4 Infrared Laser Photochemistry By R. T. BAILEY and F. R. CRUICKSHANK Department of Pure and Applied Chemistry University of Strathclyde Glasgow GI 1x1,Scotland 1 Introduction The discovery of the powerful efficient (-20°/0) CO laser induced investigations into the possibility of initiating novel chemical reactions as long ago as the late 1960’s. Since then a great deal has been done particularly with reference to isotope separation where the process promises to compete economically with the traditional uranium separation techniques. Recently a book has appeared devoted entirely to megawatt i.r. laser photochemistry.’ A number of reviews have also on data up to 1977 and the energy transfer processes which constitute the photophysics behind the chemistry have also been reviewed.4d These i.r.photophysical data now form a coherent picture and current theories are making progress in describing the processes involved. However the situation is entirely different in the i.r. photochemical work. A large number of unconnected studies have been carried out which are often reported without relevant experimental details such as laser beam flux absorption coefficient pressure dependence or cell core temperatures. Occasionally a more coherent study emerges as for SF or OsO, but even then there are some anomalies as yet unexplained. Accordingly we have attempted in this Report to assemble recent data (up to September 1978) on a variety of compounds sufficient to indicate the typical results obtained from i.r. photochemistry and the deductions usually made from such results.Most i.r. photochemistry is performed with TEA lasers and these studies are listed under the relevant compound heading. In some cases e.g. hydrocarbons all the work on one compound type is listed under one heading. Continuous-wave laser (c.w.) or chopped C.W. laser work is discussed for all compounds under the C.W. laser heading. The interpretation of these studies is entirely different from that of the pulsed laser work although it is at least as complex. ’ ‘Megawatt Infrared Laser Chemistry’ E. Grunwald. D. F. Dever and P. M. Keehn J. Wiley New York 1978. ‘Chemical and Biochemical Applications of Lasers’ ed. C. B. Moore Academic Press London 1977 VOl. 111. S. Kirnel and S. Speiser Chern.Rev. 1977,77,437. R. T. Bailey and F. R. Cruickshank in ‘Molecular Spectroscopy’ ed. R. F. Barrow D. A. Long and D. J. Millen (Specialist Periodical Reports) The Chemical Society London 1974 Vol. 2 p. 262. R. T. Bailey and F. R. Cruickshank Applied Spectroscopy Reviews 1975 10 1. ‘R. T. Bailey and F. R. Cruickshank in ‘Gas Kinetics and Energy Transfer’ ed. P. G. Ashmore and R. J. Donovan (Specialist Periodical Reports) The Chemical Society London 1978 Vol. 3 p. 109. 49 50 R. T. Bailey and F. R. Cruickshank Space does not permit an exhaustive treatment of the topic but current theories are presented together with a discussion of the many areas where further extensive research is required. A section outlining the most important features of lasers relevant to laser chemistry is also included.Since several figures use Torr as the pressure unit and since this unit is used by all principal groups in the i.r. photochemical field we have used it also (1Torr = 1mm Hg pressure = 133.3 N m-2). 2 Experimental Techniques Laser Sources.-Unlike a photon in the u.v.-visible region an i.r. photon carries relatively little energy. For example a mole of photons at 1000 cm-’ carries only an energy of 12.01 kJ. Because of this relatively low photon energy the absorption of more than one photon is needed to produce photochemical reactions. As a consequence of the small photon energy most i.r. photochemical reactions take place in the ground electronic state. However relatively high power lasers are necessary to produce the photon flux required for i.r.photochemistry. Because of this high power requirement very few i.r. lasers are suitable for photochemical studies. Some of the most important are the CO, chemical lasers such as HF-DF and CO although others such as NH3lasers are undergoing extensive development’ and should soon reach sufficient power levels for i.r. photochemistry. Tunable lasers such as the parametric oscillator or spin flip Raman laser are useful for specialized applications where their relatively low power output can be utilized. It is convenient to divide i.r. lasers into two types C.W. and pulsed. In the case of C.W. lasers the power is generally limited to a few kW cm-’ since above this level atomization occurs. Pulsed lasers are available with a wide range of characteristics but typically the output consists of -1 J cm-’ with a pulse duration of 0.15-1 ps with a beam diameter of a few centimetres.This corresponds to power levels of 1-5 J cm-2 for the unfocused beam but when focused power levels of gigawatts (GW) per square centimetre can be easily achieved near the focal point. The characteristics of the main types of laser used for i.r. photochemistry are now briefly reviewed. The CO Laser. By far the greatest number of studies have been carried out using the CO laser. This is due to its suitability in terms of energy power availability and convenience of operation. Many different versions of this laser are available com- mercially both C.W. and pulsed. Continuous powers of over 10 kW and pulsed outputs of many megawatts are currently available.Normal C.W. operation occurs near 10.6 pm but with a grating in the cavity the laser can be tuned to about one hundred discrete vibration-rotation transitions between 886 and 1096 cm-’. The C.W. laser can also be pulsed and Q-switched to provide short high power pulses typically 10 kW in pulses 300ns wide for a 2m cavity. Shorter pulses can be obtained using mode-locking techniques.* A significant advance in i.r. laser development was the introduction of the transversely excited atmospheric pressure (TEA) laser. By operating the active medium at atmospheric pressure vacuum problems are eliminated and the output ’C. K. N. Patel T. Y. Chang J. T. Nguyen Appl. Phys. Letters 1976 28 603.’P. K. Cheo in ‘Lasers’ ed. A. K. Levine and A. J. De Maria Dekker New York 1971 Vol. 3 p. 111. Infrared Laser Photochemistry power per unit volume is increased. To achieve uniform excitation at atmospheric pressure it is necessary to preionize the gas to initiate a uniform glow discharge between the main electrodes. A typical pulse from a TEA CO laser varies from 70 to 300 ns in width and powers of several megawatts are readily available. When shorter pulse widths are required active mode locking techniques can be employed.’ Single pulses can then be selected from the train by use of a suitable switching element. Typically pulses of 1-2 ns duration with an energy in the mJ range are obtained when an intracavity loss modulator is used. Passive mode-locking using SF6 as saturable absorber can also be used to produce short pulses in the 1.5 to 10 MW range.” Bonds which absorb in the CO,laser region (9.2-10.9 pm) include C-C C-0 C-F P-0 W-0 S-F Si-F and S-H.For laser chemistry it is common to use long pulse TEA CO lasers operating at a repetition rate of S1Hz since laser fluence rather than peak power seems to be the important quantity in laser photochemistry. These lasers can deliver up to about 10 J in a pulse width of between 250 and 80011sin a beam 34cm in diameter. This corresponds to an energy fluence of -1 J ern-, and a power of about 2 MW ern-,. The energy level diagram for CO showing the lasing transitions is illustrated in Figure 1. OtherLasers. Chemical lasers” such as the HF-DF laser operating between 2.8and 4.0 pm can produce large output powers and fluences.However as yet they have been little used in laser chemistry. This is due partly to operating problems which involve the handling or disposal of corrosive gases and partly to the lack of flexibility compared with the COz laser. A large number of chemical lasers have now been studied giving rise to a large number of laser lines in the mid-i.r. region. The characteristics of these lasers are generally similar to the CO laser but with lower powers. With some commercial TEA CO lasers it is possible to change the optics and electrical discharge parameters and operate the laser with a different gas mixture generating a number of different laser lines. The CO laser (5.1-5.6 pm) is generally operated C.W.but at lower powers than the CO laser. It can only be used in specific applications where its relatively low power is useful. Tunable lasers.-Rapid advances have been made in the technology of i.r. tunable lasers in recent year^,'^.'^ but their output powers are still rather too low in most cases to be of practical use in laser chemistry. Several tunable laser systems offer some promise in chemical applications; the parametric oscillator and the spin-flip Raman laser are two such lasers. Parametric oscillators utilizing powerful Nd/YAG 1.06 p m lasers as pumps are now available commercially. These devices use angle tuning of a LiNb03 crystal to tune through the 1.4-4.45 pm region of the i.r. The optical parametric oscillator can be used to achieve relatively high tunable coherent power levels in the near i.r.13,14 Optical parametric oscillation has been observed in M. C. Richardson Optics Comm. 1974 10 301. lo R. Fortin F. Rheault J. Gilbert M. Blanchard and J. L. Lachambre Cunud. J. Phys. 1973 51 414. R. Ward and T. Y. Chang Appl. Phys. Letters 1972 20 77. ’’ A. Mooradian in ‘Tunable Lasers and Applications’ ed. A. Mooradian T. Jaeger and P. Stokseth Springer-Verlag Berlin 1976. ” M. Colles and C. R. Pidgeon Reports Progr. Phys. 1975 38 329. l4 R. L. Byer and R. L. Herbot in “on-Linear Infrared Generation’ ed Y. R. Shen Springer-Verlag Berlin 1977 p. 81. 52 R. T. Bailey and F. R. Cruickshank 3000 I I J I 131 27 23 2500 -I I V 2000 J 28 24 20 16 " 1I J 28*: 'Oo0FERMI\-I I 0200 RESONANCE Figure 1 Energy level diagram showing the lasing transitions of the 10.6 pm band of COz (Reproducedby permission from 'Lasers' ed.A. K. Levine A. J. De Maria Dekker New Yorker 1971 Vol. 3 p. 111) about ten different crystals with tunable output from 0.4to almost 17 pm. The long wavelength limit is currently 16.5 pm achieved by pumping CdSe with an HF 1a~er.l~ Typical line widths are of the order of 0.1 to 1cm-'. Optical parametric oscillators are likely to find increasing application in i.r. photochemistry. The i.r. region from 2.2 to 24 prn can now be covered by difference frequency generation in various non-linear materials. However the power outputs are presently too low to be of use in i.r.photochemistry. In spin-flip Raman lasers a fixed frequency pump laser beam (CO or CO) is inelastically scattered by the stimulated Raman effect from conduction electrons in a semiconductor crystal16at 4 K. The output is tuned by varying the large magnetic field to which the semiconductor is subjected. In the use of InSb for magnetic fields Is S. Rockwood in ref. 12 p. 140. R. J. Butcher R. B. Dennis and S. D. Smith Proc. Roy. Soc. 1975,344A,541 Infrared Laser Photochemistry 53 of 25 to 100 kG pulsed output powers of the order of 1kW in the first Stokes tens of watts in the first anti-Stokes and a few watts in the second Stokes lines are typical. The output covers the regions 5.0-6.5 pm and 9.0 to ca. 14 pm; the latter limit has recently been extended to 16.8 pm by pumping with an ammonia laser at 780 cm-'.17 The spin-flip Raman laser can also be operated C.W.using the CO laser as pump. Other semiconductors such as InAs and Hgl-,Cd,Te'8 can also be used. Spin-flip Raman lasers offer moderate power narrow line widths ( cm-' C.W. and 10-4-10-5 cm-' pulsed) and a reasonable tuning range. However the equipment is complex requires liquid helium cooling and does not have sufficient output power for most photochemical applications. In some applications where the high power per unit bandwidth can be utilized the device is valuable. Although the CO laser can be line-tuned over a large number of transitions near 10 pm for some applications greater tunability is desirable. This can be achieved by operating the laser at high pressures when pressure broadening effects result in continuous absorption over the whole vibration-rotation band.Continuous tunability is then possible over the whole band with high output powers. High pressure electrical excitation of CO and other simple molecules has been demonstrated using a variety of discharge techniques.* Electron beam sustained COzlasers at pressures up to 15 atm have been operated with efficiencies of about 2% and cavity energy densities of 60 J 1F'. This technique clearly offers the potential of tunable very high power single pulse energies and is well suited to photochemical applications. Diagnostic Techniques.-Diagnostic techniques can be divided into two groups; (i) static techniques depending on the analysis of the products of a system after irradiation for a given time at a particular power level and laser fluence and (ii) dynamic techniques which monitor the progress of a photochemical reaction both in the short time regime during and immediately after a laser pulse and in the longer time regime as a function of the number of laser shots.The dynamic techniques are much better probes of the physical and chemical processes occurring in the molecular system and are far easier to interpret. The static techniques are often complicated by the absorption of laser energy and subsequent further reactions of the primary photolysis products. Standard analytical techniques such as g.1.c.-m.s. and i.r. spectroscopy are commonly used to analyse the products in a static reaction system.In the dynamic case time resolved i.r. fluorescence from both reactants and products is a valuable technique for monitoring the progress of a reaction. Since the effect is usually very weak sensitive fast response i.r. detectors and high aperture optical systems (or filters) are necessary. The techniques of i.r. fluorescence are well established in energy transfer work.6 Typically a time resolution of 10-6-10-7 s can be obtained. The spectral resolution is generally limited by the available energy and normally a wide bandpass is used; however in C.W. work with the larger energies available a resolution down to 1cm-' can be achieved. A more sensitive technique where it is applicable is to monitor the concentration of a particular species reactant or product by absorption of an i.r.laser tuned to an i.r. band in the molecule. The time resolution is again limited by the overall rise time of the i.r. detector-amplifier assembly. Tunable lasers such as the spin-flip Raman laser are particularly useful for l7 C. K. N. Patel T. Y. Chang and V. T. Nguyen Appl. Phys. Letters 1976 28,603. 18 P. Norton and P. W. Kruse Optics Comm.. 1977 22 147. 54 R. T. Bailey andR R. Cruickshank this purpose since high powers are not required. Another very sensitive and versatile technique is the visible dye laser excited fluorescence of molecular species.” The U.V. visible fluorescence from the particular species is excited by a tunable dye laser either pulsed or C.W.and monitored as a function of time and/or frequency. Since photomultipliers can be employed in this spectral region much better time resolution and greater sensitivity can be achieved. The technique is however limited to species having absorption-fluorescence bands in the appropriate spectral region. In certain cases visible chemiluminescence is observed directly from excited product species at high laser fluences. A very versatile technique for exploring the mechanisms of laser activated reactions in the collisionless or near collisionless regime uses molecular beam-mass spectrometry techniques. 20*21 The experiments are conducted in a differentially pumped beam sampling mass spectrometer designed to allow in situ optical and mass spectral analysis of the photolysis products generated by each laser pulse.A low pressure (typically 5-100 mtorr) beam of molecules is passed through the focused beam of a TEA C02laser into a quadrupole mass spectrometer (Figure 2) where the gas fLow laser beam -D /i-1 to pump I-3 x lo5 Torr L-signal beam quad .9’ divergence I I Figure 2 Schematic diagram of the mass spectrometer beam sampling system (Reproduced by permission from J. Chem. Phys. 1978,68,777) photolysis products are analysed. With this technique the primary dissociation channels can be studied directly in the absence of complicating secondary reactions. In some photochemical reactions particularly those involving laser isotope separation of heavy atoms high power densities are required at the low gas pressures employed.At these power levels power broadening of the absorption lines frequently becomes greater than the isotopic shift and isotopically selective excit‘a- l9 S. E. Bialkowski and W. A. Guillory J. Chem. Phys. 1978,68 3339. Aa. A. Sudbo P. A. Schulz E. R. Grant Y. R. Shen and Y. T. Lee J Chem. Phys. 1978,68. 1306. 21 J. W. Hudgens J. Chem. Phys. 1978,68,777. Infrared Laser Photochemistry 55 tion is lost. To overcome this difficulty a double photon technique has been used." Many substances cannot be excited directly because they do not have i.r. absorp- tion bands that match the frequencies of suitable laser systems. Such molecules can be activated by the addition of an i.r. sensitizer which absorbs the laser radiation and transfers it by V-V R/T processes to the molecule of interest.Suitable i.r. sensi- tizers include SF6 SiF4 BCl etc. SiF is particularly convenient because it absorbs strongly at 1025 cm-' and is chemically inert.' SF6is also a very strong absorber and is efficient at very low pressures but at megawatt power levels it is not inert and reacts with molecules containing hydrogen. 3 Theoretical Considerations Diatomic Molecules.-The effect of vibrational excitation on the rate of reaction of diatomic molecules depends on the sign of AHo for the reaction. In thermoneutral reactions e.g. reactions (1)and (2) HCl+ He- H2+ C1. AH" = -4.6 kJ mol-' (1) HCI + Oq3P) + *OH+ CIS AH" = + 4.2kJ mol-' (2) energy transfer processes seem to play a major role and the acceleration of reaction consequent on raising the HCl to u = 1is not large.24 With endothermic processes the acceleration of rate can be dramatic e.g.in reaction (3) K+ HCl + KCI + Ha AH" = +4 kJ mol-' (3) there is only ca. -ten-fold increase in rate if HCl is in u = 1instead of u = 0 but in reaction (4) Br. + HCl -+ HBr + C1. AH" = 65.2 kJ mol-' (4) the rate is increased by a factor of -10" if the HCl is in u = 2 instead of u = 0. This reaction is exothermic for HCl at u = 2. [AE HCl(u = 1)-HCl(u = 0) = 34.5 kJ mol-' and AE HCl(u = 2)-HCl(u = 1)= 33.3 kJ mol-'I. These results are to be expected in that for endothermic processes Hammond's postulatez5 leads us to expect a 'late' barrier in the potential surface and thus that the vibrational energy of the reactant diatomic molecule would be efficiently converted into translational energy of separation of the products.However in the simple case of the diatomic/atom reaction the density of states and the volume of phase space available to the reactants is very much less than for polyatomic systems. (Even the above discussion is over simplified in taking no account of the variations possible in the type of potential energy function operating between the various atoms.) Exothermic reactions would be expected to be slower when the activation energy is stored largely in vibrational degrees of freedom. An example is the case given in reaction (5). *O(3P)+CN-+CO+.N('D) AH"= -78 kJ mol-' (5) 22 R. V. Ambartzumian N. P. Furzikov Yu.A. Gorokhov V.S. Letokhov G. N. Makarov and A. A. Puretzky Optics Letters 1977 1 22. 23 K. J. Olszyna E. Grunwald P. M. Keehn and S. P. Anderson Tetrahedron Letters 1977 19 1609. 24 J. Wolfrum Ber. Bunsen gesellschaftphys. Chem. 1977,81 114. 2s See e.g. 'Fundamentals ot Organic Keaction Mechanisms' J. M. Harris and C. C. Wamser J. Wiley New York 1976 p. 124. R. T. Bailey and F. R. Cruickshank Already the simple arguments begin to fail however for the above result is true only for the reaction path via the complex NCO. An alternative reaction path is significant viz. reaction (6) SO(~P) + CN + CO + sN(~S) AHo= -309 kJ mol-’ (6) where the reaction cross-section increases with CN vibrational excitation. This has been rationalized on the basis of ‘induced attraction’ on the grounds that in the limit when CN is dissociated the C attracts 0 and at large vibrational amplitudes some measure of this attraction should develop.This is really a rationale for a complex potential surface. For a four-centre process (two diatomics) the activation energy is equal to several quanta of i.r. laser radiation and in practice other processes than the elementary reaction step play a significant role e.g. energy transfer. The distribution of vibra- tional energy between the molecules affects the reaction rate. In the case of reaction (7) H,+D -* 2HD (7) stimulated Raman pumping was used to excite H2 and sequential (multiple) photon absorption was used to explain the results.26 With a dye laser tuned to the t = 0 +6 transition HCl was excited but no evidence was found for reaction (8).However reaction (9) HCl(U = 6)+ D2 + DCl +HD (8) HCl(u = 6)+ C4Hs + Me3CCl (9) occurs at room temperature. Polyatomic Molecules.-Larger polyatomic molecules present a far more complex picture but for the interpretation of CO laser i.r. photochemistry several broad features appear to be common to most systems. Experiments principally on SF and MeF have been performed by many groups. Theoretical models based on several approaches now tend to support the deductions of the experimentalists. 27*28 With the CO laser one quantum at -1000cm-’ is much less than the activation energy of most molecular reactions so that -34 quanta are needed for example to decompose SF,. In addition to the complexities arising from the increased volume of phase space available to molecules such as SF or MeF there will be the complexities of energy transfer processes-very fast for species excited to v -34.In order to slow down energy transfer processes it is usual to lower the pressure i.e. the gas collision frequency. The resultant low number of joules absorbed by the system is insufficient to cause reaction and this is compensated for by increasing the intensity of the laser used until multiple photon absorption ensues. The high intensity required demands that pulsed lasers be used. A ‘rule-of-thumb’ threshold of -1 MW cm-* exists below which reaction rarely occurs at the usual 1-1000 mTorr pressures. The reason why the fluence absorbed [Jcm- or ‘dose’ as distinct from flux (J cm- s-’) all per unit path length] increases has been investigated in great detail.26 S. H. Bauer D. M. Lederman E. L. Resler and E. R. Fisher Inrernat. J. Chem. Kinetics 1973,5 93. ” M.Quack J. Chem. Phys. 1978,63 1282. J. G.Black E. Yablonovitch N. Bloembergen and S. Mukamel Phys. Rev. Letters 1977,38 1131. Infrared Laser Photochemistry It is now clear that every system studied is different in the details of this mechanism but some common features will be discussed. A multiple photon absorption is required to get -34quanta into any one molecule (N.B. not a multiphoton absorption which is a term reserved for the instantaneous absorption of several photons). Since the molecular vibrations are anharmonic 34 different sizes of quanta should be needed for this process of ascending the energy level ladder.In practice substantial reaction occurs at one laser frequency! In most cases the mechanism is a combination of the following events. There is evidence that rotational levels can compensate for the quantum mismatch at least in the lower levels. A P-branch transition in v = 0-1 will possibly match a Q-or R-branch transition in v = 1-2 etc. if the rotational quantum is of the appropriate size. Ambartzumian has ascribed this mechanism to the Os04decom-position where the luminescent decomposition failed to occur when laser lines overlapping with the R-branch of Os04were used to pump the OsO,. This was despite the significant absorption of these lines.29 The accessible states of the molecule can be considered to be relatively few at low energy i.e.up to u = 3 or 4.Thereafter a rapid rise in the density of states results in a Figure 3 Schematic energy level diagram for SF6 i.r. laser photochemistry (Reproduced by permission from Phys. Rev. Letters 1977 38 1131) 29 R. V. Ambartzumian Y. A. Gorokhov G. N. Makarov A. A. Puretzkii and N. P. Furzikov Chem.Phys. Letters 1977 45 231. R. T. Bailey and F. R. Cruickshank condition described as the quasicontinuum. Ultimately this leads to the dissociation limit (Figure 3). This model readily explains the isotopic selectivity in requiring accurate quantum matching over the first three or four quanta of the energy ‘ladder’. For a given laser line (linewidth -0.03 cm-’) matching the o = 0-1 energy gap it is clear that the 32SF6 and 34SF6absorption lines will be differentiated (isotope shift 17 cm-’); however it is necessary to explain how the narrow laser line can also match the v = 1-2 efc.In addition to the Ambartzumian mechanism which can operate at low field intensities at high laser intensities a currently accepted rationale is power broadening. The concepts involved are not easy and should be discussed briefly. The molecular model for power broadening is that of an absorbing molecular vibration surrounded by a ‘lattice’ of other vibrational modes. As for the analysis of n.m.r. linewidths the absorbing mode relaxes to the molecular ‘lattice’ of energy levels with a vibration-to-vibration energy transfer relaxation time T, (cf.spin-lattice). An absorbing mode dephasing relaxation time T2,(cf. spin-spin) is also present. The absorption linewidth of the gas (normally -0.002 cm-’ Doppler limited) due to the high incident power is increased and is given by AvR = wR/2.rrc (cm-’> Here wn is the Rabi frequency given by wR= 2rIF121EO/h where Ip12)is the transition moment (typically 0.1-0.5 D 1D = 3.3 x lop3’ C m). Eois the electrical field in V cm-’ associated with the laser beam intensity and this is the source of power broadening. It is caused by the high voltage electrical field of the laser light itself. The cycle average theorem coupled with Poynting’s theorem gives I,” = $E~c~~E(?, t)I2 where 7is the refractive index and eo the permittivity of free space.Thus Eoin the previous equation is given by Eo= 27.5 I”2 V cm-’ where I is in W cm-2. At 5 MW cm-2 Eo= 6.1 X lo4V cm-’ and AYR = 0.5 cm-’. Increasing the flux to -1GW cmP2 to make the process more efficient is not always satisfactory as the power broadening AvR becomes 2-10 cm-’ for UF6 i.e. greater than the isotopic shift of -1 cm-’ for 235UF6 and 238UF6. To avoid this problem the two frequency technique was developed (see Os04,SF6). In the case of Os04 the 956.2 cm-’ laser line was used to raise the population of the o = 1level with isotopic selectivity and the 944cm-’ laser line was used at higher power to promote the species so excited to the top of the quasicon- tinuum and then to the dissociation limit. The power broadening effect described above is not to be confused with dynamic Stark broadening due to the a.c.field. In the case of SF, where no permanent dipole exists there can be no first-order Stark effect. The second-order Stark effect gives rise to a broadening of -0.1 cm-’ for the 10 V cm-’ field of -1 GW cm-2 laser power flux. However the value of AvR is 5.00 cm-’ under these condition^.^' 30 ‘Physics of Quantum Electronics’ Ed. S. F. Jacobs M. Sargent M. 0.Scully and C. T. Walker 4 ‘Laser Photochemistry’ ‘Addison Wesley’ 1976. Infrared Laser Photochemistry 59 The Rabi frequency is the optical nutation frequency and can be obtained as a beat frequency between emission from the macroscopic dipole in the system resulting from electrical alignment of the molecules and the slightly different frequency of the incident laser itself as in a Stark pulse e~periment.~' It would be wrong to assume from the above that isotopic selectivity is entirely due to the u = 0-1step since it has been shown3 that even in the quasicontinuum the absorption coefficient varies markedly with frequency over frequency spans as small as 10 cm-'.So far the discussion has centred on SF,. Another well studied molecule is MeF. Being a symmetrical top with a large dipole moment MeF has a first-order Stark effect with a shift of several cm-' and a AvR-30 cm-' at 10 V cm-'. Also MeF has a lowest vibrational frequency of 1049cm-' and a high rotational constant B of -0.7 cm-' yielding about three lines per wavenumber as against the several hundreds per wavenumber of SF6 in the CO laser frequency region.It has been found that even 50cm-' either side of the P(20) C02 laser line resonance at 1047 cm-' MeF and C1 can be induced to react whereas even 5 cm-' off resonance SF has been reported not to react.33 However because of obvious fluence depen- dence of the level 'pumping' and absorption coefficients the differences between SF and MeF are probably not as simple as seems from the above. The validity of the quasicontinuum model is governed by Fermi's Golden In the polyatomic molecule the density of accessible states rises rapidly with total molecular energy content The Golden Rule states that the full Schrodinger equation description of the system can be replaced by a rate equation description provided that the transition rate to the quasicontinuum is such that3' liiu(~)l-' << transition rate << T,' Thus as the total molecular energy content rises u (E)rises and the Rule will be validated.This point determines the onset of the quasicontinuum. The relevant rate equation is where W is the probability of being in that group of stationary states nh v above the initial level (ground state); KE KZ are absorption and stimulated emission coefficients; and Ktiss is the reaction rate constant. The rate equation can be multiplied through by dt and thus the time evolution of the dissociation rate is seen to depend only on I dt i.e. energy fluence. This at first sight unlikely result has been confirmed now by comparison of observed reaction rate with laser pulse width.For constant fluence the reaction rate seems to vary little for sF6for laser pulse widths from 30 ps to 200 ns.36s37Because of this fact it seems likely that the spontaneous mode-locking which is almost invariably present in CO TEA laser pulses is probably of no consequence. It is still perhaps rather too soon however to be sure 31 R. G. Brewer and R. L. Schoemaker Phys. Rev. Letters 1971 27,631. 32 A. V. Nowak and J. L. Lyman J. Quant. Spectroscopy Radiative Transfer 1975 15 1945. 33 A. M. Ronn 'Laser Focus' 1976 p. 53. 34 'Fundamentals of Quantum Electronics,' R. H. Pantell and H. E. Puthoff Wiley New York 1969. 35 E. R. Grant P. A. Schulz A. S. Sudbo and Y. T. Lee Phys. Rev. Letters 1978 40 115. 36 P. Kolodner C.Winterfield and E. Yablonowitch Optics Comm. 1977,20 119. 3' H. S. Kwok and E. Yablonovitch Phys. Rev. Letters 1978.41 745. R. T. Bailey and F. R. Cruickshank that this is true for all reactions. Even if a molecule has sufficient energy to react a finite time will elapse on average before it finds the correct region of phase space for reaction. The Kassel equation for reaction rate constant X,is x=wo n!(n-rn +N -l)! (n-m)! (n+N-l)! where wois the laser frequency or the high pressure A factor in the ergodic regime m the minimum number of photons required for reaction and N the number of normal modes (N.B. n is not the quantum number of the ‘pumped’ normal mode v3 but is the level in the quasicontinuum which is a composite of all modes). If m = 34 and excess energy = n -rn = 10 the reaction occurs in about lo-’ s.Excess energy of the reaction fragments has been shown to be consistent with this Figure 4 shows the reaction yield as a function of mean excitation (n) (photons Figure 4 Reaction yield Y,for SF6 decomposition as a function of photons absorbed per molecule ((n)or (n’))for two laser pulse widths. The yields are normalized that for the 500 ps pulse being 30% greater than the 100ns value under similar conditions (Reproduced by permission from Physics Today 1978 May 25) absorbed per molecule). Since (n) corresponds to the ‘centre of gravity’ of the population distribution over energy this is an experimental measure of the shape of the energy distribution function. The shorter pulse with large Rabi broadening and energy boosts the system directly into the quasicontinuum and so the energy distribution should then be close to that of thermally excited SF6,since the ‘bottle- necking’ in the discrete levels has been avoided.The bandshape of high quantum number transitions can now be examined by laser photoacoustic or thermal lens spectroscopy and gives information on the energy distribution in those states. TIcan be thought of as the time needed for the molecule to acquire ergodic behaviour. For mode-specific reaction the activation energy would have to be supplied and the reaction would have to occur in less than TI. 38 N. J. Coggiola P. A. Schulz Y. T. Lee and Y. R. Shen Phys. Rev. Letters 1977,38,17. 39 E. R. Grant M. J. Coggiola,Y.T. Lee P. A. Schulz and Y. R. Shen Chern. Phys. Letters 1977,52,595. Infrared Laser Photochemistry 61 There are indications that Tl is of the order of a few psa4’ In practice this will require high excess energies and number of normal modes (Kassel equation). In turn this will require large energies in the laser pulse as has already been predicted. Recently cell geometries and photon ‘usage’ have been in~estigated.~~ The laser beam was admitted to the cell via a small hole (2 mm diameter) at the focus of a lens and the subsequently divergent beam reflected off the cell walls. A relatively short cell (20 cm) with multiple passes during the laser pulse (200 reflections) was found best but still lo4incident photons were needed for each SF6molecule to react.Our experience indicates that simple lens systems behave rather badly at -1000cm-’ and since no optical element is infinite diffraction effects will abound especially for apertures in the millimetre diameter range.42 Such effects can lead to very high local energy densities and these would be expected in the cell designs above. These details are included here to illustrate that the apparently simple experiment can be difficult to interpret without a full knowledge of the optics inside the cell used. Beam profile scanning is essential in this context but extremely difficult to do with a 200 ns laser pulse. There seems to be some confusion in the literature on the effects of adding inert gases to a cylindrical cell containing an absorbing gas and irradiated with a coaxial laser beam.In many cases it is assumed that the increased heat capacity of the irradiated volume will result in lower temperatures of the absorbing medium. However it has been and recent work supports the quantitative accuracy of the description to the 1% level that the relaxation time T,of the thermalized hot gas in the geometry described above is given by d2C,P 7=-4RTK where C is the heat capacity per mole of the gas mixture d is the l/e radius of the TEMoo laser beam P is the gas pressure R is the gas constant T the temperature in degrees Kelvin and K is the thermal conductivity coefficient. Clearly TOCC,P i.e. increased gas pressure and/or C (per mole) of the mixture will slow down the rate of heat loss at the cell core.Increase of P tends to accelerate vibration-translation (V-T) energy transfer and the combined effect of an increase in P is frequently to increase greatly the temperature on axis in the cell and not to decrease it. These considerations are particularly important in high repetition rate M.W. pulsed laser and in the high pressure C.W. i.r. laser experiments. Typical gas cooling times to within -1% of ambient temperature are -50 ms at a few Torr pressure. The simple argument based on C only applies where this kinetic control of the temperature is inoperative i.e. low sample pressures. 4 Sulphur Hexafluoride Sulphur hexafluoride is probably the most studied molecule in i.r. laser photo- chemistry. This is the result of its similarity to UF and its strong absorption of CO 40 J.D. Rynbrandt and B. S. Rabinovitch J. Phys. Chem. 1971,75,2164. 41 J. L.Lyman S. D. Rockwood and S. M. Freund J. Chem. Phys. 1977,67,4545. 42 R. T.Bailey F. R. Cruickshank D. Pugh and W. Johnstone Chem. Phys. Letters 1978,59,324. 43 R. T.Bailey F. R. Cruickshank D. Pugh and W. Johnstone ‘Lasers in Chemistry’ ed. M. West Elsevier London 1977,p. 257. 44 R. T. Bailey F. R. Cruickshank D. Pugh and W. Johnstone unpublished data. R. T.Bailey and F. R. Cruickshank laser radiation. SF6 is in other ways an unfortunate choice having a very complex i.r. absorption spectrum extending from 936-953 cm-'. Additionally the thermal decomposition of SF6 is poorly characterized. Coupled with the difficulties of high field intensity spectroscopy the above problems are the reason why after five years study there is still considerable doubt about the details of the decomposition mechanism.In most studies now a TEA C02laser pulse on the P(20)line (944.2cm-') is used to 'pump' the SF,. Absorption occurs from the ground vibrational state to the v3(u=1,944 cm-') level. At 300 K the z16 (v =1,363 cm-') level contains -0.52 of the population and can also be 'pumped' to the v3+ v6 (u = 1 1328cm-') There is little doubt however that most molecules will be 'pumped' up the v3 vibrational ladder for a few discrete levels before attaining the quasicontinuum of the Bloembergen-Yablonovitch model." Table 1lists the calculated anharmonic level frequencies of the SF6 v3ladder.46 The exact level at which the quasicontinuum can be considered to begin is obviously of interest and work is still being carried out to verify the existence of this quasicontinuum.Table 1 Calculated anharmonic level frequencies of the SF6 v3 ladder v3quantumnumber Level type 1 quantumnumber Frequency /cm-' 0 1 A',Flu 0 1 0 947.97 0 1886.4 2 {gig 2 1889.0 3 F2n F;I{FL F2u 2 1 3 3 1896.9 2820.3 2837.5 2839.0 '42U 3 2846.9 3744.0 3744.4 3767.8 4 3773.4 A tg 3778.5 3780.5 3789.0 Above v3 (u =lo) the vibrational level density is such (10' per cm-') that there is complete overlapping of the vibrational lines within their natural linewidth of -lO-'cm-'. In the zero electric field condition it is reasonable to assume that Au = 1 AJ = 1 transitions are -8 cm-' wide in the P-branch and -5 cm-' wide in the R-branch by extrapolation from the absorption spectrum.Within this total band contour width of -13 cm-' it can be that there are several resonant paths in the v3ladder from u =0 to u =4-5 by Av =1 processes. At fixed fluence plots of SF6 dissociation rate as a function of laser line frequency typically maximize at -P(24) (-941 cm-'). This result is consistent with the u =4-5,u =5-6 absorp-tions and the above argument that the number of resonant paths is an important D. S. Frankel and T. J. Manuccia Chem. Phys. Letters 1978 54,451. 4h J. Dupre J. Dupre-Maquaire. P. Pinson and C. Meyer Infrared Physics 1978.18 185. Infrared Laser Photochemistry 63 feature. Rabi power broadening is invoked to bridge the gap between v =4-5 and u = 10.This study further quantifies the Bloembergen-Yablonovitch and certainly explains the success and observations of the twin laser experiment^.^^'^^ In these a low power laser (100kW cm-2 at 947 cm-') was used to promote the SF6 to the v3(v =4-5) level. A second CO laser pulse coincident in time was used at higher power (58 MW cm-*) and not at a resonant absorption frequency (1084cm-'). This would not broaden the lower levels but had sufficient field intensity to give the necessary Rabi broadening to promote the molecules excited by the first laser beam from the v =4-5 levels to the quasicontinuum. Given that the v3 mode is excited to high energies as above it still remains to explain how this energy can be rapidly channelled into the other modes even at low pressures (collisionless regime),49 and giving a variety of products (At 150 K and a molecular beam pressure of Torr SF5 SF3,SF2,S and F are all detected by mass spectrometer).A series of experiments were performed4' in which fluores- cence at -600cm-' was monitored when SF6 (0.002Torr) was excited by a long (-2ps) CO laser pulse on the P(20) line. The risetime of this fluorescence was faster than the 2 ps pulse width even when the time between collisions was as long as 60 ps! Under these conditions this fluorescence risetime was also faster than the rotational relaxation rate5' (p~ = 36 ns Torr) and 300 times faster than the collisional V-V energy transfer rate." With 0.028 Torr SF6 and 2 Torr He the fluorescence relaxes at the V-T rate but not to zero showing that significant bulk heating occurs after 40 ps.It is stated that the amplitude of this part of the signal (due to thermalized hot SF6)decreases as more He is added due to the increased heat capacity of the system consequent upon the rare gas addition. However this frequently used explanation may not be correct. From observation~~~ at even lower laser powers it is found that the effect of added rare gas buffers is to increase the V-T rate and decrease the thermal diffusivity. The result is to increase the temperature of the thermalized gas at the cell core. This effect can overcome that of increased C,. The conclusions of Frankel etal. may stem from the fact that as the thermal diffusivity is reduced it becomes far less easy to tell when the fluorescence intensity has returned to the 'baseline' (zero).At laser repetition rates in excess of -10 Hz this heating effect will integrate. At high repetition rates the absorbing gas temperature could be quite high. This would lead to a significant increase in population of the 363 cm-' level and a change in the 'pumping' mechanism over a period of minutes. This added complication is best avoided by low laser repetition rates. The importance of the fast risetime of the 600 cm-' fluorescence lies in that it is a clear demonstration of fast collisionless intramolecular energy transfer. It may also be that the fluorescing state is directly pumped. (The 2v4 state is within 60 cm-' of the v3+ v6 state which can be directly 'pumped' from the 363 cm-' level.Fluorescence at 600 cm-' would then arise from the v3+ v6/2v4+ v4 transition. At the 100kW cm-' laser intensity and low (50-100 mJ) energy used Rabi broadening and quasicon- tinuum processes should be unimportant in the Frankel experiment. Thus there 47 R. V. Arnbartzumian N. P. Furzikov Y. A. Gorokhov V. S. Gorokhov V.S. Letokhov G. N. Makarov and A. A. Puretskii Optics Comm. 1976 18 517. 48 M. C. Gower and T. K. Gustavson Optics Comm. 1977,23,69. 4y F. Brunner T. P. Cotter K. L. Kompa and D. Proch J. Chem. Phys. 1977,67 1547. P. F. Moulton D. M. Larsen J. N. Walpole and A. Mooradian Optics Letters 1977,1 51. 5' J. T. Knudtson and G. W. Flynn J. Chem. Phys.. 1973 58 1467. R. T. Bailey and F.R. Cruickshank seems to be a mechanism of fast vibrational thermalization in the ‘discrete’ level v = 0 +t = 5 region. Clearly this requires further investigation. The dependence of the probability P of dissociation of an irradiated molecule on incident laser flux I,(W cmP2) depends on whether measurements are made above or below the critical flux I,,at which this probability saturates. Below I,,P varies as 13,suggesting that v = 3 is the average excitation of molecules stuck in3the discrete level region,52 this agrees with Ambartzumian’s value.53 Above I,,PK I2as expected from saturation and the conical geometry. When a 50 ns strongly self mode-locked pulse was rather different results were obtained. It should be noted that in this study a ‘uniform portion’ of the laser beam was ‘selected by a 9.5 mm diameter aperture’ and the laser beam was not focused.This aperture will give rise to Fresnel diffra~tion.~~ Without knowledge of the distance from cell to aperture it is not possible to assess the significance of this diffraction but it could lead to very high local intensities in a complex pattern in the cell. Low pressures (10-3Torr) of hydrocarbon were added to give a luminescent reaction. The intensity of this luminescence was interpreted as a measure of rate of reaction of F atoms with hydrocarbon and thus of pressure of SF6 dissociated It was found that for the P(20) C02 laser line the luminescence intensity varied as (n)43*’3((n) = average number of photons absorbed per molecule).The variation of (n)with fluence data agrees with the values of the Harvard group2* for their 100 ns pulse. Given the complexities of the chemistry involved and the length of the laser pulse which will cause ‘bottlenecking’ the very high value of the exponent of (n) cannot be regarded as a reliable measure of the energy distribution. That there is a 1ps delay after the laser pulse before luminescence begins to be observed and that it peaks 200 ps after this suggests that the luminescence rises with a time constant very like a translational heating process. We should not therefore like to use these data to examine the nature of the energy distribution and we believe the results of the Harvard group are more reliable i.e. that a Planck type distribution prevails.The behaviour in the quasicontinuum has been outlined in Section 3. Here transitions are those of incoherently ‘pumped’ oscillators and a Planck distribution is to be expected with a temperature which can be deduced from the energy absorbed from the laser beam. Carefully calibrated optoacoustic measurements of the vibra- tional temperature in short pulse (500 ps) experiments2’ give results in agreement with this model. It should be realised however that significant Fermi-resonance type mixing of wave functions must occur in this region in order that i.r. inactive states can be ‘pumped’ (i.e.all levels except v3and v4). Vibrational thermalization is achieved in the quasicontinuum in 30 ps or less.37 The absorption cross-section CT of the SF molecule can be expressed in terms of the average number of photons absorbed per molecule with short laser pulses (n’) viz CT =d((n’)hv)/dF ’* D.Tal U. P. Oppenheim G. Koren and M. Okon Chem. Phys. Letters 1977,48,67. ” R. V. Ambartzumian Y. A. Gorokhov V. S. Letokhov G. N. Makarov and A. A. Puretskii J.E.T.P. Letters 1976 23 22. 54 S. Speiser and J. Jortner Chem. Phys. Letters 1976 44 399. ” M. Rothschild W. S. Tsay and D. 0.Ham Optics Comm. 1978,24 327. Infrared Laser Photochemistry 65 where F is the fluence (J cm-’). As the molecule is energized in the quasicontinuum this absorption cross section falls from lo-’* cm’ to well below cm2 at F = 10 J cm-’. This drop is due to the broadening and shift in the absorption as the temperature of the molecule rises.32 Clearly if F is raised to for example 100 J cm- (1GW cm-2 for a typical CO laser pulse) the background i.r.absorption cross section (-lo-’’ cm’) will begin to absorb significant amounts of energy. Thus at this power and energy level any molecule can be dissociated by C02laser radiation even if it has ‘no’ i.r. absorption frequency in the C02 laser region. It must be noted that for CO laser pulses longer than 500ps at the threshold fluence of 1.4 J cm-’ considerable ‘bottlenecking’ will occur in the lower levels because insufficient Rabi broadening will have increased the significance of anhar- monicity. If under these conditions (n)(average number of photons absorbed per molecule with ‘bottlenecking’) is used to calculate vibrational temperature the calculated a will disagree with the thermal value contrary to the 500 ps pulse data.Additionally departures from RRKM behaviour will be shown if (n)is used instead of (n’). The difference between (n) and (n’) is attributed to the fraction f of molecules which cannot reach the quasicontinuum from their initial vibration- rotation state under long pulse conditions f-((n‘) -(n))/((n)- 3) where it is assumed that 3 hv is the average energy of those molecules which are always stuck in the discrete levels (see above). The value of f is about $ at 1.4 J cm- and tends to zero at > 10J cm-*. In the quasicontinuum vibrational temperatures ((n)hv/Nk)of -1700 K can be reached. A careful study has been made of 32SF6 decomposition where the 32SF6 pressure was monitored by absorption of a weak C.W.C02 P(20),laser beam as a continuous function of time.52 Since the stable photofragments SF4 S2F2 S2FI0 S02F2 and SOF show no i.r. absorption in the 944 cm-’ region only the 32SF6 was monitored The data are summarized in Figure 5. Obviously there is a process in the case of pure SF6 which very slowly restores some of the 32SF6 to the monitoring beam path. The fraction of 32SF6 which reacts varies with the number of laser pulses (Figure 6) in a different way depending on whether H2 scavenger is present or not. The curves in Figure 6 intersect at high numbers of laser pulses so that enrichment of 34SF6 with respect to 32SF6 becomes very difficult to interpret unless monitored as a function of time.This study underscores the need for detailed careful work on all aspects of the physics as well as the chemistry of multiple photon laser decompositions. The quantum yield (molecules dissociated per photons absorbed) of 32SF6 dis- sociation for a single pulse is 6 X lop5for pure SF6. Addition of a scavenger reduces the quantum yield. It has been known for some year^^^.^' that intense CO laser radiation could selectively remove for example ?3F6 by reaction to leave the 34SF6species having undergone reaction to a lesser extent or not at all if the pressure was sufficiently low (Figure 7).49,58 The recent revelation that the reaction with or without a scavenger ’‘ R. V. Ambartzumian Y. A. Gorokhov V. S. Letokhov and G. N. Makarov J.E.T.P.Letters 1965,21 171.57 J. L. Lyman R. J. Jensen J. P. Ruik C. P. Robinson and S. D. Rockwood,Appl. Phys. Letters 1975,27 87. 58 H. N. Rutt Culham Laboratory ‘Preprint CLMlP470’ 1977. R. T. Bailey and F. R. Cruickshank loolo Ii 0.10 b 0.20 4 0.30 ' 40.40 5 10 TIME /minutes (a) 0.05 +. v) 0.10 k LL 0 W 3 a -0.15 % W0z W a s !z3i zU I- 40 n Q-0.30 ' -0.20 ~ t-Lz - 40.40 0 5 10 TIME / minutes (b) Figure 5 The variation of SF transmission of a low power C02 laser probe beam as a function of initial SF6 pressure and time (a)with no scavenger present (b)with -1 Torr H2 as added scavenger. Laser pulse (5 J) repetition rate 25 per min (Reproduced by permission from Chem. Phys. Letters 1977 48 67) Infrared Laser Photochemistry 1 o 0.7 z \ E z -0.5 0 + 2 0.4 z 0 F 0.3 a a t- Z W u g 0.2 0.1 torr SF \ 0 e.g.HZ, is time variant in a complex way has necessitated a complete reappraisal of the studies of isotope enrichment factor (34SF6/32 sF6 after reaction 34SF6/32SF6 before reaction) as a function of number of laser pulses. The data of Figure 6 could explain why Dupre et al.59 report a higher enrichment factor with scavenger after 1000 pulses than observed without a scavenger. At a small number of pulses the dissociation rate of 32SF6without a scavenger is higher than with a scavenger so that under these conditions the Cotter and Fuss6' data which appeared to show that the scavenger decreased the 32SF6dissociation rate are consistent with the apparently contradictory Dupre results.A scavenger can also appear to cause higher 32SF dissociation if analysis is carried out several minutes after irradiation because reverse processes then become very significant. In an isotope separation plant it would be useful if the SF6pressure could be raised to improve throughput. Predictably it is observed61 that the excited 32SF6,absorbing 59 J. Dupre P. Pinson J. Dupre-Maquaire and C. Meyer Comptes Rend. 1976,282 B 357. "' T. P. Cotter and W. Fuss Optics Comm. 1976,18 31. 61 S. T. Lin S. M. Lee and A. M. Ronn Chem. Phys. Letters 1978. 53 260. R. T. Bailey and F. R. Cruickshank WAVENUMBEP. (cm-! ) 1000 900 1000 900 -n n n n n l -1 -1 500 1000 1500 2000 2500 3000 3500 LASER PULSES Figure 7 The i.r.spectra of 1Torr SF6+ 1Torr H2irradiated for a series of numbers of (2 J x 1 ps) laser pulses. The 947.9 cm-' absorption is due to 32SF6 and that at 930.5 cm-' to 34SF6 (Reproduced by permission from Chem. Phys. Letters 1978 53 260) the P(2O)CO2laser line gives up its energy to 34SF6more readily as pressure is increased so that the 32SF6percentage decomposition is reduced while the 34SF6 decomposition rises. Long time analysis was used in this study so that all reverse processes can be assumed to have occurred. However the CO laser pulse used was unusually long at 1ps (2 J fluence 50 J ern-,). In agreement with the results of Tal et a1 the decreased observed dissociation of 32SF6and 34SF6with H2 added increases towards the collisionless values at large numbers of laser pulses.It was also shown that addition of for example 20 Torr Ar does not prevent isotopic enrichment. It should be noted however that sample heating under these conditions will be severe,44 altering the rotational band contour and thus absorption coefficient. This effect will confuse the V-T/R energy transfer argument used by Ronn et al. to interpret the results. As a further example of the experimental artefacts that can arise it is appropriate to mention here the paper by Turner et a1.62 in which CO laser irradiated SF in Ar or CO matrices at 12 K was apparently enriched isotopically in 34SF6. This reproduced the data of Ambartzumian et al.63 and was shown to be an artefact due to ablation of the matrix.'* B. Davies. M. Poliakoff K. P. Smith and J. J. Turner Chem. Phys. Letters 1978 58,28. 63 R.V. Ambartzumian Y. A. Gorokhov G. N. Makorov A. A. Puretskii and N. P. Furzikov in 'Laser Spectroscopy' 3rd Edn. ed. J. L. Hall and J. L. Carlsten Springer Verlag Berlin 1977. Infrared Laser Photochemistry 69 5 Hydrogen Halides The interaction of vibrationally excited molecules with reactive atoms will frequently involve competition between reaction and relaxation pathways. In some cases the reactive channel may account for a large fraction of the total vibrational depletion so that vibrational excitation may be employed to overcome the endothermicity in a reaction between a molecule and an atom. Thus vibrationally enhanced selective photochemistry becomes a possibility with isotopic selection in appropriate cases.Hydrogen Chloride.-The competition between reaction and relaxation in vibra-tionally excited HC1 was first studied by Arnoldi and W~lfrum.~~,~~ These authors investigated the reactions of HCl(u) with H D C1 and Br atoms using the laser excited fluorescence technique combined with a discharge flow reactor. By deter- minations of the absolute concentration of the vibrationally excited molecules time resolved observation of the atom concentration and mass spectrometric analysis of the products from laser induced reactions the reactive and inelastic pathways could be distinguished. A HCl laser was used to provide vibrational excitation. In the case of Br atom reaction there was a dramatic increase in the rate constant (k3)when HCl was excited to the u = 2 level (i.e.ca.lO"-fold). The main processes are given in reactions (lo) (1l),and (12). HCl(u = 1)+ Br(2P:) -+ HCI(u = 0)+ Br(2P:) (10) kl = (2.5 *0.6) x 10" cm3mol-' s-l + HCI(u =O)+Br*(*P:) (11) k2 = (1.7* 0.4) x 10l1cm3mol-' s-l HCI(u =2)+Br(2P:) + HBr(v =O)+Cl k3=(9*6)x1O1'cm'mol-*s-' (12) Reaction (12) was used to provide an enrichment of 35Cl of up to 90% using the sequence reaction (13) reaction (14). H35Cl(u= 2)+ Br + HBr(u = 0)+ 'kl (13) 35 CI+Br -+ Br35C1+Br (14) A similar study on the HCl (IJ= 1)system was carried out by Leone et a1.66 Again a HCl chemical laser was used to produce HCl(u = l) but a tunable parametric oscillator was used to excite HCl directly from u = 0 to IJ = 2.With direct 0-2 excitation the kinetics of the deactivation of HCl(tl= 2) by Br atoms are simple and the rate constant reliable. Also the high excitation efficiency of the parametric oscillator allowed a working pressure as low as 0.04 Torr to be employed making the V-V rates comparable to those of atom deactivation. The fluorescence from the HCl(v = 2 -+ 1)was used to monitor the HCl (u = 2) population. The rate constants for the deactivation of HCl(v = 1)and HCl(u = 2) by Br atoms at 294 K were found to be (2.8f0.52)x cm' molecule-' s-' and (1.8f0.33) x cm' molecule-' s-' respectively. The rate constant for deactivation of HCl(t = 1)by Br 64 D. Arnoldi K. Kaufman and J. Wolfrum Phys.Rev. Letters 1975 34 1597. 65 D. Arnoldi and J. Wolfrum Ber. Bunsen gesellschuft Phys. Chem. 1976,80 892. " S.R.Leone R. G. MacDonald and C. B. Moore J. Chem. Phys. 1975,63,4735. 70 R. T. Bailey and F. R. Cruickshank was also measured and found to be (3.26*0.13) X cm3 molecule-'s-l at 295 K. The vibrational relaxation of HC1 by Br is about ten-times faster than by Br2 and about 105-times faster than by Ar67 illustrating the importance of the reactive pathway in the case of Br. The results of this study support the conclusions that the deactivation of HCl(u = 1) occurs throbgh V -+ T R energy transfer while the deactivation of HCI(u = 2) occurs largely by reaction. Attempts to produce isotope enrichment of 35Cl were not successful in the case of HCl(u = 1).Hydrogen Bromide.-A large rate enhancement was also observed for the reaction of vibrationally excited HBr with iodine atoms by Badcock et aL6*The rate of reaction (15) HBr(u)+I + HI+Br (15) was found to increase by at least a factor of lo9when HBr(u = 0) is excited to HBr (u22). The bromine atoms produced in this reaction are rapidly scavenged by undissociated I2 molecules by reaction (16) kl Br+12 -+ BrI+I which produces a stable easily detected bromine product and kl 2 3 X cm3 molecule-' s-l. Ground state iodine atoms were produced in the reactor by 0.4 to 0.8 J cm-' pulses from a pulsed dye laser. Vibrational excitation of HBr was achieved using a TEA laser operating on H2 and Br2. With multiline operation both u = 1 and t = 2 levels were populated but single line operation populated only the u = 1level.Reaction products were analysed using a quadrupole mass spectrometer. Acceleration in reaction rate was observed for both multiline (u> 1) and single line (u = 1)excitation. Since reaction occurs from HBr (u > l) acceleration also occurs after collisional pumping which populates the higher vibrational levels. The acceleration is sufficient to allow the reaction to compete effectively with the rates of vibrational isotopic equilibration and isotopic scrambling reactions in order to produce the observed small bromine isotopic enrichment in BrI. 6 Halogenated Hydrocarbons The C02 laser induced decomposition of the halogenated hydrocarbons is one of the most extensively studied areas of i.r.laser photochemistry. From this work it has become obvious that conflicting results can be obtained for the dissociation of even simple molecules like CCI2F2 and CFC13. These difficulties arise mainly from the different experimental conditions employed by various groups of workers. These include different laser lines laser powers and energy fluences and sampling the products at different intervals after irradiation so that further reactions/dissociations can occur to varying degrees. A variety of gas pressures have also been employed which affect the amounts of laser energy absorbed by the gas and the kinetics of the reaction. The decomposition of CC12F2 at low pressures by a pulsed TEA C02 laser has been studied by Hill Grunwald and Keehn.69 This molecule absorbs at 921 cm-' 67 R.V. Steele and C. B. Moore J. Chem. Phys. 1974,60 2794. 68 C. C. Badcock W. C. Hwang and J. F. Kalsch Chem. Phys. Letters 1977 50 381. 69 G. A. Hill E. Grunwald and P. Keehn J. Amer. Chem. Soc. 1977 99,6521. Infrared Laser Photochemistry when a CCl stretch is excited and at 1088cm-' where a CF stretching mode is excited. Two competing reaction channels are expected for CC12F2 as shown in reactions (17) and (18). CC12F2 -+ CClF2+ C1 AH = 350 kJ mol-' (17) CC12F2 -+ CF2+ C12 AH = 3 10 kJ mol-' (18) These two reactions should be easily distinguished by the nature of the reaction products. For the first case the formation of ClF,CCClF and other free radical products such as CClF would be expected whereas in the second case C2F4 would be the predicted product.Laser irradiation at both 921 cm-' and 1088cm-' produced identical results with ClF2CCClF2 and CClF accounting for -83% of decomposition. There was no evidence for the formation of C2F4. Thus the primary reaction mechanism involves the breaking of the C-Cl bond. When CBr2F2 was irradiated the only product detected was CBrF2-CBrF indicating the primary step to be C-Br bond fission. Also when a mixture of CC12F2 and CBr2F2 was excited at 921 cm-' (CBr2F2 does not absorb at this frequency) the dominant products were CClF,CClF, and CBrF2CBrF2. There was no evidence for the formation of CC1F2CBrF2 which should have been formed had the free radical species -CClF2 and *CBrF2 been present simultaneously.The evidence thus suggests that the reaction occurs in two stages the photochemical decomposition of CC12F2 occurring very soon after excitation from a non-equilibrium molecular energy distribution while CBr2F2 reacts thermally to produce CBrF after V-T/R relaxation of the absorbed energy. CC12F2 does not react during the thermal reaction period due to the markedly greater dissociation energy of the C-Cl relative to the C-Br bond. Using a molecular beam-mass ~pectrometer~~ Sudbo et al." have studied the multiple photon dissociation of a number of halogenated hydrocarbons with pulsed CO laser radiation. Both angular and time-of-flight distributions were measured for the fragments to identify the major dissociation channels. Some of the molecules studied are listed in Table 2 together with results and some experimental parameters.The nozzle was heated in some experiments to increase hot-band absorption and thus shift the absorption frequency closer to a CO laser line. In the case of the CF3X series (X = C1 Br or I) the products were CF and X with velocity distributions correlated by the conservation of linear momentum in the dissociation process. The molecules CF2C12 CF2Br2 and C,HCl were also found to dissociate mainly by rupture of a carbon-halogen bond in agreement with the theory by Hill et al. For CFCl however as previously reported for SF6,71 an ionization pattern was observed that depended on laser energy fluence and the product translational energy. At moderate (-5 J cmP2) fluence the products were identified as CFCl and C1*.At higher energy fluence however ( -20 J cm-') further dissociation of -CFC12 to CFCl and .C1 was indicated. With some of the other molecules listed in Table 2 HC1 elimination was observed. In the case of molecules with two carbon atoms competing dissociation channels were observed but the dissociation always pro- ceeded along the channel of lowest threshold. Thus the results were consistent with 'O Aa. S. Sudbo P. A. Schulz E. R. Grant Y. R. Shen and Y. T. Lee J. Chem. Phys. 1978,63 1306. '' E. R. Grant P. A. Schulz Aa. S. Sudbo Y. T. Lee and Y. R. Shen in 'Multiphoton Processes' ed. J. H. Eberly and P. Lambropoulos Wiley New York,1977. R. T. Bailey and F. R. Cruickshank Table 2 The results and some of the experimental parameters used in the study of the multiple photon dissociation of halogenated hydrocarbons with pulsed CO laser radiation Absorption Exciting Beam nozzle Molecular Dissociation frequency /cm-' frequency /cm-' temperature 1°C CH3Cl + CF3*+C1* 1106 1090.0 240 CF3Br + CF3-+Br.1082 1078.6 25 CF31 -+CF3.+I. 1076 1073.3 25 CF2C12 + CF2Cl*+C1. 1098,923 1089.0,925.1 25 CF2Br2 + CF2Br.+Br. 1090 1084.6 25 CHFzCl + CFZ +HCl 11 16,1160 1082.3 280 CHFC12 4 CFCl +HCl 1070 1055.6 290 CFC13 + CFCl,.+CI* CFC12 + CFCl +C1. } CZF2+ HCl 1090 1074.6 25 /* CHClCF2 970 967.7 25 L 'CHCF2 + C1. C2HC13 -B C2HC12 + C1* 930 929.1 80 CF3CF2. + C1. /* CF3CF2Cl 982 978.5 25 L CF2Cl. + CF3. CH3CF2C1 -+ CH2CF2 + HCl 963 956.2 280 CH3CC13 -+ CH2CC12+ HCl 1075 1073.3 25 a random distribution of the excitation energy and the statistical theory of uni- molecular reactions appears to hold.Hudgens7' also observed the multiple photon decomposition of CF2C12and CFC13 by a strong i.r. laser field under collisionless or near collisionless conditions. Here the pressure ranged from 5-80 mTorr and laser fluences at the focus between 10 and 140 J cm-2. The techniques used were similarly to those employed by Sudbo et ala70 and generally similar conclusions were reached. However in the case of CF2C12,a small quantity (-3%) of :CF radicals were produced by reaction (19). CF2C12 -B :CF2 + C12 (19) When CFCl was irradiated it was found to decompose exclusively according to reaction (20).CFC13 -+ *CFC12+Cl. (20) No molecular C1 was detected in contrast to the result of Dever and Gr~nwald.'~ This illustrates the difficulties involved in postulating reaction mechanisms from an analysis of secondary reaction products in a static system. In the case of CF2Cl2 a '' J. W. Hudgens. J. Chem. Phys. 1978 68,777. 73 D. F. Dever and E. Grunwald J. Amer. Chem. Soc.. 1976,98,5055. Infrared Laser Photochemistry 73 laser energy threshold has been shown to This was found to be about 23 MW cm-2 which is similar to the value found for SF6. The i.r. laser induced decomposition of CHClF2 has also been studied by Grun- .~~ wald er ~ 1 Pressures from 5.8 to 100Torr and laser energy fluences up to 0.5 J cm-2 at 1088 cm-’ were used.Only C2F4 and HCl products were observed in agreement with other ~ork.~’,~~ By means of monitoring the concentration of :CH2 radicals as a function of time by U.V. absorption a delay of about 0.7 ps was noted between the laser pulse and the rise in :CF2 concentration. This is consistent with a photochemical rather than thermal mechanism for the dissociation of CHClF,. Further evidence for the formation of :CF2 radicals in CF2C12 decompositions was found in a preliminary of the laser induced reactions of 02,HCl NO and Me2C=CH2 with CF2C12. Isotopic selectivity was indicated arising from the initial isotopically selective formation of this reactive intermediate difluoro-carbene (:CF,). In a further study,77 the CO laser induced reactions of CF2C12 and CF2Br2 with isobutene propene and ethene were investigated.Maximum power densities of the order of 100 MW cm-’ in P(36) R(18) R(24) and R(26) C02 laser lines were used. In the Me2C=CH2-CF,Br2 system evidence for the formation of laser produced difluorocarbene was provided by the identification of the :CF2-olefin addition compound in the products. The yield of this material from the Me2C=CH2-CF2C12 system was dependent upon the laser irradiation frequency. No analogous gem-difluorocyclopropanes were detected in the CF2Br2-MeCH=CH2 and CF2Br2-C2H4 systems. Carbon isotopic segregation was found in all the experiments. Thus it appears that the :CF2 radical is only produced at the higher laser energy fluences. Evidence for difluorocarbene radical formation was also found in the laser induced dissociation of difluoromethane to give C2F4 as a major This conclusion was substantiated by scavenging with O2 when CF20 was found to be the major product.In the presence of molecular chlorine at least two distinct mechanisms in different pressure regimes were found. The principal product was CHF2Cl which was formed very efficiently. No power threshold was obvious since the reaction was driven by the unfocused laser beam towards a single product. With large numbers of pulses other products such as C2F4C12 appeared by further reaction. Products were also observed when the laser was tuned away from resonance by 10cm-’ to the R(34) 9.6 pm line as well as 30 cm-’ away to the P(20) 9.6 pm transition. No strong dependence of product concentration or nature on laser frequency was detected.Although energy transfer information” predicts a more reactive C-F channel the experimental evidence favours the C-H channel almost exclusively and these products are thermodynamically preferred. The selective i.r. photoisomerization of 1,2-dichloroethylene has been studied by .~~ two groups. Ambartzumian et ~1 have observed the selective isomerization of 74 M. C. Gower and K. W. Billman Appl. Phys. Letters 1977 30 514. 75 E. Grunwald K. J. Olszyna D. F. Dever and B. Krishkown J. Amer. Chem. SOC.,1977 99,6515. 76 J. J. Ritter and S. M. Freund J. C. S. Chem. Comm. 1976 811. 77 J. J. Ritter J. Amer Chem. SOC.,1978 100 2441. S. T. Lin and A. M. Ronn Chem. Phys. Letters 1977 49 255. 79 L.A. Gamss and A. M. Ronn Chem. Phys. 1975,9 319. R. V. Ambartzumian N. V. Chekalin V. S. Doljikov V. S. Letokhov and V. N. Lokhman Optics Comm. 1976,18,400. 74 R. T.Bailey and F. R. Cruickshank trans-C2H2C12 with 10.6 pm radiation from a TEA CO laser at power levels greater than lo9W cm-'. At these power levels however other processes such as dis- sociation of truns-C2H2C12 to give acetylene :CH and C2 radicals also occur. Photoisomerization of both cis- and trans-C2H2C12 have also been observed when SF is added as a sensitizer." The direct photoisomerization was also observed for the trans which absorbs at 898 cm-' overlapping the P( 16-20) lines of the 10.6 pm band of the CO laser. The reaction rate of the sensitized photo- isomerization was about six times that of the direct process owing to more efficient pumping of the 898 cm-' band.The C02 laser induced decomposition of chloroethylene to acetylene and hydrogen chloride has been studied by Willis and Back.82 Since chloroethylene could be selectively decomposed by the 1041.34 and 1045.04 cm-' laser lines deuterium enrichment was achieved. Deuteriated chloroethylenes are transparent to laser radiation at these frequencies. At pressures below 1 Torr however C-Cl bond scission appears to increase at the expense of molecular fragmentation and gives rise to radical processes which are isotopically non-selective. The CO laser photolysis of dilute samples of various alkyl halides in helium (0.5 to 2 Torr in up to 50 Torr He) was studied using the P(36) line of the 10.6 pm CO band.83 The pyrolysis of the isobutyl halides yielded isobutene methyl acetylene acetylene and other compounds.The ethyl halides yielded ethylene and acetylene but the former is also photolysed with acetylene as the only product. The reaction pathways were always dissociation into the lowest thermal dissociation channels of the molecule. The molecules photolysed were found to be not thermally equilibrated with the bath or with each other and the molecular-specific nature of the laser excitation was demonstrated. The multiple photon dissociation of CF31 has been achieved with a CO TEA laser producing a maximum of 1J single line output in a 60 ns A I3C enrichment factor of nearly 600 was obtained by irradiating 0.1 Torr of CF31 at 193 K with the R(14) line of the 9.6 pm CO laser transition.The dependence of enrichment factor on gas pressure indicated that collisions during the dissociation were effective in destroying the selectivity. The multiple photon dissociation was found to be quite efficient; at a laser fluence of 1.2 J ern-, one in every eleven absorbed photons contributed its energy to the breaking of the C-I bond. Octafluorocyclobutane absorbs CO laser radiationg5 and its thermal decom- position has been well characterized both thermally and chemically. Thus the thermal reaction yield can be accurately predicted and any increase due to vibra- tional (laser induced) enhancement should be detected. The photolysis of C4Fs at low pressures using the P(14) line of the 10.6 pm band of a TEA C02 laser has been shown to yield C2F4 cleanly according to reaction (21) CyClO-C4F8 -+ 2C2F4 (21) both thermally and by i.r.photolysis.g6 Addition of 20 Torr of argon to 1 Torr of " K. Nagai and M. Katayarna Chem. Phys. Letters 1977,51 329. 82 A. Gandini C. Willis and R. A. Back Canada. J. Chem. 1977 55,4156. 83 W. Braun and W. Tsang Chem. Phys. Letters 1976,44 354. 84 S. Bittenson and P. L. Houston J. Chem. Phys. 1977,67,4819. '' T. G. Roberts Rev. Sci.Instr. 1976 47 257. 86 J. M. Preses R. E. Weston and G. W. Flynn Chem. Phys. Letters 1977,46 69. Infrared Laser Photochemistry C4F8 was found to decrease the decomposition by about one-sixth by increasing the heat capacity (sic). However for the thermal reaction a decrease of 10-7-10-s was predicted and the above result was interpreted as showing that vibrationally excited molecules were involved in the reaction.As indicated in Section 3 however the above argument is not necessarily valid. With unfocused pulsed CO laser radiation hexafluorocyclobutene (1)decom-poses to the less stable isomer hexafl~orobutadiene.'~ unfocustd F -aF With focused beams however other reaction products appear in addition to C4F6 including C2F4 and polymers. It is not clear if these products result from decom- position of the C4F6 or are formed as a parallel reaction to the formation of the diene. At a pressure of 1Torr the yield of diene product approaches 60% but on the addition of 16 Torr of helium to 1Tom of the reactant the reaction goes to completion.This is interpreted as showing that collisional deactivation of the diene by helium atoms is so effective that the reverse reaction is quenched. However a temperature increase could also be responsible. The thermal decomposition of octafluorocyclobutane (3)'' occurs smoothly between 360 and 560°C but side reactions with the wall of the vessel give in addition to C2F4 [reaction (22)] CO CO, and SiF,. By contrast however laser induced decomposition of (3) at 949 cm-' with laser powers in the megawatt region leads to clean decomposition. The example illus- trates one of the major advantages of i.r. laser induced chemistry; the virtual absence of wall effects in gas phase reactions. 7 Hydrocarbons Many different kinds of CO laser induced reactions have been carried out with hydrocarbons.These include dissociation rearrangement and isomerization reac- tions. Reactions have also been carried out under a variety of conditions; for example the reactions of ethylene have been studied at atmospheric pressure using a C.W. CO laser to excite the CH2-wagging mode." Products obtained in a 20 :80 ethylene :butadiene mixture at 1atm included cyclohexane at 25 W and cyclo- pentene and cyclopentadiene at 40 W. On the other hand the i.r. multiple photon 87 A. Yogev and R. M. J. Benmair Chem. Phys. Letters 1977,46 296. J. N. Butler J. Amer. Chem. SOC.,1962,84 1343. 89 J. W. Robinson P. J. Moses and P. M. Boyd. Spectroscopy Letters 1974,7 395. 76 R. T. Bailey and F.R. Cruickshank dissociation of C2H4 at low pressures and high powers suggested that C2 was eliminated intact possibly by atomization of all four H-atoms in a collision-free time.90 In the low pressure regime (<1Torr) unimolecular reactions usually predominate and isotopic selectivity can sometimes be achieved with appropriate choice of i.r. laser wavelength. An early i.r. laser study of hydrocarbons involved the gigawatt cis-trans iso-merization of but-2-ene91 [reaction (23)]. Me Me Me H \ / 10.6wm c=c e‘c=c / (23) . H ’ H / ‘H ‘Me cis trans This process was accompanied by partial decomposition. When a 1:1 mixture of both isomers was irradiated at 4Torr a 15% enrichment of the cis isomer was observed. At 14Torr however no change in the ratio was observed.Ther-modynamic data show that the cis :trans ratio should be less than unity at all temperatures. In the presence of SiF as sensitizer allene can be isomerized to methyl on irradiation with a C02 laser at 1025 cm-’ [reaction (24)]. SiF, CHz=C=CH?-MeC_CH 1025 cm-’ The equilibrium constant for this isomerization is 13.4 at 298 K 3.4 at 1000 K and 2.7 at 2000 K. On irradiation of a mixture of 20 Torr allene with 5 Torr SiF4 at an energy fluence of 0.73 J cmP2 the ratio methylacetylene :allene reaches about 1.6. In both these cases the i.r. photostationary state was different from the thermodynamic ratio. The retro-Diels-Alder reaction of 1-limonene (4) to isoprene (5) was one of the first laser induced organic reaction The symmetry selection rules dictate that this reaction is allowed only in the electronic ground state confirming the laser induced reaction to be a ground state reaction.The early work was done with a 5 W C.W. laser at 943 cm-’ resulting in 90 R. V. Ambartzumian N. V. Chekalin V. S. Lotokhov and E. A. Ryabov Chem. Phys. Letters 1975,36 301. 91 A. Yogev and R. M. J. Lowenstein-Benmair J. Amer. Chem. SOC., 1973,958487. 92 P.Keehn and C. Cheng J. Amer. Chem. SOC.,1977,99 5808. 93 A.Yogev R. M. J. Lowenstein and D. Amar J. Amer. Chem. SOC.1972,94 1091. Infrared Laser Photochemistry other products including benzene toluene and several other compounds as well as the major product isoprene. The chemistry was found to be less complicated when the reaction was conducted with added SiF sensitizer under pulsed megawatt conditions.Norbornadiene (6)94also undergoes retro-Diels-Alder reaction when sensitized with SiF at 1025 cm-' and a fluence of 0.3 J cmP2 as shown in reaction (26). Irradiation of a C7Hs-SiF4 mixture (12.5 Torr-5.5 Torr) resulted in almost complete reaction. However under the same conditions in a C5H6-SiF4 mixture (10Torr-13Torr) 68% of the diene was decomposed. A Since C-H and C-D vibrations occur at different i.r. frequencies deuterium labelled compounds may sometimes be cleanly rearranged by i.r. laser radiation so that the D atom moves to another site in the molecule. For example the Cope rearrangement of hexa-lS-diene (7) reaction (27),95 has been induced by focused gigawatt i.r.laser irradiation at 926 cm-'. D D When an equilibrium mixture of the two isomers is irradiated only (7a) absorbs and the mixture is greatly enriched in (7b). The reaction is clean at 5-16 Torr but at higher pressures decomposition leading to acetylene formation is observed. The mole fraction of (7b) in the equilibrium mixture is greatest at a pressure of 5 Torr. Studies with other deuterium labelled isomers of hexa- 1,5 -diene have given consis- tent results and confirmed that the reaction was a genuine Cope rearrangement rather than dissociation to two alkyl radicals followed by recombination. The thermal decomposition of cyclopropane as well as the U.V. photolysis has been well ~haracterized.~~ Several vibrational modes including the CH2-wag CH3-rock and ring deformation may be pumped by the CO laser.The i.r. laser photolysis of cyclopropane was carried out using focused gigawatt power from a TEA C02laser at 9.552 pm P(20).96 Two processes were observed an apparently non-Boltzmann high energy decomposition giving rise to acetylene propylene methane and ethylene as major products and a typical flame reaction characterized by the luminescence of the C2+(d3wg)Swan band. The latter process played only a minor 94 D. Garcia and P. M. Keehn personal communication quoted in ref. 1 p. 77. 95 I. Glatt and A. Yogev J. Amer. Chem. SOC.,1976 98 7087. 96 M. L. Lesiecki and W. A. Guillory J. Chem. Phys. 1977 66. 4317. 78 R. T.Bailey and F. R. Cruickshank role in terms of the percentage consumption of cyclopropane.Both temporal and frequency resolved spectroscopy were used for the identification of the emitting species as well as in the characterization of the elementary reactions producing them. When mixtures of cyclopropane and NO were photolysed CN' C2' CH' and NH' were the observed emitters whereas with cyclopropane-0 mixtures only C2+ and CH emissions were observed. Previous work has indicated that C2 was eliminated intact by the multiple photon dissociation of ethylene." To obtain information of this process Hall ef af.97 have examined the mechanism for the production of C2 generated by the laser induced photolysis of 12CH2 13CH2 as a function of initial sample pressure. The laser induced fluorescence spectra of l3.l3C2 12*13C2 and 12*12C2 indicated that at pressures greater than -0.2Torr the C2 resulted from collisional processes such as C=C scission followed by recombination of carbon fragments or a possible four-centre reaction between a pair of highly excited ethylene molecules.At pressures below -0.2 Torr Cz was produced primarily by direct elimination of C2 from a single C2H4 molecule. Photolysis of a 0.30Torr sample of C2H4 (in natural abundance) resulted in an almost 100% enrichment of 12CHi2CH2. The delay times between the CO laser pulse and the onset of observed C2 were studied as a function of pressure and extrapolated to the collisionless pressure regime to give an estimated energy fluence of 50* 12 J cm-2 for direct elimination of C2. 8 Alcohols Ethers and Esters The i.r.muhiple photon dissociation of MeOH has been studied in two different pressure In the relatively high pressure regime (1.7-10 Torr) the focused output of a TEA C02laser was used to dissociate both pure MeOH and with added NO as a free radical scavenger.98 The energy fluence was calculated to be about 150 J cm-2 at the 0.05 cm diameter focal point resulting in both molecular and free radical initiated products. The decomposition was assumed to follow the course in reactions (28) and (29) CH30H -P CH20*+H2 (28) CH2O* + CO+H2 (29) which accounted for -90% of the consumed MeOH and the radical initiated process in reaction (30) CH30H + CH3.+OH (30) which ultimately results in stable products C2H4 C2H2 and CH4 accounted for the remaining -10% of the MeOH consumed.Visible luminescence observed from the focal zone was associated with the decay of OH* CH* C2* and possibly CH20". The concentration of stable products as well as the visible luminescence due to the electronically excited diatomic radicals were followed as a function of pressure time and the addition of the free radical scavenger gas NO. Both major photodecom- position routes appeared to be non-Boltzmann. 97 J. H. Hall M. L. Lesiecki and W. A. Guillory J. Chem. Phys. 1978,68,2247. 98 S. E.Bialkowski and W. A. Guillory J. Chem. Phys. 1977,67 2061. 99 S.E.Bialkowski and W. A. Guillory J. Chem. Phys. 1978,68,3339. Infrared Laser Photochemistry In the low pressure regime the collisionless dissociation of MeOH between 1000 and 10 mTorr was examined using similar laser energy fl~ences.~~ The laser-induced fluorescence technique was used to monitor the concentrations of product species.The appearances of OH 50* 20 ns after the laser pulse independent of initial MeOH pressure (50-200 mTorr) suggested that the primary dissociative channel was reaction (3l) CH3OH -+CH3*+OH (31) although CH3*could not be detected. The appearance of CH 70k20 ns after the appearance of OH independent of initial MeOH pressure (70400mTorr) suggested secondary collisionless dissociation. The observation of the relaxation of OH over a lops interval after radical appearance allowed the separate charac- terization of collisional rotational relaxation and translational diffusion.Ethyl vinyl ether has been shown to undergo two thermal reactions a retroene reaction [reaction (32)] and homolytic cleavage of the C-0 bond [reaction (33)]. H Recently Rosenfeld et af.loo proposed that the radical disproportionation k4 in reaction (33)alone competes with reaction (32) when ethyl vinyl ether is subjected to an intense i.r. radiation field at 10.6 pm (0.3 J per pulse focused). By measuring the ratio acetaldehyde :ketone as a function of pressure they found that the reaction was independent of pressure and consistent with a thermal distribution of energy. In subsequent work Brenner"' evaluated the effect of energy fluence and pulse duration on the branching ratio and yields of reactions (32)and (33). The conditions used for the photolysis were different from those used previously and different results were obtained.At a pressure of 0.4Torr using the 1041 cm-' laser line at a fluence of 0.91 J cm-2 acetaldehyde butyraldehyde and ethylene were the major products. At a high energy fluence the butyraldehyde decomposed and ketene trapped as ethyl acetate was observed. Evidence that the butyraldehyde was derived from reaction (33) was provided by the observation that the addition of NO quenched its formation. Under these conditions the yield of acetaldehyde decreased by only a minor extent consistent with the concerted nature of (32). By changing the laser pulse duration it was found that the yields and the branching ratio were varied and also that both reaction channels were observed at threshold when the pulse duration lo" R.N. Rosenfeld J. I. Braurnan J. R. Barker and D. M. Golden J. Amer. Chem. Soc. 1977,99,8063. lo' D.M.Brenner Chem. Phys. Lerters 1978.57 357. R. T.Bailey and F. R. Cruickshank was 0.2 ps. There was also evidence to suggest that a statistical energy distribution was not achieved during laser pumping when T,,= 0.2 ps. When ethylacetate is irradiated with a pulsed CO laser ethylene and acetic acid are the sole products under both partially focused and non-focused conditions. lo2 Under non-focused conditions Daren et a1.'02 found that at a fluence of 0.7-0.8 J cm-* ethylene was produced at 12-16% conversion per pulse nearly all of which was due to thermal processes. The elimination of HBr from isopropyl bromide was used as an internal standard to monitor thermal effects.Under partially focused conditions (fluence of 0.8-8 J cmP2) laser induced non-equilibrium chemistry was demonstrated by showing that ethylene was produced in an amount that was greater than expected via the thermal route. The results indicated that both low pressure and high energy fluences were necessary for the non-equilibrium pathway. The comparative single pulse shock tube-laser induced reactions of ethyl acetate were examined by Gutman et al.'03 Laser energy fluences of 4.5J cm-2 at 9.28 pm were used for the laser induced studies. In both cases ethylacetate was mixed with a second compound either allylmethylether or isopropyl bromide so that relative unimolecular rate constants could be determined.The decomposition of these compounds yields stable products reactions (34)-(36) under these conditions iso-C3H7Br -* C3H6+ HBr (34) C3H50CH3 -+ C3H6 + HCHO (35) CH3COOC2H.j + C2H4 +CH3COOH (36) avoiding the complicating effect of secondary reactions. It was found that although the products of the thermal and laser induced reactions were identical the relative rate constants showed variations. In the ethylacetate-isopropyl bromide system only the former compound absorbed laser radiation and the ratio of unimolecular rate constants almost reached the thermal value at the highest pressures studied (50 Torr). At lower pressures however (2 Torr) the decomposition of the pumped reactant became dominant in the laser experiments. A closer adherence to the thermal rate constant was observed where both molecules absorbed the laser radiation (allylmethylether with ethylacetate).The results suggest rapid intramolec- ular energy equilibrium prior to the onset of decomposition with identical mechanisms for both the thermal and laser induced reactions. Tetramethyl-1,2-dioxetane.-The competition between collisional energy transfer processes and chemical reaction plays a crucial role in determining the mechanism of a laser initiated chemical reaction. Tetramethyl-l,2-dioxetane(TMD) is an interes- ting molecule for studying such processes since it possesses some unique features. These include the quantitative i.r. photochemical decomposition to produce acetone reaction (37) accompanied by the emission of blue light at about 410nm and the 0-0 0 II I1 Me-C-C-Me + ,C + hv (37) I1 Me Me Me Me "'* W.C. Daren W. D. Munslow. and D. W. Setser J. Amer. Chem. SOC.,1977,99,6961. ln3D. Gutman W. Braun and W. Tsang J. Chern. Phys. 1977,67,4291. Infrared Laser Photochemistry 81 well established thermochemistry of the decomposition. The reaction dynamics can be conveniently probed after excitation by monitoring the time resolved visible emission from the acetone. Two groups of workers'04*'05 have looked at different aspects of the laser induced decomposition. Haas and Yahavlo4 used focused 0.5 J 80 ns 10.2 pm pulses from a TEA C02 laser to decompose TMD at a pressure of 0.35 Torr. Visible chemi- luminescence was observed with a monochromator-photomultiplier system with a time constant of 5 ns.The results indicated that TMD dissociated to yield an electronically excited product which decayed by at least two mechanisms a truly unimolecular one and another involving collisions. In subsequent work,'06 these authors used -3 ns pulses from a mode locked CO laser and replaced the sample in the cell after each shot. The gas pressure was reduced to 0.1 Torr with no buffer gas present. Two distinct emissions were observed in this case at 420nm attaining maximum intensity simultaneously with the laser pulse and at 460 nm appearing 140 ps after the laser pulse. The latter emission was assigned to triplet acetone and the former to singlet acetone fluorescence mixed with some triplet emission. In a related experiment Farneth eta1.'05 used 9.6 pm radiation from a TEA CO laser to initiate TMD decomposition by energy transfer from MeF.In this case unfocused radiation (300 mJ in 1ps pulses) was used so the power density was far below that used by Haas and Yahav.'06 The reaction was followed by monitoring the time resolved visible emission from acetone the time resolved spontaneous i.r. emission from MeF and the time resolved translational temperature changes (using the thermal lens technique). Attempts were made to generate luminescence using a variety of other sensitizers but only SF6 was effective. Typically pressures of -1 Torr TMD and 2-30 Torr MeF were employed so that collision induced processes probably dominated the mechanism. This was supported by the results which indicated that the reaction was initiated by i.r.absorption into MeF and that the visible light generated by the decomposition of TMD was produced on an energy transfer timescale. 9 Miscellaneous Compounds Hydrazoic Acid (HN3 and DN3).-The i.r. multiple photon dissociation of HN3 and DN has been studied at low pressures using the focused output of a TEA CO laser.lo7 The D-N bending fundamental in DN is centred at 954 cm-' while the v2-Y4 hot band in HN3 is found at 950 cm-'. Excitation within the P-branches of this band is possible with many P-branch transitions of the 10.6 pm CO laser. Hart- fordlo7 used the P(18) line for excitation using 100mTorr of DN3 and 150 mTorr of HN3. In the case of DN relatively strong emission was observed in the visible region at wavelengths greater than 560 nm.This was attributed to emission from ND,(*A,) produced by reactive collisions of ND(a 'A) produced in the primary photolysis step (38). 104 Y. Haas and G. Yahav Chem. Phys. Letters 1977,48 63. ")' W. E. Farneth G. Flynn R. Slater and N. J. Turro J. Amer. Chem. SOC.,1976,98,7877. '06 Y. Haas and G. Yahav J. Amer. Chem. SOC.,1978,100,4885. In' A. Hartford Chem. Phys. Letters 1978 57 352. R. T. Bailey and F. R. Cruickshank The multiple photon dissociation of HN by irradiation of the vZ-V~ hot band also produced emission attributable to However owing to the small cross section the hot band emission could only be observed at pressures greater than 150 mTorr of HN,. The smaller dissociation of HN3 compared with DN3 under identical conditions was suggested as a possible basis for a deuterium isotope enrichment process.Ammonia.-The dissociation of ammonia into ground electronic state NH2 radicals using high intensity CO laser radiation has been observed by Campbell et a1.1°8 This was accomplished using the P(32) CO laser line which is close to resonance with a line in the v2 band of NH,. In subsequent work this group reported results on the dissociation of NH3 into ground state NH using different CC2 laser lines.lo9 Many of the 60 laser lines used were out of resonance with absorption features in the NH3 spectrum. Letokhov et al. previously studied the multiple photon dissociation of NH under relatively high pressures (90Torr) using a series of CO laser lines and found emission from the electronically excited NH fragment only when the laser frequency was in coincidence with an absorption line of NH,.In the experiments of Campbell et a1.l" ammonia at pressures ranging from 0.05 to 7 Torr was dissociated with the focused radiation of the multimode TEA CO laser line tunable over the range 9.2-10.9 pm. The laser output ranged from 1.3 to 4.5 J per pulse about 100ns wide. Ground state NH radicals were detected by laser fluorescence excitation using a tunable dye laser at 597.7 nm to excite the radicals and a photomultiplier to detect the fluorescence. The maximum fluorescence intensity was found to occur for CO lines within the (00'1-10'0) band close to the densely packed v2( +) and v2( -) band heads but not in direct resonance the laser excited fluores- cence maxima being apparently shifted to lower wavenumbers.Signals observed from CO lines in the (00'1-02'0) band were of lower intensities. The fluorescence intensities for the sequence of lines R(6) to R(16) of the (00'1-10'0) CO laser band are shown in Table 3 together with the observations of Letokhov."' The laser energy kept constant at 3 J per pulse was focused into NH at a total pressure of 6 Torr. Letokhovll' only observed strong fluorescence with the laser lines which were close to NH3 absorption features whereas Campbell et al.lo9 observed comparable intensities for all the lines. These differences are probably due to the higher power employed in the latter work with a consequent increase in power broadening effects.The laser powers used (-10 GW cm-*) would give rise to a power broadening of about 6 cm-' for the strongest NH lines. This is sufficient to account for substantial pumping of the v2 vibrational level by virtually all CO laser lines at these intensities. Furthermore the observation of collisionless dissociation implies that the CO laser frequency and the broadened vibration-rotation NH lines must overlap significantly to enable the molecule to climb the vibrational ladder. 1')8 J. D. Campbell G. Hancock J. B. Halpern and K. H. Welge Optics Comm. 1976 17 38. '09 J. D. Campbell G. Hancock J. B. Halpern and K. H. Welge. Chem. Phys. Letters 1976 44 404. V. S. Letokhov E. A. Ryabov and 0.A. Tumanov. Soviet Phys.J.E.T.P. 1973.36 1069. Infrared Laser Photochemistry 83 Table 3 Fluorescence intensities of some lines of the 00'1-10'0 CO laser band Fluorescence intensities Campbell et al.'09 Letokhov"o 2.3 strong 1.3 weak 1.1 weak 1 .o none 1.2 strong 1.3 none The pumping and relaxational processes in the lower levels of NH have been studied by Tablas et al.' " Various levels of the v2mode were pumped by a CO laser and the level populations monitored by U.V. absorption spectroscopy. The ground state depletion was found to correspond well to the excited state populations but only a fraction of the absorbed photons were present in excited molecules indicating very fast V-T relaxation. Direct optical pumping appears to pump primarily the 1(+) level but strong collisional coupling with the 1(-) and 2( -) levels populates these also.Boron Trich1oride.-In addition to the considerable amount of C.W. laser work on boron compounds some high pressure pulsed laser studies have been carried out with particular reference to isotope separation. As expected from the C.W. laser results 'OB is enriched. The reactions have the additional merit that only the enriched 'OB species remain in the gas phase. Two separate studie~"~*~'~ give similar 10 B isotopic enrichment factors of 1.5-1.7. In one case'' BC13 (10 Torr) hydrogen (20 Torr) and Ti metal (-1 g used as a product scavenger) were irradiated (1.5 h) with the 947.7 em-' CO TEA laser line with -7 MW pulses at a 5 Hz repetition rate.This was absorbed by "BC13 only. In the other case,'13 BC13 (2 Torr) and H,S and D,S (2-40 Torr) were similarly irradiated except that the laser was focused by a 25 ern focal length lens at the centre of a near spherical 500 cm3 cell. The 5-10 h reaction times were typical. It was also found that if the 982 cm-' laser line was used being selectively absorbed by "BCl, a "B isotopic enrichment factor of 1.4 could be produced. The product of this latter study was HSBCI,. The chemistry is however not apparently simple. When D,S was used instead of H2Sin the 947.7 cm-' irradiation experiment the 'OB 'enrichment' factor became 0.8 i.e.the opposite effect to that of the H2S experiment was observed. The reversal did not occur in the 982cm-' irradiation experiments.It is not clear how these results correlate with those of Arnbart~umian''~et al. who report two excitation processes in TEA CO laser irradiation of BC13 and Houston et al.'" who report efficient energy transfer between "BC13 and "BC13 in less than 0.5 ps. ''I F. G. M. Tablas W. E. Schmid and K. L. Kompa Optics Comm. 1976 16 136. T. Lin T. D. Z. Adams and F. B. T. Pessine J. Appl. Phys. 1977,48 1720. S. M. Freund and J. J. Ritter Chem. Phys. Letters 1975 32 255. R. V. Ambartzumian N. V. Chekalin V. S. Doljikov V. S. Letokhov and E. A. Ryabov. Chem. Phys. Letters 1974 25 515. 'Is P. L. Houston A. V. Nowak and J. I. Steinfield J. Chem. Phys. 1973 58 3373. R. T. Bailey and F. R. Cruickshank OsmiumTetroxide.-A C02 TEA laser has been used to excite Os04at several laser frequencies and fluence levels from -0.02 to 1J cm-2.The oso4so excited reacts to give luminescence products and it has been shown'I6 that this luminescence intensity is proportional to the dissociation rate. This unusual but important result makes Os04a very convenient compound to study. The experimental conditions optically as well as chemically have been very carefully m~nitored.~~ The laser beam is collimated not focused so that spatial effects are eliminated. Only about 10% of the laser energy is absorbed in the 100cm long cell so that (n)can be calculated with good precision. The dissociation rate as a function of laser frequency is shown in Figure 8. Clearly no reaction occurs if R-branch absorption lines are used where the I I 1 1 I 0.2Torr Os04 I3 65 MW cm-2 Q / I\ I /I II \ \ \ \ \ 1.0 950 970 v/cm" Figure 8 The Os04 luminescence intensity (IF)dependence on laser frequency at constant laser power flux. The dotted line is the low i.r. intensity absorption spectrum (After Chem. Phys. Letters 1977 45 231 with permission) small signal absorption coefficient of these frequencies is quite as large as for the P-branch lines. As discussed in Section 3 a rotational compensation for anharmonicity is pro- posed for the multiple photon absorption. A P-branch line (u = 0 + 1)may match a P- or Q-branch line (u = 1+ 2) and a Q-or R-branch line (u = 2 +3)thus allowing pumping of the discrete levels. For the proposed mechanism P+Q + R tran-sitions or Q -P R transitions it is necessary that 12 BJ-Aval<AvR.For P+ R transitions 14BJ-AvaI < AuR is required. Ava is the frequency shift of the u = 2 level due to anharmonicity and AvR is the power broadened width of the transition. Above u = 3 the transition width is of the order of the interstate spacing so that a quasicontinuum can be assumed to exist above this level. A two pulse laser technique has been used to probe the details of the proposed mechanism. The lower power (-0.2 J cm-2) TEA laser (El)was tuned to a P-branch transition frequency and was expected to 'pump' the Os04 to u = 3. The higher power TEA laser (E,)(-0.7 J cm-2) was expected to 'pump' the oso4from u = 3 to the dissociation limit i.e. it was to 'pump' in the quasicontinuum region.Figures 9 'I6 R. V. Ambartzumian V. S. Letokhov. G. N. Makarov and A. A. Puretzkii Optics Comm. 1978,25,69. Infrared Laser Photochemistry lo4 n u) Y .-5 lo3 5 2 .w .-2 .2 Id 1. c .-ul al c .-9 10 al u) C .-5 JI 910 920 930 940 950 960 970 980 Exciting frequency 9, /cM' Figure 9 The Os04 luminescence intensity dependence on El laser frequency (vl) with El and E2fluencesfixed at the values shown and v2= 932.96 cm-' as indicated by the arrow. The Os04pressure is 0.18 Torr. The upper and lower curves are on log and linear scales respectively and the low i.r. intensity absorption spectrum is shown for comparison (Reproduced by permission from Optics Comm.1978 25,69) al 10 al u) .-C s dl 910 920 930 940 950 960 970 980 Dissociating frequency Se/CM-' Figure 10 The Os04 luminescence intensity dependence on E2laser frequency (v2)with El and E2fluencesfixed at the values shown and v1 = 954.5 cm-' as indicated by the arrow. The Os04pressure is 0.18 Torr. The laser pulse shapes were identical to those used for Figure 10 (Reproduced by permission from Optics Comm. 1978 25 69) R. T. Bailey and F. R. Cruickshank and 10show clearly that the rate of reaction is strongly dependent on the frequency of El, and this yield spectrum is highly structured. The yield spectrum of E2is much less structured and is strongly shifted to the low frequency end of the small signal spectrum.This is as expected for both Eland Ezfrom the proposed mechanism. The low frequency shift of the EZspectrum is characteristic of a high anharmonic level being ‘pumped’ to some upper state. The structureless appearance is characteristic of a wide choice of upper states with small energy gaps being favoured over larger but with otherwise little difference in transition probabilities i.e. a quasicontinuum does indeed exist here. When the delay between the two laser pulses El and E2was lengthened from the 0.5 ps used in the above experiments it was possible to probe the decay kinetics of the upper states in the discrete level region; Figure 11shows the result. The El laser Delay time /ps Figure 11 The oso4 luminescence intensify dependence on the delay between El and E2 laser pulses.Laser El was set at 954.5 cm-’ E2 at 924.97 cm-’. The oso4 pressure was 0.2 Torr (Reproduced by permission from Optics Cornrn. 1978 25,69) pulse was shortened to 15f5 ns FWHM and El = 0.03 J cm-’ with E2=0.3 J cm-* (90ns FWHM 1ps tail). The upper discrete level population obviously decays on a ps timescale. It was calculated that only 2/7 of the initial molecules were excited by the El laser pulse at the fluence used and that an energy level equivalent to 7 CO laser quanta was achieved in these molecules. This work is in its early stages yet but the technique promises to reveal a great deal about the details of the mechanism. It is unfortunate that the chemistry of OsO is not better characterized. Fe(C0)4 (isolated in low temperature matrices).-In contrast with room tempera- ture i.r.photochemistry low temperature (-20 K) inert gas matrix isolated compounds can undergo reactions of very low activation energy to products which will be stable. Reaction rates will be very slow for activation energies considerably lower than one i.r. quantum of CO laser radiation. Infrared Laser Photochemistry The most systematically studied reaction series of this type is that involving Fe(CO),. 117-'19 A C.W.CO laser was used (1.5-3 W bandwidth<0.001 cm-' 1919-1880 cm-'). The Fe(CO) was produced by prolonged U.V. photolysis of Fe(CO)Sin an Ar matrix. This reaction is only partially reversible if the matrix is irradiated with broadband i.r. from a Nernst glower and this is interpreted as a demonstration that most of the CO ejected from the Fe(CO) has diffused away from the Fe(CO) product.These 'isolated' Fe(CO) molecules enriched in 13C'80could be induced to isomerize as follows as shown in reaction (39) where X is 13C'80. These species are distinguishable being of C2"symmetry with bond angles of -145" and 120". It was shown that dimer participation or reaction with the CO ejected from the Fe(CO) was extremely unlikely. Only one CO laser photon at 1880 cm-' (-22 kJ mol-') seems to be necessary to isomerize the mole-cule with no phonon-vibration energy transfer occurring so that multiple photon transitions are not involved. The possibility that the absorbed energy resulted in a - local 'melting' of the matrix reaction (40) was considered for 3ci8o).When the laser beam was plane polarized substantial dichroism developed during the photolysis i.e. the Fe(C0)4 molecules did not rotate during the reaction timescale (hours). Thus orientation-specific laser photolysis occurred ruling out localized melting which would allow free rotation. With a tunable spin flip Raman laser it was possible to irradiate Fe(C0)4 in specific matrix sites since the site differences were reflected in a fine splitting of the i.r. absorption band. The reaction induced was between Fe(CO) and the CO ejected from the parent Fe(CO)5after only 15 s of U.V. photolysis i.e. the reverse of the preparative reaction. The tunable laser enabled the absorption linewidth of the matrix isolated Fe(C0)4to be measured (0.2 cm-') and these lines were found to be so narrow that they are less than the slit-widths of most spectrometers.Thus the degree of overlap between an i.r. absorption line (measured on a highest quality spectrometer) and for example a CO laser line is extremely difficult to estimate. Clearly the continuously tunable spin-flip Raman laser will greatly facilitate i.r. photochemistry of matrix isolated species. B. Davies A. McNeish M. Poliakoff and J. J. Turner J. Amer. Chem. SOC., 1977,99,7573. 'I8 B. Davies A. McNeish M. Poliakoff,M. Tranquille,and J. J. Turner Chem.Phys. Letters 1977,52 477. 'I9 (a)M. Poliakoff N. Breedon B. Davies A. McNeish and J. J. Turner Chem. Phys. Letters 1978 56 474; (b)M. Poliakoff Chem. SOC.Revs.1978,7 527. R. T.Bailey and F. R. Cruickshank Sulphur Pentafluorochloride (SF,Cl).-Although there are many similarities between SF,Cl and the more extensively studied SF6 there are several differences which serve to illuminate the mechanism of absorption and reaction. First the S-Cl bond in SF,Cl is weaker than the S-F bond in SF6 (255 versus 385 kJ mol-') and secondly if the decomposition of SFsCl involves the breaking of a S-Cl bond intramolecular vibrational energy transfer from the excited S-F to the S-Cl bond must occur. The lower symmetry of SFsCl also results in a different vibrational spectrum. Isotopically selective reactions of SF,Cl induced by the focused radiation from a TEA COz laser at several frequencies near the S-F stretching frequency at 909 cm-' were observed by Leary et al.'" The experiments were performed at low pressures (0.25-4.0 Torr) with pure SF,Cl and with several SF5Cl-diluent mixtures using pulse energies of the order of 1J for all the lines.The reaction yield product distribution and isotopic selectivity were determined for various reaction conditions using a quadrupole mass spectrometer. Isotopic selectivity was found to be poorer than for SF6 due in part to intermolecular V-V energy transfer and other scrambling processes. The primary photolysis reaction involved the breaking of the weakest bond reaction (41) a reaction which is isotopically selective. Thus the V-V transfer processes are fast enough even at the lowest pressures to transfer excitation from the S-F to the S-C1 modes before S-F bond cleavage can occur.Mode selective chemistry does not therefore appear to be possible with this molecule at least under the conditions used and the presence of 'collisionless' V-V energy transfer rate processes is in agreement with SF6results. Methyl Cyanide (MeCN).-The MeCN molecule is interesting in that it exhibits two different absorption bands (Me rock and C-C stretch) within the tuning range of the CO laser. The effect of mode selective pumping on the decomposition can thus be studied. The i.r. laser induced photolysis of MeCN has been studied by Lesiecki and Guillory.l2l The MeCN at pressures of 0.5-5.0 Torr was pumped with the focused output of a TEA C02 laser using the P(20) line of the 00'1-02'0 transition to pump u7 (Me rock) of MeCN or P(32) of the 00'1-10"0 transition to pump u4 (C-C stretch).The powers at the focus were estimated to be about 1GW cm-'. Both electronically excited and ground state CN are produced by the dissociation of MeCN. The former was detected by luminescence and the latter by its dye laser excited fluorescence.'22 A pulsed N2-laser pumped dye laser was used to excite the fluorescence. It was found that excitation of the Me vibrational mode was at least three-orders of magnitude more effective in producing fragmentation of MeCN than was the excitation of the C-C stretch. This could not be accounted for by the simple effects of differences in absorption coefficient and power differences. However the products and relative product ratios were the same irrespective of the mode pumped; that is there was no detectable mode specificity.I2O K. M. Leary J. L. Lyman L. B. Asprey and S. M. Freund J. Chem. Phys. 1978,68 1671. 12' M. L. Lesiecki and W. A. Guillory Chem. Phys. Letters 1977,49,92. '22 M. L. Lesiecki and W. A. Guillory J. Chem. Phys. 1977,66,4239. Infrared Laser Photochemistry Methyl Isocyanide (MeNC).-The thermal isomerization of MeNC to MeCN known to be exothermic to about 14.7 kcal mol-' has been suggested as an ideal reaction for testing thermal explosion the0~ies.l~~ It has been the subject of extensive kinetic studies as a result of which it has emerged as a model unimolecular reactant. Recently the states of MeNC having sufficient energy to effect iso- merization were pumped directly by a C.W.dye 1a~er.l~~ This experiment led to an unambiguous determination of the unimolecular rate constant k for the states selected. In a subsequent MeNC vapour was irradiated with a TEA CO laser tuned to coincide with the v4 fundamental of this molecule. At pressures between 10and 100Torr at room temperature more than 50% conversion of the gas to its isomer MeCN was observed. For a given pressure a sharp threshold in CO laser power was found to exist above which isomerization occurs. The threshold for thermal isomerization was also shown to be pressure dependent (Figure 12). These 1 I T1 I 50 40 \ -3 E 30 -\ -5 W 20 10 0 1 I I I I 0 20 40 60 80 I00 PRESSURE (Torr) Figure 12 Variation of threshold energy with pressure; 0 threshold energies at the entrance window of the cell A threshold energies at the beam focus in the centre of the cell (Reproduced by permission from Chem.Phys. Letters 1978 57 479) results were interpreted in terms of a thermal explosion triggered by laser induced heating (cf. EtC1 EtI). 10 Continuous (Low Power) I.R. Laser Photochemistry As in the case of high power pulsed i.r. laser photochemistry few systematic studies have as yet been carried out with C.W. i.r. lasers which by their nature must have lower powers typically in the 50-400 W range. One well-studied system is the reaction between ozone and nitric oxide. The H. 0.Pritchard and B. J. Tyler Canad. J. Chem. 1973 51 4001. 124 R. V. Reddy and M.J. Berry Chem. Phys. Letters 1977,52 11 1. 12' D. S. Bethune J. R. Lankard M. M. T.Loy J. On and P. P. Sorokin Chem. Phys. Letters 1978,57,479. R. T.Bailey and F. R. Cruickshank effects of vibrationally (CO laser) excited'26 03(001)and vibrationally (CO laser) excitedlz7NO on the rate of reaction have been examined separately. The reaction is highly exothermic and electronically excited N02('B1)is produced as well as ground state N02(2A1).For reactions (42)-(45) O,+NO A N02(2Bl)+02 (42) B' +NO -03* N02(2B1)+ 0 (43) A O,+NO -N02(2Al)+02 (44) A' 03* + NO -+ NO,(~AJ + 0 (45) the room temperature rate constant enhancement factors where 03*is very probably O,(OOl) are kBl/ kg = 4.1f2.0 and kA'/ kA = 17.1f4.3. A flow system was used to study these reactions with -2.4 Torr O, 0.7 Torr NO and 0.1 Torr 0 excitation by a CO laser (2 W P(30) 9.6 pM,modulated at 240 Hz).It is calculated from the observed enhancements that 50% of the vibrational energy in the O3is used in B' and 85%of the available energy is used in A. In a similar flow system at similar pressures except that the NO pressure was typically an order of magnitude smaller a CO laser was used [up to -0.4 W cm-2 1884 cm-' P(13) modulation 100 Hz] to excite the NO to o = 1. For reactions (46)-(49) NO+O -!+ N02(2Bl)+02 (46) NO* + o3-L NO,(~B,)+ 0 (47) NO+03 2N02('A1)+02 (48) NO* + o3-2 NO,(,A 1) + 0 (49) the enhancement factors are kl,/kl= 4.7* :: and k2,/k2s 18. While the NO fluorescent state may not be created by thermal heating the arguments used in this paperlZ7 to show that thermal effects are absent are not too convincing.For example it is stated that the maximum temperature rise that could occur is given by (energy absorbed)/C of the irradiated volume and that addition of up to 4 Torr N2 has little effect on the fluorescence of the NO although this increased the heat capacity of the system by a factor of 3.3. As we have pointed out already in connection with the high power laser photochemistry the highest temperature will occur on the laser beam axis and will be much larger than that calculated by Stephenson et al. Additionally the effect of added heat capacity is to slow the thermal diffusivity which tends to preserve the high thermal gradient to which reaction exothermicity also contributes.We feel that a thermal contribution cannot be excluded. From these results it is deduced that in the transition state the NO vibration must transfer energy to the 02-0reaction co-ordinate. The fact that the NO vibration contains nearly twice the quantum energy of the 03(001),but gives similar rate 126 R. J. Gordon and M. C. Lin Chem. Phys. Letters 1973 22 262. 12' J. C. Stephenson and S. M. Freund J.Chem. Phys. 1976,65,1893. Infrared Laser Photochemistry enhancements is ascribed to several factors e.g. the probability of energy flow into the reaction co-ordinate could be low or the NO quantum energy which is rather less than twice the 03(001)quantum energy may not be sufficient to excite the mode of the complex into u = 2 (This is almost certainly true since the laser energy is too low to allow such a multiphoton transition any large probability.) Similar experiments on reaction (50) o3+ SO -+ SO,('B~) + o,(~c,) (50) using vibrationally excited O3Iz8yield an enhancement factor for the resultant SO2 fluorescence of 2.5 stO.6 at 300 K.The 630*200 cm-' blue shift of fluorescence was interpreted in terms of vibrational energy transfer from the 0,to the SO2('Bl) vibrational manifold. C.W.i.r. laser radiation is known to cause explosions in for example the irradiation of C2HsC1'29 and C~HSI.'~~ These are purely thermal processes and almost certainly rely on no phenomena unique to laser deposition of energy. The escalating reaction rate following an induction period and leading to the explosion results in both cases from the increased i.r.absorption coefficient of the product ethylene. Thus the reactant temperature increases with extent of reaction and all the observed phenomena can be explained. There remains only a difficulty in measuring the temperature in such systems e.g. -1100 K from i.r. emission band contours and -900 K from reaction rate in the case of ethyl chloride. Product distributions differ from the isothermal distributions but this is not surprising given the large thermal gradients in the system. Similarly in the reactions between atomic oxygen and C.W. CO laser vibrationally excited C2H4 and OCS no rate enhancement was observed which could not be explained on the basis of heating effect^.'^' In a study of the C.W.C02 laser [400W P(20) 10.6 pm] promoted decomposition of CF2C12,132 calculated temperatures Teqof -1300 K are attained. The 300 K CF2C12 laminar flow system operates at just over one atmosphere pressure with CFzClZ-50% and the shield gas Ar -50% of this pressure. Conditions were so arranged that the energy input to the CF2Cl was kept constant at 100k 0.2 kJ mol-' ( Teq-1300 K). When 31'/0 SF6 was added to the system the CF2C12 decomposition was totally inhibited. It was concluded that this was evidence of a laser specific effect. However at these high pressures of Ar and CF2C12 and at > 1Torr SF6 the penetration of the COJaser beam into the CF,C12 stream would be confined to a very thin boundary layer adjacent to the laser input port.Even if all the CF2C12 in this layer decom- posed it would represent a trivial overall yield. This totally thermal explanation seems to us the most probable reason for the observed phenomena. Work with such an optically thick sample always makes reliable interpretation difficult. In more A. Kaldor W. Braun and M. J. Kurylo J. Chem. Phys. 1974,61 2496. ''' R. T. Bailey F. R. Cruickshank J. Farrell D. S. Horne A. M. North P. B. Wilmot and Tin Win. J. Chem. Phys. 1974 60 1699. 130 J. C. Bellows and F. K. Fong J. Chem. Phys. 1975 63 3035. ''I R. G. Manning W. Braun and M. J. Kurylo. J. Chem. Phys. 1976 65 2609. '32 M. P. Freeman D. N. Travis and M. F. Goodman J. Chem. Phys. 1974,60 231. 92 R. T. Bailey and F. R.Cruickshank recent on CH2Cl2 and CC1F3 all the laser intensity was absorbed within the sample cell. A higher conversion rate resulted from C.W. irradiation of CF2C12 with the R(40) (9.4pm) laser line than when the P(36) (10.6 pm) line was used. This observation has been as evidence of the non-thermal nature of the reaction since the extinction coefficient of the R(40)line is lower than that of the P(36) line. However the lower extinction coefficient will allow greater penetration of the optically very thick sample and since the CF2C12 can only be dissociated this could explain the increased conversion. A measure of the relative irradiated volumes of sample above the reaction laser flux threshold would have been very helpful in this work. As the CF2C12 is converted into CF3Cl light is emitted.It was found that the intensity of the emission correlated very poorly with reaction rate. This is almost certainly true of similar work both in short pulse M.W. laser studies and in C.W. studies. In the SF sensitized decomposition of C2H6134 (static system) promoted by CO laser radiation [300-600 W ern-, P(20) 944.2 cm-' c.w.] a pressure was observed at which the reaction rate maximized; at 263 Torr C2H6,3.3 Torr SF, and 51.7 Torr A this occurs at 600 W cm-2 laser power; the thermalization factor x,is found to minimize at this pressure. x is given by r/h where T =E/Eo (E =thermocouple reading just outside the irradiated zone with Ar SF6 and C2H6 present Eo= the same measurement with only Ar and SF in the cell).A = (Po-P)/Po (P=transmitted laser power for the Ar SF, C2H6 mixture Po=transmitted laser power for the Ar and SF only mixture). The coincidence of these pressures is held to be evidence of a laser specific process. However an examination of heat flow in the cell would indicate that as the pressure of C2H6 is increased the heat capacity increases and thermal diffusivity decreases. Thus over the 1 s irradiation period used T will decrease and A will rise since P drops as C2H6 pressure increases due to the relaxation of the absorbing SF6 by the C2H6 as discussed by the authors. Thus a minimum in x is to be expected and must coincide with a maximum in the cell core temperature i.e. in reaction rate. In view of these doubts we do not feel that speculation on the photochemical mechanistic details of this work is justifiable.Isotope separation in C.W. i.r. laser supported reactions has also been reported. Deactivation processes can apparently be used successfully to compete with level 'pumping' of a particular isotopic species. Early work almost certainly involved substantial thermal eff ects13' as already discussed. 136*137 Bauer et al.I3' have shown that H D and "B "B can be separated in multiple i.r. photon induced dissociation of D3BPF3using a CO laser. All precautions were taken to minimize thermal effects. Deactivation of the laser excited molecule MeBr reacting with atomic chlorine was arranged to be faster than V-V energy exchange between MeBr decreased quantum efficiency of course occurs. The P(10) and R(14)lines of the 133 V.Slezak J. Caballero A. Burgos and E. Quel Chem. Phys. Letters 1978 54 105. 134 J. T. de Maleissye F. Lempereur C. Marsal and R. I. Ben-Aim Chem. Phys. Letters 1976 42,46. S. W. Mayer M. A. Kwok R. W. F. Gross and D. J. Spencer Appl. Phys. Letters 1970,17 516. 136 C. B. Moore Accounts Chem. Res. 1973,6 323. 13' C. Willis R. A. Back R. Corkum R. D. McAlpine and F. K. McClusky Chem. Phys. Letters 1976.38 336. 13* K. R. Chien and S. H. Bauer J. Phys. Chem. 1976,80,1405. 139 T. J. Manuccia M. D. Clark and E. R. Lory J. Chem. Phys. 1978,68 2271. Infrared Laser Photochemistry 10.6pm CO laser band were used to excite (intracavity 160 Wcm-,) pre-dominantly Me8'Br and Me79Br respectively. The reaction scheme is that given in reactions (51) to (53).laser MeBr ___* MeBr* C1. +Me8'Br* + HCl+ sCH,~'B~ (preferentially) (52) sCH,~'B~ +C12 -+ CH2C18'Br+C1. (53) The 79Br/81Br enrichment factor was found to be 1.05 (183K) for the R(14) line and for the P(10) line the 81Br/79Br enrichment factor was 1.04 (183K). Typical conditions for the separation were MeBr = 0.0023 Torr Ar =0.7 Torr C1 = 0.028 Torr. The estimated cost of 6000 eV per enriched molecule was calculated by the authors for a suitable reaction. This would represent a significant advance over present methods. This work was almost certainly true i.r. photochemistry since varying the laser frequency altered the isotope enriched. Very similar experiments with the U.V. initiated bromination of Me irradiated with a C02 laser showed that the 12C or 13C molecule could preferentially react with a suitable choice of laser freq~ency.'~' Typical conditions were; Br2 0.075 Torr Ar 2.9 Torr MeF 0.01-0.15 Torr CO laser power 10-100 W.A considerable number of useful syntheses in boron chemistry have been carried out using low power C.W. C02laser radiation. There appears to be strong evidence that several of these are essentially i.r. photochemical processes with little or no thermal contribution. Thermally H2S reacts with diborane to give (HBS), a polymeric solid. However irradiated at 973.3 cm-' by a 6-7 W C.W. CO laser where B2H6 is the only absorber the reaction sequence of (54) to (57) is initiated.I4' BH3.+H2S + HSBH,+H2 (55) HHH H2S +HSBH2 + (HS)2BH+H2 (57) CO laser radiation of 1.5W 973.3 cm-' produces icosaborane &OH16 from 200 Torr B2H6 by a laser-triggered chain ~eaction.'~' The p-HSB2H5 which is otherwise difficult to synthesise was obtained in up to 30% yield in about 30 minutes.Y. N. Molin V. N. Panifilov and V. P. Strunin Chem. Phys. Letters 1978 56,557. H. R.Backmann F. Backmann K. L. Kompa H. Noth and R. Rinck Chem. Ber. 1976,109,3331. '42 H.R. Backrnann H. Noth R. Rinck and K.L. Kompa Chem. Phys. Letters 1974 29,627. 14' R. T.Bailey and F. R. Cruickshank A temperature probe was used in a trimethyl boron reaction sequence in order to estimate the thermal contribution. For the sequence (58),(59) (60) BMe3+HBr + BMezBr+CH4 (58) (970.5 cm-' laser thermal reaction temperatures 150-1 80 "C) BMe2Br+HBr + BMeBr +CH4 (59) (1039.4 cm-' laser thermal reaction temperatures >250 "C only) BMeBr2+HBr -+BBr3+ CH4 (60) (970.5 cm-' or 1039.4 cm-' laser thermal reaction temperatures >450 "C only) reaction (60)occurs only at temperatures significantly above that at which reaction (59) is significant.BMeBr (100 Torr) was irradiated (60 min 4.5 W 970.5 cm-') with HBr (200 Torr) and BMe,Br (20 Torr). Only the BMeBr absorbed the laser radiation. Although BBr3 was produced in significant yield the probe gas BMe2Br was unchanged in concentration clearly demonstrating the non-thermal nature of the process. Further evidence of the synthetic utility and i.r. photochemical nature of these C.W. C02 laser boron reactions is provided by the BCI promoted trimerization of C2C14.Only the BC1 (150-200 Torr) absorbed the -6 W 940 cm-' laser radiation; 0.1 mmol C2C14 per hour were converted c&& being recovered in 88% yield. BCl loss was 0.015 mmol h-'. This reaction of czCl4 occurs thermally between 700 and 725 "C. However when SF6 is used instead of BCl no reaction whatever is observed despite the increased absorbed energy.144 In the presence of 02,c2c14 produces phosgene at 1200 "C via the intermediate C2C12. With BCI, O, and C2C14 under laser irradiation however CO was the main product and no phosgene was found. If SF was used in these experiments instead of the BCl then the phosgene was produced. This seems to demonstrate that the BC13 promoted reaction is truly i.r.photochemical whereas the SF6promoted reaction is most probably thermal. A further example of this type of reaction is the removal of phosgene from BCl (15-100 Torr 100W C.W. C02 laser 944.2 cm-'). Only the BCl absorbed the laser radiation and in 3 4 s all the COCl (1.6% in BCI,) had reacted to CO and C1,. No appreciable loss of BCl was detected. Replacement of BCl by the similarly absorbing C2H4 produced no reaction between the C2H4 and COC1,. A reduced rate of decomposition of the COCl was observed This work does of course explain why COC1 was absent in the previous study. In a study of reaction (61) 2BC1 +BMe3 + 3MeBCl2 (61) induced under similar conditions to the above by C.W. CO laser radiation a long wavelength shift was observed in the laser frequencies giving optimum yield compared with the BCl absorption spectrum.145 Irradiation of the v3 absorption band of "BCl produced a more efficient con- version than when the "BC13 band was irradiated. These results were interpreted in 143 H. R. Backmann H. Noth R. Rinck and K. L. Kompa. Chem. Phys. Letters. 1975,33,261. 144 H.R. Backmann R. Rinck H. Noth and K. L. Kompa Chem. Phys. Letters 1977,45,169. 14' F.Backmann H. Noth R. Rinck W. Fussand K. L. Kompa Ber. Bunsen gesellschaft Phys. Chem. 1977 81.313. Infrared Laser Photochemistry terms of a rate determining absorption step by vibrationally excited BCl,. The vibrational absorption maximum of these molecules derived from the yield spec- trum indicated that the energy distribution in the excited BCl was far from Boltzmann.11 Conclusion In the pulsed TEA laser studies there has been considerable interest in whether the reacting molecules are in Boltzmann equilibrium with respect to their internal degrees of freedom particularly vibrational. Some studies seem to show that the internal distribution of energy is non-Boltzmann e.g.cyclopropane ethyl vinyl ether (in long pulse work) SF (with long laser pulses only) and methanol. Other work (e.g. ethyl vinyl ether and SF6with short laser pulses) seems to show that the reactant is in Boltzmann equilibrium internally. It could be unwise to attempt a general conclusion i.r. photochemistry still being in its infancy but it seems likely that the reactant molecule is in Boltzmann equilibrium internally.The rapid V-V energy transfer in the collision-free regime e.g. in SF, occurs by an as yet unknown mechanism. However its occurrence seems effectively to remove the possibility of mode specific chemistry in pulsed laser work (e.g. selective bond breaking by ‘pumping’ the molecule with a frequency corresponding to that ‘bond stretching’ vibration). More subtle C.W. ‘pumping’ of vibrational populations in a system where kinetic control allows establishment of a photostationary state may well succeed in promoting mode specific i.r. photochemistry. If the reactant molecule is in thermal equilibrium internally and also translation- ally with a substantial number of its nearest neighbours then its chemistry ceases to fall under the title of i.r.photochemistry and becomes thermal instead. Considerable effort has been expended in many studies to show whether or not the chemistry was thermal albeit with substantial thermal gradients in the reaction cell. That this proof is not easy is obvious from the work described above. In the case of pulsed laser work it is difficult to see how a reaction occurring before any collisions could be thermal as defined here. On the other hand it is difficult to see how any C.W. laser study at pressures up to tens of Torr and higher could be other than thermal. Nevertheless in both types of work as we have indicated phenomena occur which seem to dispute this simple picture. Clearly much more work must be done on the careful characterization of the optics of the system its thermal history and its chemical history with the timing of these events accurately correlated.The extremely low quantum yield (-6 x lo-’ for SF6)must also be critically examined. The conversion efficiency seemed to be much higher in the molecular beam experi- ment,49 but otherwise very low values obtain even given that the reaction is a multiple photon process. At these low levels careful consideration will have to be given to the number of collisions the reactant molecule is likely to undergo at ‘collisionless’ pressures given that very few energised molecules react. Wall collisions may also be important especially when high efficiency reaction cells are used where the laser beam impinges on the walls of the vessel. As outlined above simple heat capacity arguments are totally inadequate for estimating a mixture’s thermal behaviour particularly in sensitized systems with added non-absorbing gas.Bulk heating is known to occur with SF6(0.028Torr)+ R. T.Bailey and F. R. Cruickshank He(2Torr) and the C effect of added He must be tempered with consideration of kinetic control of the core temperature. Additionally the pressure dependence of the extinction coefficient is rarely mentioned. Figure 13 shows the variation of extinc- *O I 0 100 200 P/ Torr-Figure 13 Absorption of CO2 laser Q-switched pulses at -10 Hz (100 W 942.4 cm-') by -0.040 Torr SF6 as a function of added argon pressure (P) tion coefficient of SF6 with pressure of added argon.44 The SF6pressure was very low (-40 mTorr) and as yet not accurately known.However the change in absorbed energy as argon is added to the constant SF pressure is very large. Although obviously important in i.r. photochemistry such observations are never reported. Statements that absorbed power is kept constant require amplification. Unless the absorbed power is measured for each run the statement could be quite untrue. It is not enough to assume that the behaviour of such systems is totally predictable. As for SF6 any molecule operating near to saturation of its laser energy absorbing transition will have an extinction coefficient strongly dependent on pressure due to kinetic factors. The value of the fluence is frequently calculated from assumed optical charac- teristics of lenses.This is rarely adequate. Experience of simple lens systems indicates to us that the laser beam diameter and profile must be measured to ensure the validity of fluence calculations. At the long wavelengths involved in i.r. photo- chemistry diffraction effects can be very important. Thermal lens effects parti- cularly at high laser pulse repetition rates will alter the fluence as a function of reaction cell length. These effects depend on refractive index temperature coefficient heat capacity laser beam profile and diameter and thermal diff usivity. They will be expected to be very important in C.W. laser photochemistry. In many of the systems examined by Grunwald et al. the conversion per laser pulse has been shown to follow an Arrhenius type expression A exp -@a/&,) where Eab is the laser energy absorbed and A and Eaare constants.Eavalues near the threshold Infrared Laser Photochemistry energies of the thermal reaction systems were required to fit the data and this has been interpreted as indicating that the relevant temperature calculated from Eabis the Boltzmann equilibrium value not the higher value associated with a restricted number of vibrational modes less than the maximum. This would be indicative of ergodic reactant molecules. However in view of the above experimental problems it is probably better to reserve judgement on this deceptively simple argument. The applications of i.r. photochemistry are certain to spread rapidly. Even if mode selective reactions are rare molecule specific processes definitely occur for example in isotope separation.Freedom from wall effects will be attractive in many reactions. Equilibrium forcing in either direction i.e. independent of thermodynamic con- straints is therefore also possible. Far more subtle possibilities will no doubt be exploited soon. Already C02 laser pumping of levels gives sufficiently large photo- stationary excited state populations for them to lase in the far i.r. The i.r. lasers used in i.r. photochemistry can be readily scaled up to give very large beam diameters (several inches) and their efficiency is already sufficiently high to attract commercial users. We can undoubtedly look forward to some exciting developments in the i.r. photochemistry field in the near future.