has proved to be a difficult task.In recent years, however, considerable progress has been made so that, at least for small molecules, agreement between experiment and theory can be considered satisfactory. Because of the availability of Sileo and Cool's extensive infrared emission intensity data, hydrogen fluoride has been the subject of many theoretical calculations of dipole moment functions. Recently Werner and R o ~ m u s ' ~ have performed several ab initio calculations of the dipole moment functions of hydrogen fluoride, comparing their results to those of previous calculation^^^*^^ and of e~periment.~ Werner and Rosmus concluded that MCSCF and SCEP/CEPA calculations predict spectroscopic constants and dipole-moment functions with com- parable accuracy, each method giving deviations of a few percent.In such a context our revised value of pe is of little significance, although its inclusion may cause one to favour the MCSCF method. In an earlier paper Werner and Rosmusls reported SCEP/CEPA calculations using a slightly different basic set. These calculations agreed with the experimental dipole-moment function of hydrogen fluoride to within 0.3% for the first few vibra- tional states. Incorporating the present experimental correction to p e improves the agreement to 0.1 % which is somewhat better than our experimental uncertainties. This agreement, although impressive, is of course dependent upon the choice of basis set. This situation is not satisfactory for ab initio theoreticians but does suggest the possibility of optimising a basis set to obtain agreement with available experi- mental data, and then using this basis set to predict dipole-moment functions in less experimentally accessible regions. This work was carried out with the financial support of the Natural Sciences and The experiments were performed with the Engineering Research Council of Canada.assistance of D. A. Gravel. J. S. Muenter and W. Klemperer, J . Chem. Phys., 1970,52,6033. J. S. Muenter, J. Chem. Phys., 1972, 56, 5409. E. W. Kaiser, J . Chem. Phys., 1970, 53, 1686. R. N. Sileo and T. A. Cool, J . Chetn. Phys., 1976, 65, 1 17. T. E. Gough, R. E. Miller and G. Scoles, Appl. Phys. Lett., 1977, 30, 338. T. E. Gough, R. E. Miller and G. Scoles, J . Mol. Spectrosc., 1978, 72, 124. ' T. E. Gough, R. E. Miller and G. Scoles, J . Chem. Phys., 1978, 69, 1588. L. F. Mollenauer and D. H. Olson, J . Appl. Phys., 1975, 46, 3019. R. Beigang, G. Liffin and H. Welling, J . Opt. Soc. Am., 1978, 68, 636. lo C. H. Townes and A. L. Schawlow, Microwave Spectroscopy (McGraw Hill, New York, 1955). l1 J. H. Scharpen, J. S. Muenter and V. W. Laurie, J. Chem. Phys., 1967,46, 2431. l2 A. D. Buckingham, J. Chem. Phys., 1962, 36, 3096. l3 R. M. Herman and S. Short, J . Chem. Phys., 1968, 48, 1266. l4 C. Schlier, Fortschr. Phys., 1961, 9, 455.T. E . GOUGH, R. E . MILLER AND G . SCOLES l5 H. J. Werner and P. Rosmus, J. Chem. Phys., 1980,73, 2319. l6 W. Meyer and P. Rosmus, J. Chem. Phys., 1975, 63, 2356. l7 R. D. Amos, Mol, Phys., 1978, 35, 1765. H. J. Werner and P. Rosmus, J. Mol. Strucf., 1980, 60, 405. l9 D. V. Webb and K. N. Rao, J. Mol. Spectrosc., 1968, 28, 121. 2o D. H. Rank, D. P. Eastman, B. S. Rao and I. A. Wiggins, J. Opt. Suc. Am., 1961, 51, 929. 21 G . Tomasevich, Ph.D Thesis (Harvard University, 1972). 85

ISSN:0301-7249    

DOI:10.1039/DC9817100077

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Metastable States in Some Transient Molecules by High-resolution Laser Spectroscopy BY EIZI HIROTA Institute for Molecular Science, Okazaki 444, Japan Received 26th November, 1980 The A”’A” (050) t %’A’ (000) band of the chlorocarbene molecule was observed by laser excitation spectroscopy. For the upper (050) state only one series Jo,J of rotational levels ( J = 0-25) was identified. The series was found to be heavily perturbed at J = 8-10; two lines were observed for each of these three J values. The perturbed spectral lines were very susceptible to an external mag- netic field; the observed Zeeman coefficients were as large as 1.0 GHz T-’ for the J‘ = 9 lines. The origin of the perturbation is discussed, based upon the observed term values and Zeeman effects. Two models, i.e.electronic Coriolis interaction and singlet-triplet mixing, were examined in detail ; the spectra calculated assuming the latter mechanism were found to reproduce qualitatively the observed spectra. It has been realized that metastable electronic states having multiplicities different from that of the ground state often play unique roles in chemical reactions. Triplet states have also attracted much attention in the field of molecular science, because molecules excited to higher electronic states are often transferred to these triplet states by the so-called intersystem crossing. For simple molecules, however, a very limited amount of data has been accumulated on low-lying metastable states, especi- ally on the excitation energies of such states, because spin conservation gives us little chance to observe transitions between two states of different multiplicities.Herzberg and Herzberg’ have established that the ‘Ag state of the oxygen molecule is located at 7918.1 cm-’ above the 3Cg- ground state. However, it is only quite recently that the corresponding states of SO (‘A), and S , (‘Ag)3 were located with respect to the ground states. For another series of molecules NX (X = F, C1 and Br) the location of the ‘A state has been established only for NF.4 The detection of metastable states, particularly in transient molecules, is one of the most challenging problems for high-resolution laser spectroscopy; it obviously requires high sensitivity, and yet high resolution is necessary for an unambiguous identification of the detected state, or even of the molecular species.Although the carbenes play extremely important roles in chemical reactions, spectroscopic data on their metastable states have been very fragmentary. For the parent species, the methy- lene radical CH,, both singlet-singlet and triplet-triplet transitions have been re- ported, but the separation between the singlet and triplet manifolds has long been the subject of controversy; even two recent values for the &‘A, and T3B1 separation (2203 & 280 cm-l and 2833 are still subject to large uncertainties and do not agree well with each other. In contrast to CH2, the two halogenocarbenes, HCF and HCCI, are known to have singlet ground ~ t a t e s , ~ and Merer and Travis have analysed the Z’A” t z l A ’ transitions observed using a 35 ft (10.7 m) grating ~pectrograph.**~ However, nothing has been reported on the lowest triplet states.We have recently repeated the observation of the x+- 3 transitions of both HCF’O and HCC1,l’ by exciting them with a dye laser of high spectral purity. The resolution we achieved, 28088 HCCl LASER SPECTRA although still Doppler limited, is higher than that of Merer and Travis, and may un- ravel the details of perturbations reported in the previous paper^.^^^ Because low- lying triplet states may cause such perturbations, analyses of perturbed spectra may give us a clue to the nature and position of the unknown triplet states. In the present work we concentrated mainly on the HCCl A’A” (050) +- X’A’ (000) band, because this band showed one of the most conspicuous perturbations.EXPERIMENTAL The experimental set-up we used in observing laser excitation spectra was described in detail in a previous paper.12 A Varian 15 in. (0.38 m) magnet, which delivers a magnetic field up to 2.3 T, was employed in measuring Zeeman effects of the observed spectra. Optics of telescopic configuration focused the fluorescence light onto a photomultiplier placed out- side the electromagnet, Polarization of the laser light was chosen such that the AM = 0 and AM = f l selection rules were satisfied, respectively, for Q and P branch transitions. This choice is most convenient, because larger JMJJ components showing larger Zeeman effects appear most strongly. RESULTS Just as Merer and Travis found,g we also could identify only transitions that ter- In addition to the pP1, PQ1, and By using minated in one of the Jo,J levels in the 3 (050) state.PR1 branches, two axis-switching branches “Qo and O R 2 were observed. TABLE TE TERM VALUES AND MOLECULAR CONSTANTS OF THE J I A ” (050) Jo,J LEVELS OF HCCl (IN cm-’) J obs.” obs. -calc. J obs.’ obs. - calc. 1L 2L 3L 4L 5L 6L 7L 8L 8U 1.210 3.628 7.255 12.086 18.130 25.377 33.817 43.425 43.954 0.000 - 0.001 -0.002 - 0.006 - 0.004 -0.001 -0.000 - 0.004 - 0.01 5 9L 9u 10L 1ou 11u 12u 13U 14U 15U 54.188 54.706 65.980 66.681 79.928 94.41 3 110.1 13 127.023 145.130 0.022 0.025 -0.013 - 0,000 - 0.009 - 0.005 0.000 0.005 - 0.002 parameters in eqn (16)b parameters in eqn (6)b B 0.596 52 (53) B 0.596 52 (53) R 0.000 000 006 (11) IT 0.000 000 006 (11) SE 1.464 (68) SE 1.549 (63) E‘ 0.250 9 (59) U 0.027 12 (62) b -0.000 005 2 (35) D -0.000 005 2 (36) 6 8 -0.017 60 (69) SB -0.017 61 (70) a Uncertainties are 0.005 cm-’ or less.Values in parentheses denote standard errors and apply to the last digits of the constants. ground-state parameters determined from ground-state combination differences, the upper-state term values1’ reproduced in table I were calculated. As shown there we observed two J’ = 9 lines of nearly equal intensities, and also one satellite each for J’ = 8 and 10. These observations clearly indicate that, with increasing J , a series of per-E . HIROTA 89 turbing levels approaches the Jo,s series from the high-frequency side, crosses the latter near J = 9, and goes to lower frequencies.Then we examined the effects of the magnetic field on the observed spectra; we inspected only the pP, and "Q1 branches carefully, because they are the strongest among the observed branches and free from overlapping. The two J' = 9 lines were found to be split into two groups of unresolved Zeeman components for magnetic fields higher than 0.8 T, and we could follow the peaks of the two groups up to ca. 2 T. Fig. 1 illustrates the Zeeman patterns observed for the two J' = 9 Q and P lines. The T 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . o 1 . 2 1 . 4 1 . 6 1 . 8 Q P L T 0.0 0.2 0 . 4 0 . 6 0 . 8 1.0 1 . 2 1 . 4 Q P U FIG. I .-Observed Zeeman patterns for the "Ql(9) and pPI(lO) transitions of the HCC1A""A" (050)t Z'A' (000) band. The magnetic field is varied from 0 (top) to 1.8 or 2.0 T (bottom).The abscissa scale is 0.049 983 cm-'/division. The higher- and lower-frequency lines are designated as U and L. higher-frequency lines showed a marked asymmetry in the Zeeman pattern; the lower Zeeman components move to lower frequencies much faster than the upper Zeeman components move to higher frequencies. The asymmetry is much smaller for the lower-frequency J ' = 9 lines. The Zeeman splitting between the two groups of components is ca. 1.5 GHz at 1 T. For J ' = 8 and 10 the Zeeman effects could be examined only for the main lines; the satellites were too weak to record their Zeeman " broadened '' spectral patterns. The J ' = 8 lines showed some peculiar Zecman shifts at ca. 0.8 T ; the Zeeman components jumped to higher frequency by ca.0.02 cm-'. After 1 T they stayed at nearly the same frequency without showing any appreciable further broadening. The Zeeman effects of the J' = 10 lines were found to be even smaller; the Zeeman components moved to lower frequencies as the mag- netic field was increased. The Zeeman coefficients were ca. -0.2 GHz T-', an order of magnitude smaller than those of the J' = 9 lines.90 HCCl LASER SPECTRA The analysis was started by fitting the observed term values to the followingex- pression : F(J) = FJ(J + 1) + F2J2 ( J + 1)2 + F3J3(J + 1)3 +h* I!= [fo +flJ(J + 1) + f 2 J 2 ( J + 1>”13, (1) where the + and - signs apply, respectively, to the higher- and lower-frequency levels with the same J value (referred to as J U and JL). The energy origin was taken to be at the J = OL level, because the observed term values were given relative to this level.The observed term values of J up to 15 were thus analysed by a least-squares method, and were well-reproduced by eqn (1) except for J = 8U, 9L, 9U and lOL, as listed in table 1. A pair-wise energy-level interaction model, as exemplified by eqn (l), is obviously insufficient to account for the observed Zeeman effects, because both of the J’ = 9 levels showed Zeeman patterns that moved more to lower frequency than to higher frequency with increasing magnetic field. Eqn (1) comprises two models to be discussed later. DISCUSSION Perturbations in the A’A’’ excited electronic state can be ascribed to one or more of the following three mechanisms: (i) interactions with other vibrational states with- in the manifold of the same electronic state, the $A” state in the present case (here the interaction is of either Coriolis or Fermi type), (ii) interactions with highly excited vibrational states associated with the ground electronic state, ZlA’, and (iii) inter- actions with higher vibrational states of the lowest triplet state, G3A”.We may easily eliminate the first mechanism, because it cannot explain the observed Zeeman effects. As discussed by Merer and T r a v i ~ , ~ the HCCl molecule is isoelectronic with HNO, and thus we expect one lA’ and one ’A” as the low-lying singlet states which are derived from a ‘A state in the limit of a linear configuration. Therefore, when the two states come close, we might have a large magnetic moment due to the orbital motion of the electron.This is the mechanism (ii) mentioned above, which is also referred to as an electronic Coriolis interaction. The two states interact with each other by the per- turbing Hamiltonian : H’ = -2CR,J,L,, 9 which is derived by expanding the rotational Hamiltonian HR = zRg(Jg - L,)2. In eqn (2) R, denotes a rotational constant, Jg and L, the total and orbital angular mo- menta, respectively, and g the gth principal axis. The Zeeman Hamiltonian takes the form 9 where 2 denotes a space-fixed axis taken parallel to the applied external magnetic field H, g , the g factor for a magnetic moment associated with the orbital motion, .B, the Bohr magneton, and mZg a direction cosine. Because all vibronic states in the A manifold belong to A” symmetry and those in 8 to A‘, only L, and Lb have non-zero matrix elements between 2 and 8.Since the interaction takes place even for zero magnetic field, we need to retain only AJ = 0 matrix elements of QZg, in so far as weE. HIROTA 91 consider only one perturbing level for each perturbed level. proximation the interaction matrix element is given by In a symmetric-top ap- + (g,P/2)(~IL,lJ)[J(J + 1) - K(K ic l)l+M/H/[J(J + 1)1 ( 5 ) for AK = A l , where A, B and C are the rotational constants and (f( . . . 12) denotes a vibronic matrix element. Rotational functions with K, = even are A' and those with K, = odd are A". Therefore, we may list candidates for the perturbing levels as follows: perturbed level in 2 80,s 90,9 100,lO Comparison of these combinations with eqn (4) and (5) indicates that the most prob- able candidate for the perturbing level is J1,J-l.Again using a symmetric-top approxi- mation the term values including Zeeman energies are given by the following matrix form: F(J) = BJ(J + 1 ) + DJ2(J + 1)2 + HJ3(J + + 6E/2 I? (6) SE/2 + SBJ(J + 1)/2, U[J(J + l)]" + U,M,H/[J(J + 1)]* +[ symmetric, - 6E/2 - SBJ(J + 1)/2 where B denotes the average of the effective rotational constants of the two interacting states, i.e. B = (Bx + B A ) / 2 , SB = B, - BA, 6E = Exo - Eao, where Exo and EAo are energies of the two unperturbed series, f J1,j-1 and 2 Jo,j, in the limit of J = 0 and b and are higher-order terms representing contributions from molecular asymmetry and centrifugal distortion effects. The two constants in the off-diagonal position are given by and The term values in the limit of weak field thus become F(J) = BJ(J + 1) + DJ2(J + 1)2 + HJ3(J + 1)3 + 6E/2 j-{[SE + SBJ(J + 1)]'/4 + U2J(J + 1) + 2UU,M,H])*, (9) where the + and - signs apply, respectively, to 2 Jl,J-l and 2 J0.j.An analysis using eqn (1) mentioned earlier gave the parameters in eqn (9), which are listed in table 1 . The Zeeman coefficient in the weak-field limit, i.e., -J(UU,/A)M,H, is obtained by expanding the square root in eqn (9), where A2 = [SE + SBJ(J + 1)12/4 + UzJ(J + 1). (10)92 HCCl LASER SPECTRA The Zeeman effect observed for J' = 9, ca. 1.0 GHz T-I, thus requires that I(R(L,IZ)l - 0.10. On the other hand, from the observed U value, 0.027 12 (62) cm-', we obtain I(fIBL,JZ)I = 0.019 18 (43) cm-'.If we may take the B constant of about 0.6 cm-I out of the integral which seems rea- sonable for a strong (vertical) vibrational transition, the vibronic integral of L, be- comes I (f ILb IX) I - 0.032, (12) which is smaller than that [eqn (Il)] obtained from the Zeeman effect by a factor of three. Furthermore, eqn (6) requires that the Zeeman effects of the higher- and lower-frequency lines be of the same magnitude, but opposite in sign. This predic- tion does not agree with the observation that negative Zceman effects predominate for both J' = 9 lines. Eqn (6) also fails in reproducing the Zeeman effects observed for the J' = 8 and 10. It is obvious that a pair-level interaction model such as mechanism (ii) cannot ex- plain the observed Zeeman effects of the J' = 9 levels. Mechanism (iii), singlet- triplet mixing, does better.In this scheme the Zeeman effect mainly arises from the magnetic moment associated with the electron spin S = 1 in the triplet state. The Zeeman Hamiltonian is thus given by Hz = -gsPSzH = -gsPMsK (1 3) whereg, isclose to the free-electron g factor, 2.0023, and Sz and Ms denote, respectively, the 2 component of the spin angular momentum operator and its eigenvalue. The Zeeman energy in the limit of weak magnetic field is given by Ez = -gsP(Ms)H, (14) where ( M s ) = ([J(J + 1) - S(S + 1) - N(N + 1)1,"2J(J + 1)]>MJ- (15) A coupling scheme J = N + S is used, where N denotes the angular momentum of the molecular rotation. The average value of M, is thus equal to M,/Jfor N = J - 1, M,/[J(J + l)] for N = J , and -MJ/(J + 1) for N = J 4- 1, in the case of S = 1.For a near-degenerate case such as the J' = 9 levels, the Zeeman energy is divided equally between the two levels. Therefore, the Zeeman coefficient is 13-14 GHz T-l for N = J & 1 and 1.4 GHz T-l for N = J . The last case reproduces well the ob- served Zeeman coefficient of J ' = 9. In the following we will thus assume N = J for the perturbing triplet levels. Following Hougen's discussion'3 of the symmetry of the spin functions, we may choose candidates for the perturbing levels among rotational levels in the ii3A" state as follows: perturbed level in 2 8 0 . 8 90.9 ~ 0 0 , I O According to Stevens and Brand,14 first-order or direct spin-orbit interaction is allowed for HCCl, because of its low symmetry.We will refer to this interaction as (11. AsE . HIROTA 93 second-order interactions they have discussed the spin-orbit and orbital-rotation coupling [referred to as (2.1)] and spin-orbit and vibronic coupling [referred to as (2.2)]. The selection rules for (1) and (2.2) are AN = 0, & 1, AK = 0, & 1 , and those for (2.1) are AN = 0, & l , AK = 0, &2. An exact selection rule for the singlet- triplet interaction is AJ = 0 and, as the observed Zeeman effects indicate, N is equal to J for the perturbing triplet levels. Therefore, we arrive at a selection rule AN = 0. Because the perturbed levels in 2, have K (i.e. K,) nearly equal to zero, AK = 0 matrix elements, which are proportional to K, will be small. The AN = 0 and AK = & I interaction, which primarily couples 2 Jo,J with a" Jl,J, gives a matrix element that is nearly independent ofJ. The term values are then expressed in the following matrix form: We will designate it as €'.F ( J ) = BJ(J + 1) i- D J ~ J t i y + R J ~ ( J + 113 + [ ( m / 4 ) + EQI+ I, +[""2 + S m J + 1)/2, E' C.C., - 6€/2 - GBJ(J + 1)/2 where the constants are defined in a way similar to the case of eqn (6), except that the ground-state x is replaced by a". The parameters obtained by the least-squares analysis are included in table 1. Finally the AN = 0 and AK = A2 interaction will mainly couple A J0,j with a" J2,J-2 and gives a matrix element nearly proportional to [(J - I)(J -f 2)]+. Therefore, the term values will take a form that is very similar to the case of the electronic Coriolis interaction, eqn (6).To calculate the Zeeman effects we have to enlarge the 2 x 2 matrix of eqn (16) so as to take into account the Zeeman interactions between the J = N and J = N 1 levels of the triplet state. (ln the following we fix the N value at No.) The J = No 1, N = No levels (called F, and F3) are mixed by the spin-spin interaction with J = No &- I , N == No $r 2, so that the Zeeman effects must be modified as follows: ( J = No f l(H,(J = No t 1) = g,p[cos20,+,/(N + 1) - sin20,+,/(N + 2)]M,H, ( J = No - lIH,IJ = No - 1) = - g,P[cos20,_,/N - sin20,-,/(N - l)]MJH, ( J = No + 1 IHZ/J = No) = - g,/J[N/(N -1 I)] { [ ( N + 1)' - MJ2I/"(2N + 1)]>".os 0 N + 1 , {(N' - MJ2)/[(N + 1)(2N + I)]>+:cos ON-1, ( J = No/Hz/J = No - 1) = - gSP[(N + 1)/N] where No has been replaced by N in the right-hand sides for simplicity, and 0, is given by 0, = (1/2)tan-'( Y / X ) , (17) with X = B(2N + 1) - (3/2)~[1/(2N + l)], Y = 3a[N(N + 1)]*/(2N + l), and o! denoting the spin--spin interaction constant.more familiar notation, corresponds to ( 3 / 2 ) ~ . ] Fl - F2 separations were estimated from [Note that A, which may be a For simplicity, the F3 - Fz and and F3 - F2 = - [ N / ( 2 N - I)]. Fl - F2 = - [ ( N + 1)/(2N + 3 ) ] ~94 HCCl LASER SPECTRA [see eqn (23) of Raynes]15. Other spin-spin and spin-rotation interaction terms have been neglected. It was also assumed that fluorescence intensities are proportional to the amount of singlet character in the resulting levels. Because the MJ components were not resolved, each component was Doppler-broadened using a calculated f.w.h.m. of 0.029 cm'l and intensities of neighbouring lines were summed to give the cal- culated patterns in fig. 2. 1 0.0 0-2 0.4 0.6 Q P L Q P U F-G. 2.-Calculated Zeeman patterns for the "Q1 (9) and pP1 (10) transitions of the HCCl xlA" (050)t 2"A' (000) band, which are to be compared with the observed patterns of fig. 1. The observed Zeeman effects of J' = 9 require the I;; and F3 levels to be placed above the F2 level interacting with the singlet level. This means that the spin-spin coupling constant a must be negative, although the 2 constant of NCl, an isoelectronic molecule, is + 1 .776 cm-1.16 The molecular constants listed in table 1 lead us to expect that theX9,,, level is higher than the a" 91,9 or 92,7 level when the perturbation is absent ; the unperturbed separation between a" 91.9 or 92,7 and 29,,, is calculated to be -0.120 or -0.036 cm-l, although the standard errors, 0.130 or 0.126 crn-l, are larger than their respective calculated values.On the other hand, the observed fluorescence in- tensity is obviously stronger for J' = 9L than for J' = 9U, and the observed Zeeman patterns shown in fig. 1 are better reproduced with a positive unperturbed separation than with a negative separation. The spectra shown in fig. 2, which are to be com- pared with the observed spectra of fig. 1, were obtained using an a of -7 cm-I and an unperturbed separation of +0.165 cm-l. The Zeeman effect of the J' = 9L level is fairly well-reproduced, but the agreement is less satisfactory for the J' = 9U level.The remaining discrepancies, including those in the least-squares fitting for the J = 8U, 9L, 9U and 1OL levels mentioned earlier, may indicate the presence of otherE . HIROTA 95 perturbing levels. The present model also fails to explain the Zeeman effects observed for J' = 8 and 10, which thus still remain to be explained. The author thanks Dr. M. Kakimoto and Mr. Y. Endo for taking laser excitation He is also grateful to Jon T. Hougen for critical reading of the Calculations in the present work were carried out at the Computer spectra of HCCl. manuscript. Center of the Institute for Molecular Science. L. Herzberg and G. Herzberg, Astrophys. J., 1947, 105, 353. ' I. Barnes, K. H. Becker and E. H. Fink, Chem. Phys. Lett., 1979,67, 310. I. Barnes, K. H. Becker and E. H. Fink, Chem. Phys. Lett., 1979, 67, 314. K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules (Van Nostrand Reinhold, New York, 1979). J. Danon, S. V. Filseth, D. Feldmann, H. Zacharias, C . H. Dugan and K. H. Welge, Chem. Phys., 1978, 29, 345. R. K. Lengel and R. N. Zare, J. Am. Chem. Soc., 1978, 100, 7495. A. J. Merer and D. N. Travis, Can. J. Phys., 1966, 44, 1541. A. J. Merer and D. N. Travis, Can. J . Phys., 1966,44, 525. M. Kakimoto, S. Saito and E. Hirota, unpublished. ' M. E. Jacox and D. E. Milligan, J. Chem. Phys., 1969,50, 3252; 1967, 47, 1626. lo M. Kakimoto, S. Saito and E. Hirota, J. Mol. Spectrosc., in press. '' M. Kakimoto, S . Saito and E. Hirota, J. Mol. Spectrosc., 1980, 80, 334. l3 J. T. Hougen, Can. J . Phys., 1964, 42, 433. l4 C. G. Stevens and J. C. D. Brand, J. Chem. Phys., 1973, 58, 3324. l5 W. T. Raynes, J. Chem. Phys., 1964, 41, 3020. l6 R. Colin and W. E. Jones, Can. J . Phys., 1967, 45, 301.

ISSN:0301-7249    

DOI:10.1039/DC9817100087

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Laser Stark and Interferometric Studies of Thioformaldehyde and Meth yleneimine BY GEOFFREY DUXBURY * AND HIROSHI KATO Department of Natural Philosophy, University of Strathclyde, 107 Rottenrow, Glasgow G4 ONG AND MICHEL L. LE LERRE School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 ITS Received 22nd December, 1980 Laser Stark and optical-optical double resonance experiments have been carried out on HzCS and on D2CS, and rotational and vibrational constants obtained for the v3 bands. The dipole mo- ments in the v3 and v4 states of HzCS, and in the vj and ground states of D,CS have been measured. Interferometric spectra have been obtained and analysed for the v4, v5 and vg bands of CHZNH, and some laser Stark and interferometric spectra obtained in the perturbed 10 pm region.Very few high-resolution infrared spectra of short-lived molecules have been ob- served using conventional grating spectrometers owing to their inherently low sensi- tivity. Two molecules, H2CS and HBS have been observed in the 3 pm region using these methods, but their observation was time-consuming and difficult. Most recent experiments to observe the infrared spectra of transient molecules have used laser spectrometers, because of the high brightness and narrow linewidths of the sources. The laser spectrometers fall into two categories, those which use fixed-frequency, high- power C02 or CO laser^,^ and those which use tunable low-power lasers such as diode4 and colour-centre lasers. The fixed-frequency spectrometers make use of electric or magnetic fields to tune the molecular transitions into resonance with the lasers, and are known as laser Stark and laser magnetic resonance spectrometers, respectively.The high power of the CO, and CO lasers allows sub-Doppler resolution to be obtained, but only a limited frequency coverage. The diode and F-centre lasers possess good frequency coverage, but the low powers mean that, in general, the resolution is Doppler limited. In the present work we will demonstrate the application of laser Stark spectroscopy, optical-optical double resonance and high-resolution interferometry to the study of short-lived species. High-resolution Michelson interferometers of the Connes type have considerable advantages over grating instruments both in light throughput, the Jacquinot advantage, and in the multiplcx (Fellgett) advantage.Although they do not possess quite the sensitivity of diode laser spectrometers, they allow much greater spectral regions to be covered with comparable resolution, and with very great preci- sion. The use of lhser Stark spectroscopy and of optical-optical double resonance to * Visiting Scientist, Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy. Inc., under contract with the National Science Foundation.98 SPECTRA OF HZCS, D2CS AND CH2NH characterise short-lived molecules will be exemplified with the 10 ,urn spectra of H2CS and of D,CS. Methyleneimine, CH,NH, will also be used as an example of the com- bined use of laser Stark spectroscopy and of interferometry, in order to show the com- plementary nature of the methods.EXPERIMENTAL LASER STARK AND OPTICAL-OPTICAL DOUBLE RESONANCE The basic principles of laser Stark spectrometers are described by Freund et al.5 and of this particular spectrometer by Caldow ct a1.6 The use of an acousto-optic modulator to produce a single frequency-shifted sideband is described by Bedwell and D ~ x b u r y . ~ The main differences in the present work are that a more efficient Phillips acousto-optic modulator has been used, so that up to 60% of the laser power can be obtained in the sideband with a radiofrequency drive power of 10 W, and that the sideband is at higher frequency than the carrier. The Edinburgh Instruments semi-sealed PL3 laser used was run with l2CI6O2, 12C1'02 and 1 3 P 0 2 isotopic species.The small frequency-difference region can be probed if desired by using amplitude modulation of the sideband at 1-5 MHz, via a double balanced mixer before the r.f. power amplifier. A similar method has recently been used in the visible region by Dixon and Noble,' and Orr and Oka have successfully used an electro-optic modu- lator to perform similar experiments. The effective spacing of the Stark electrodes was deter- mined using laser Stark and OODR spectra of I2CH3F and 13CH3F, for which the magnitude of the Stark effect is accurately known.6*'0 The thioformaldehyde, H,CS, and the fully deuterated thioformaldehyde, D2CS, were produced by the pyrolysis of dimethyl disulphide and fully deuterated dimethyl disulphide, respectively.The niethyleneimine, CH2NH, was produced either by the pyrolysis of methyl- amine or of diaminopropane. The pressures used in the Stark cell ranged from 5 to 60 pm, depending upon the Stark voltage used. INTERFEROMETRY The interferometric spectra of CH2NH were obtained using the Solar Telescope Fourier transform spectrometer (FTS), of Kitt Peak National Observatory, Tucson, Arizona. This is a high-resolution rapid scanning machine developed by Dr. J. W. Brault," and which has a maximum unapodised resolution of 0.006 cm-' in the 10 pm region. Spectra were ob- tained between 11 and 2 pm using a KBr beam splitter and various combinations of filters. The ability of the machine to provide rapid scans at medium resolution was crucial in optimis- ing the production of the imine, so that the concentration of methylamine was minimised.Methylamine possesses a very complex spectrum in the region of many of the fundamental bands of CH2NH, so that its concentration must be kept low to prevent the imine spectrum being masked. These molecules possess strong spectra in the 11-2 pm region, but the absorption lines are widely spaced, and the high resolution and precision of the interferometer allowed their lines to be easily assigned. The optimum pressure for the production of CH2NH using an S-bend pyrolysis tube was ca. 500 pm. A total of sixteen traversals of a 60 cm multiple-pass absorption cell were used. The scan time of the interferometer was between 5 and 10 min, depending upon the number of sample points needed. The minimum number of scans used was 8 and the maximum number 16.The Fourier transformation was carried out at KPNO, and the data analysed using the Honeywell Multics system at BristoI University. The line positions were determined by fitting a parabola to the highest three sample points of each line. The accuracy of this procedure was tested on the data run with the InAs filter, with a sample spacing of 0.0027 cm-'. Ground-state combination differences generated from these data had an accuracy of ca. 2 x cm-'. The other impurities produced by the pyrolysis are HCN and NH3.G . DUXBURY, H . KATO AND M . L . LE LERRE 99 RESULTS LASER STARK A N D OPTICAL-OPTICAL DOUBLE RESONANCE SPECTRA OF H2CS A N D OF D2CS The high-resolution infrared spectrum of thioformaldehyde, H2CS, was first ob- served in the 3 pm region by Johns and Olson.' More recently the 10 pm region has been studied by Bedwell et aZ.12 and the lowest frequency fundamental bands, v,, v4 and v6 located.Bedwell and Duxbury also observed optical-optical double reson- ance, OODR, signals between ground-state levels of H2CS. We have extended the study of the v3 band using 12C'80, laser lines, so that improved molecular constants have been derived, and we have also observed several new OODR signals. In symmetric tops or near-symmetric tops, when the asymmetry splitting is negli- gible, the OODR signals form a characteristic doublet pattern of the type first des- cribed by Brewer.13 When a single-sideband modulator is used, the double-resonance condition for signals involving co-propagating beams, and the overall selection rule AMJ = &2 is given by where E is the electric field, SZRF the modulator drive frequency and ,u is the molecular dipole moment.6 Certain simple relationships between the pair of signals hold for the following cases, where E2 > El.(a) Parallel infrared transitions with AKa = 0: El E2 PI (i) QQ lines with no asymmetry splitting, - = (ii) QR or QP lines, 5 !z J (for low J ) . E2 J + 2 (b) Perpendicular infrared transitions with AKa = & l , and AJ = 0: El - Ka E2 Ka + 1 * -- In transitions where the asymmetry splitting is comparable with the r.f. frequency, signals showing resolved M structure can be seen. An example of this is the pQ( 11, 3) transition in the v g band of H2CS reported by Bedwell and D ~ x b u r y .~ Two pairs of OODR signals observed in H,CS are very interesting, since one pair allows the dipole moment difference between the ground state and v3 to be checked, and the second pair allows the dipole moment of the v4 state to be determined accur- ately. The signals involving v4 are shown in fig. 1, and illustrate the sub-Doppler resolution achievable in this type of experiment. By a serendipity both infrared tran- sitions involve the same ground-state rotational level, so that a test of the internal con- sistency of the data can be made. The infrared spectrum of D,CS is much less well-characterised than that of H2CS, and only approximate values of the ground-state vibration frequencies have been determined by Jacox and Milligan l4 using matrix-isolation spectroscopy, We have located and analysed the v3 band using lines from a 13C1602 laser.One of these lines, the 10R32 line at 936.1359 cm-', lies very close to the v3 band origin, and allows the characteristic QQ Stark patterns to be observed, as shown in fig. 2. The intensities of the K, even and odd transitions are reversed from those of H2CS given by Bedwell and The results are summarised in table 1.100 SPECTRA OF H2CS, D2CS AND CH2NH 500 1000 EjkV cm-’ FIG. l.-OODR spectrum of the RR(12, 7) transition of HzCS using the 9P(28) line of a 1zC1s02 laser. The path length was 3 m, the total gas pressure was 40 mTorr, the modulation field 1 V cm-l and the time constant of the detection system 100 ms. The radio frequencies used were (a) 52.06, (b) 60.03 and (c) 68.05 MHz.Duxbury,12 as expected from the nuclear spin statistics when H2 is replaced by D2. The band was analysed with the aid of the computer program DEFIN, which is suitable for C2, molecules, and which was used for H2CS.l2 The Hamiltonian is given in eqn (1)-(5) of ref. (1 5 ) , and uses the Watson centrifugal distortion constants, including the HK term. In the analysis of the v3 band of D2CS, 23 vibration-rotation transitions were used with 61 AM components. The data are suniinarised in table 2 and the derived vibra- tion-rotation frequencies in table 3. The ground-state rotation constants were fixed at the values given by Judge et a1.,16 and the excited-state centrifugal distortion con- stants were fixed at the ground-state values. The initial value of the ground-state TABLE 1 .-OBSERVED RESONANCE ELECTRIC FIELDS FOR OPTICAL-OPTICAL DOUBLE RESONANCE SIGNALS IN H,CS, WITH OVERALL SELECTION RULE \bhf.~I = 2 laser line transition r.f./MHz E‘/V cm-’ p’/D E”/V cm-l ,a”/D 1161“9R10 QR(12, 7 ) ~ 3 52.06 810.6 60.03 937.2 68.05 1060.3 1.654 699.3 1.658 806.2 1.657 915.6 1.648 1.648 1.645 average 11 81 “9P28 RR(12, 7)v4 52.53 732.1 60.41 841.6 68.07 948.1 1.656(3) 1.621 705.9 1.622 811.6 1.622 915.6 1.647( 3) 1.647 1.647 1.646 average 1.622( 3) 1.646(3) a 1161 l2Cl6O 2, (181 12c’80,.G .D U X B U R Y , H . KATO AND M . L . LE LERRE 101 TABLE 2.-oBSERVED STARK RESONANCES IN THE V 3 BAND OF D2CS USING A 13c02 LASER laser line laser frequency resonant field assignment 1cm-l /kV cm-l J ' K t K c t t Jg attK,tr M'-M" lOR(38) 939.9500 1.293 * 1.929 * 3.874 * 0.889 1.249 2.477 * 7.517 * 2,576 * 7.190 * 6.646 10.032 4.394 5.720 8.068 13.178 * 13.926 lOR(32) 936.3705 34.993 * 14.370 * 14.998 * 17.743 * 18.776 * 20.984 * 21.436 * 21.901 * 22.386 * 21.484 * 21.718 * 21.949 * 22.178 * 22.386 * 22.619 * 15.420 * 15.493 * 15.569 * 15.640 * 15.712 * 15.782 * 15.864 * 15.936 * b 0.700 * 24.056 * 24.74 1 SO9 * b 2.905 * 32.73 5.01 3 * 8.063 * 12.216 * - 110 +- 110 111 +- 111 220 f- 220 221 +- 220 33, +- 331 43 +- 43 55 t 55 5 4 +- 5 4 65 +- 65 64 +- 64 75 t 75 76 +- 76 8 5 +- 85 95 +- 95 lo5 t lo5 66 66 3 t 3 24-2 1t1 2 t 3 1-5-2 O t l 4 t 3 3 t 2 1t0 2t1 3473 2 t 2 2 t 3 1t2 O t l 4 t 3 1t0 1t0 1t0 O t l 1t0 O t l 2 - 1 1 - 0 0 - 1 1 - 2 3 t 2 2t1 1t0 04-1 1t2 2 t 3 4 t 3 3 t 2 2 + 1 1t0 04-1 1t2 2 t 3 3 + 4102 SPECTRA OF HZCS, DZCS AND CHzNH TABLE 2.-Continued laser line laser frequency resonant field assignment /cm- /kV cm-I J I K a t K c t -+ JKattKcN M‘-M’’ 1 OR(30) 935.1359 10.926 1 OR(24) 93 1.3093 26.88 16.783 * 34.312 * 29.634 * 38.757 * 29.848 * 39.562 * 20.42 24.445 30.741 43.614 * b 432f- 533 38.730 * 20.1 1 23.730 10R20 39.131 32.060 41.319 * 21.49 25.280 3 1.020 39.900 928.6568 19.520 24.140 * 32.058 * 27.717 * 38.551 * 37.807 12.775 14.880 17.848 * 22.641 * 30.031 * b 441 +- 542 c 734 + 835 C 735 836 744 f- 84s 16.341 20.035 25.774 35.719 * 26.164 * 743 +- 844 36.601 * O t O O t l 4 - 4 3 - 3 4 t 5 3 t - 4 4 - 4 3 - 3 4 - 3 3 - 2 2 - 1 1 - 0 4 - 4 3 - 3 4 - 3 3 - 2 2 - 1 1 - 0 4 - 4 3 - 3 4 - 3 3 - 2 2 - 1 1 - 0 7 - 7 6 - 6 5 - 5 4 - 4 3 - 3 7 t 8 6 6 7 04-1 l t 2 7 - 7 6 - 6 5 - 5 4 - 4 3 t 3 7 t 6 6 + 5 5 t 4 4 + 3 3 t 2 24-1 1 c o 7 t 8 6 t 7 O t l 1 t - 2 * Lamb dip: a components unresolved: overlapped; very weak.G .D U X B U R Y , H . KATO AND M . L . LE LERRE 103 TABLE 3.-THE V 3 BAND OF CDZS transition Vobs/Cm-' vcatclcm-' (v, - v,)/cm-' laser line 940.0049 a 940.0047 939.9586 939.9548 936.3673 936.3673 936.283 1 936.2833 936.281 1 936.28 12 936.21 69 936.21 72 936.1698 936.1700 936.1889 936.141 8 936.141 3 936.141 3 936.13 10 935.1864 931.5480 93 1.4788 93 1.431 1 93 1.2869 940.0042 940.0042 9 3 9.9 5 8 8 939.9549 936.3682 936.3682 936.2832 936.2832 936.2806 936.2806 936.21 70 936.2170 936.1699 936.1699 936.1890 936.1419 936.141 5 936.1415 936.1320 935.1851 931.5477 931.4783 931.431 1 93 1.2867 0.0007 0.0005 - 0.0002 -0.0001 -0.0009 - 0.0009 -0.0001 0.0001 0.0005 0.0006 - 0.0001 0.0002 -0.0001 0.0001 -0.0001 - 0.0001 - 0.0002 - 0.0002 -0.0010 0.0013 0.0003 0.0005 0.0000 0.0002 11 3 I lOR(3 8) I 1 3 I 1 OR( 30) 11311OR(24) 9 9 7 9 7 9 a Obtained by adding the calculated Stark shift to the laser-line frequency.dipole moment was taken from recent work of Cox and Hubbard,17 but was finally allowed to be determined in the least-squares fit. The standard deviation of the fit- ting was 0.0005 cm-'. The vibrational and rotational constants are given in table 4, where they are compared with those of H,CS. The dipole moments of H2CS and of DzCS are compared in table 5. The ground-state dipole moment is taken from the molecular-beam experiments of Fabricant et aZ.19 TABLE 4.-MOLECULAR CONSTANTS OF H2CS AND D2CS (Cm-') HzCS DZCS E" A B c 0 9.726 991 (216) 0.590 391 9 (33) 0.555 451 4 (33) 0.7622 (207) 0.1784 (64) 0.659 (13) 0.1109 (89) 0.406 (83) a v3 1059.2051 (11) 9.715 75 (22) 0.587 34 (36) 0.552 34 (37) 0.7558 (76) b b b b a 0 4.882 06 0.497 186 0.450 168 0.168 2 0.119 3 0.435 6 0.086 68 0.662 4 C v3 936.132 65 (3) 4.891 54 (3) 0.497 12 (9) 0.449 72 (9) b x 10-3 b x 10-4 b x 10-4 b x 10-7 x Ref.(12); fixed at ground-state values; ref. (16).104 SPECTRA OF HZCS, D2CS AND CH2NH I- --- I C (10,5) 1 Q (3,3) I I 1 t 20 25 30 E/kV cm-' 3 5 FIG. 2.-Observed AM = The laser line used is the 1131 lOR(32) line at 936.3705 cm-'. The time constant for detection was 100 m s, the It can be seen that the dipole moment difference, aP = pV3 - po, is much smaller in D2CS than in H,CS.This contrasts with the result for the analogous bands of H2C0 and D2C0,19 but is similar to the results obtained for CH3F and CD3F.20 A similar explanation to that given in the methyl fluorides is proposed, namely that since the v2 and v3 states are separated by ca. 400 cm-' in the hydride, but by ca. 200 cm-' in the deuteride, the increased mixing between Y , and v3 results in a fall in the value of 1 transitions for the Q-branch series of the v3 band of D,CS. modulation amplitude 5 V cm-', and the sample pressure ca. 10 mTorr. TABLE 5.-DIPOLE MOMENTS (D) FOR THIOFORMALDEHYDE AND DEUTERATED THIOFORMALDE- HYDE IN VARIOUS VIBRATIONAL STATES vibrational state H2CS ref.D2CS ref. ground state 1.6491 (4) 18 1.658 (3) this work 1.647 (3) this work v 3 = 1 1.6576 (12) 12 1.661 (3) this work 1.656 (3) this work v4 = 1 1.622 (3) this work v6 = 1 1.642 ( 5 ) 12 8p/8q3. This fall does not appear to occur where the C-O(S) stretching vibration is " isolated ", as it appears to be in H,CO.l9 INTERFEROMETRIC AND LASER STARK SPECTRA OF METHYLENEIMINE Although the infrared spectrum of methyleneimine, CH,NH, has been studied by matrix-isolation spectroscopy by Jacox and Milligan,,' the only high-resolution gas-G . DUXBURY, H . KATO AND M . L . LE LERRE 105 phase spectrum reported has been the laser Stark study of the v, band at 1638.3 cm-' by Allegrini et aLZ2 In the latter paper they also reported an attempt to observe the infrared spectrum using a grating spectrometer, with which they were just able to identify the QQ branches of the v, band.In the present study we have observed the high-resolution gas-phase spectrum of CH2NH using both interferometry and laser Stark spectroscopy. The interferometric spectra spanned the region from 11 to 2 pm, and the laser Stark spectra the 10 ,urn region. Unfortunately the spectra observed in the 10 ,urn region are very complex owing to a strong Coriolis interaction between the v7 and vg vibrations, and a weaker interaction with the v8 vibrational state. The Stark spectra are therefore much less easy to interpret than those of thioformaldehyde described in the previous section, and only fragments of these spectra are so far assigned. All the band assigned here are of A' symmetry, and so in principle possess both type A and type B electric dipole transitions.The v4 band has been obtained using the interferometer with a resolution of 0.006 cm-I. Whereas the transitions seen by Allegrini et a1.22 had J' and K,' values < 3, the interferometric spectra extend up to much higher values, and hence allow a better evaluation of the rotational and centrifugal distortion constants in the v4 state. The 11 l:* a 7 6 ; 5 ' 4 3 t I ' K = 0 3 4 5 6 7 1655.4 1656.2 1657.0 1657.8 wavenumber/cm- FIG. 3.-Interferometric spectrum of the QR(8, K ) transitions of the v4 band of methyleneimine, CH2NH. This spectrum was obtained using an InAs filter, with the fore-optics purged, with sixteen scans, and with a sample spacing of 0.0027 cm-'.The total gas pressure of the pyrolysis products in the 60 cm multiple pass absorption cell was ca. 500 mTorr, and sixteen traversals were used. Peaks marked with an asterisk are those identified by a peak-finding computer program. The pattern seen resembles that of a high-resolution microwave spectrum, with obvious K doubling for K, = 1 and 2 transit ions.106 SPECTRA OF H2CS, D2CS AND CH2NH QQ branches were easily identified, but as the changes in rotation constants are very small, the central Q feature is very compact and hence only partially resolved. The low J and K, QR and QP lines in the spectrum were easily identified using the para- meters of Allegrini et al., but the high J lines were difficult to assign. A stepwise approach was therefore used when the improved constants were used to extend the predictions, until finally 446 QR and QP transitions were assigned.The line positions and assignments of the v4 band are given in Supplementary Publication no. SUP 23024 (11 pp).* A portion of the spectrum is shown in fig. 3, where the asymmetry splitting of the transitions with low values of K, can be seen easily. No type B transi- tions with AK, = &1 were observed, so that the infrared transition moment of this band, which is primarily associated with the C-N stretching vibration, must lie al- most completely along the a molecular axis. In the final least-squares fitting of the data the ground-state parameters were frozen at the values determined from the fit to the microwave spectral data.23,24 The vibrational and rotational constants are given in table 5.The v5 and v g bands at ca. 1453 and 1347 cm-' have not previously been observed in the gas phase. Portions of the spectra in the QQ branch regions are shown in fig. 4 and 5. It can be seen that these branches are considerably degraded, and are similar K = l 11 1 3 2 1 1 2 3 101 I 5j q 33 ! t 2 1 I ~ = 2 8 7 6 5 4 4 , 5 6 7 8 K = 3 1347.2 1348 0 1348.8 FIG. 4.-QQ transitions of the v6 band of CH2NH. The experimental conditions are as those for fig. 3. The K, = 1 and 2 patterns are dominated by asymmetry splitting, and those with large values of J are considerably degraded. in the two bands. The asymmetry splitting of the sub-bands with K, = 1 and 2 can be easily seen in the v6 band. In both bands the transition involving high values of IC,' are perturbed, so that the overall fit is not as good as for the v4 band.The assignments were made with the aid of the ground-state combination differences, which have a very high accuracy as stated in the experimental section. No type B component could be found in the vs band, but one could be identified in the v6 band, where the type B lines were a factor of five to ten times weaker than the corresponding * See Notice to Authors, J. Chem. SOC., Fnrnday Trans., 1980, 76, index issue.G . D U X B U R Y , H . KATO A N D M. L . LE LERRE 107 TABLE 6.-MOLECULAR CONSTANTS OF METHYLENEIMINE (Cm- ') ' ground state V4 v5 v6 ).'O 1638.298 8 (56) A 6.544 896 6.549 45 (11) b6.549 56 (19) B 1.155 557 1.151 3847 (92) d1.151 3766 C 0.979 028 0.972 767 0 (76) A K 213.6 215.3 (41) A j K 19.4 17.54 (12) A j 2.0 1.860 (9) d K 0.3 0.247 (8) 65 16.6 14.1 (7) HK 0.556 0.101 (66) 1638.297 6 1452.053 (12) 1453 6.589 4 (18) 1.593 (18) 0.978 4 (18) 444.(120) -64. (30) d d d 3.6 (2.1) 1344.261 2 (23) 1347 6.778 48 (81) 1.158 335 (86) 0.972 788 (70) 53.7 (12) x 10- 400. (36) x 2.01 (16) x 0.28 (12) x 30. (14) x -1.1 (8) x ~~ Uncertainties in parentheses are three standard deviations in units of the last digit of the para- meter; ref. (22); ref. (21); fixed at the ground-state value. K- 3 4 5 I" ! HCN 2v2R(13) 1.452.2 1.4 53.0 1453.8 1454.6 wavenumber/cm- FIG. 5.-QQ transitions of the vs band of CH2NH, with the same experimental conditions as in fig. 3. Note the strong resemblance to the structure seen in the v6 band.108 SPECTRA OF HzCS, D2CS AND CHZNH ,M .J G 1 - - --___1__- 1 25 30 35 40 EjkV cm-I FIG.6.-AM = fl transitions of a QQ(3, 3) line of CHzNH observed using the 9P(22) line of a COz laser. The time constant was 100 ms, the gas pressure was 5 mTorr, the modulation field 5 V cm-' and both first (If) and second (2f) derivative spectra are shown. This pattern is very similar to those observed in CH3F ( 5 ) and in HzCO.I9 L -__i_ ~ V J 4.0 4 5 5 ,O E= 4.405 V cm-' 824MHzl- 1 FIG. 7.-Saturation spectrum of a QQ(5, 4) transition of CHzNH observed using the 9P18 line of a 12C1B02 laser. The frequency offset is cn. 381 MHz, and the radio frequency 60.09 MHz. (a) Spec- trum produced by carrier and sideband, the central group comprises the counter-propagating double- resonance signals.(h) Saturation spectrum (Lamb dips) with the sideband only. (c) Frequency scan of the laser through the Stark pattern at fixed electric field. The longtiudinal mode spacing of the laser is indicated. The electric field used was 4405 V cm-'.G . DUXBURY, H. KATO AND M. L . LE LERRE 109 type A transitions. The preliminary constants derived from the analyses are given in table 5. The region of the v, and vg bands contain complicated patterns of lines, but some transitions with recognisable structure have been obtained in the laser Stark spectra. In particular a QQ(3, 3) transition has been observed with both the 9P(22) line of the 12C'60, laser, as shown in fig. 6, and with the 9P(48) line of the 12C180, laser. It has also been identified on the interferometric spectrum.Similar transitions, OQ(6, 4) and QQ(5, 4), have been observed using the 9RlO line of the 12C1602 laser and the 9P18 line of the 12C1s0, laser. The frequency offset has been measured using the acousto-optically generated sideband, as shown in fig. 7. The outer and inner patterns of Lamb dips correspond to the transitions induced by the sideband and by the carrier, whsreas the central pattern is due to counter propagat- ing double-resonance signals of the type observed by Mattick et aZ.24 The sub-Doppler Lamb dip pattern can also be observed by scanning the frequency of the laser using a ramp voltage on the end piezoceramic as shown in the lower part of fig. 7. Rackley and Butcher 25 have recently made use of radiofrequency Stark modulation of the sample gas to produce sidebands with a known r.f.separation. However, in the present example only a single sideband is generated, and hence the three sets of signals described above are easily identified. It would be much more difficult to inter- pret a complicated Lamb dip pattern of this type had the multiple-sideband method been used. CONCLUSION In this paper we have compared some of the methods used for studying short-lived molecular species, in order to show the relative merits of fixed-frequency laser spectro- scopy and of interferometry. The laser based spectrometers are capable of much higher resolution and of higher sensitivity, but possess a very limited tuning range. The interferometers can give an overall picture of the spectrum with good sensitivity, precision of measurement and with reasonable intensity information.It can also be seen that high-resolution Michelson interferometers are very competitive with diode laser spectrometers for studies in this region, particularly since the resolution achieved is comparable, and the region covered in a single experiment is an order of magnitude greater . A complete study of any one system would probably utilise all three spectroscopic approaches. We are indebted to the S.R.C. for the provision of much of the equipment, and for the support of two of us (M. L. L. and H. K.). We also wish to thank Dr. J. W. Brault and Mr. R. Hubbard of K.P.N.O. for their help in obtaining the Fourier transform spectra, and Mr. A. J. Fox of Phillips Research Laboratories, Redhill, for assistance with the A.O.M.We are also grateful to the Royal Society and the U.S. Army for travel support. We would also like to thank Mr. M. Taylor for his help with the 12C's02 Stark spectrum of H,CS, and Dr. A. R. W. McKellar for his unpublished data on the v4 band of CH,NH. J. W. C . Johns and W. B. Olson, J. Mol. Spectrosc., 1971, 39, 479. R. L. Sams and A. G. Maki, J. Mol. Struct., 1975, 26, 107. J. W. C . Johns, A. R. W. McKellar and M. Riggin, J. Chem. Phys., 1978, 68, 3957. C . Yamada and E. Hirota, J. Mol. Spectrosc., 1979, 74, 203. S. M. Freund, G. Duxbury, M. Romheld, J. T. Tiedje and T. Okay J. Mol. Spectrosc., 1974,52, 38.110 SPECTRA OF HZCS, D2CS AND CH2NH G . L. Caldow, G. Duxbury and L. A. Evans, J . Mol. Spectrosc., 1978,69,239. R. N. Dixon and M. Noble, Chern. Phys., 1980, SO, 331. B. J. Orr and T. Oka, Appl. Phys., 1980, 21, 293. ’ D. J. Bedwell and G . Duxbury, Chem. Phys., 1979, 37, 445. lo R. L, Shoemaker, S. Stenholm and R. G . Brewer, Phys. Reu. A , 1974, 10, 2037. l 1 J. W. Brault, Proceedings of the Workshop on Future Solar Optical Observations-Needs and l2 D. J. Bedwell and G . Duxbury, J . Mol. Spectrosc., 1980, 84, 531 ; M. Taylor, B.Sc. Thesis(Uni- l3 R. G. Brewer, Phys. Rev. Lett., 1970, 25, 1639. l4 M. E, Jacox and D. E. Milligan, J. Mol. Spectrosc., 1975, 58, 142. l5 J. W. C. Johns and A. R. W. McKellar, J . Chem. Phys., 1977, 66, 1217. l6 R. H. Judge, D. C. Moule and G. W. King, J . Mol. Spectrosc., 1980, 81, 37. l7 A. P. Cox and S. Hubbard, personal communication. B. Fabricant, D. Krieger and J. S . Muenter, J. Chem. Phys., 1977, 67, 1576. l9 J. W. C. Johns and A. R. W. McKellar, J . Mof. Spectrosc., 1973, 48, 354. 2o G. Duxbury, S. M. Freund and J. W. C. Johns, J . Mol. Spectrosc., 1976, 62, 99. 21 M. E. Jacox and D. E. Milligan, J. Mof. Spectrosc., 1975, 56, 333. 22 M. Allegrini, J. W. C. Johns and A. R. W. McKellar, J . Chem. Phys., 1979, 70, 2829. 23 W. H . Kirchoff, D. R. Johnson and F. J. Lovas, J . Phys. Chem. Ref. Data, 1973, 2, 1. 24 A. T. Mattick, A. Sanchez, N. A. Kurnit and A. Javan, Appl. Phys. Lett., 1973, 23, 675. 25 S. A. Rackley and R. J. Butcher, Mol. Phys., 1980, 39, 1265. Constraints, Florence, Italy, November 7-10, 1978. versity of Bristol, 1980).

ISSN:0301-7249    

DOI:10.1039/DC9817100097

出版商:RSC    

年代:1981

数据来源: RSC