Spectroscopic Studies of Open-shell Organic Cations in the Gaseous Phase : Chlorodiacetylene and Dichlorodiacetylene Cations BY JOHN P. MAIER, OSKAR MARTHALER, LIUBOMIR MISEV AND FRITZ THOMMEN Physikalisch-Chemisches Institut der Universitat Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Received 1st December, 1980 The application of the techniques of emission, laser-induced excitation and photoelectron-pho- ton-coincidence spectroscopies to open-shell organic cations which decay radiatively are illu_strated by such studies on chlorodiacetylene (A”%* ++ X’IIfi) and dichlorodiacetylene (A”211,q, ++ X’IT,,,) cations. The emission and the laser-induced excitation spectra yield the vibrational frequencies of most of the totally symmetric fundamentals for these cations in their 2 a n d xstates.Higher-resolu- tion emission spectra reveal further structure. The detection of coincidences between cnergy- selected photoelectrons and emitted photons show that the radiative process depletes Ievels at least up to 3000 cm-l within their xstates and the data give their fluorescence quantum yields and cascade- free lifetimes. It is shown how these methods provide complementary information on the spectro- scopic structure and decay behaviour of open-shell cations. The cations of chlorodiacetylene, Clf CEZC>~H+, and of dichlorodiacetylene, C ~ ~ C E C ~ ~ C I + , belong to the hundred or so open-shell organic cations which have been found to decay radiatively in the gaseous phase.’P2 This has been established by recording the emission spectra of such cations and by identifying the transitions by reference to photoelectron spectroscopic data.In all these cases the band systems are the result of electronic transitions from one of the two lowest excited doublet states (2x, or 28) to the ground state (2y) of the cations. The detection of the radiative decay has opened up the possibilities in the application of well-established and of newly developed spectroscopic techniques to the structure and decay behaviour of such organic open-shell cations. High-resolution emission spectroscopy, which historically has played a vital role in elucidating electronic and geometric structures of gaseous species, belongs to the former ~ategory.~ By this means a variety of triatomic cations has been investigated4 as well as the cations of diacetylene and recently of the halogenated benzenes.6 The newer methods include laser-induced fluorescence which was first applied to the molecular cation N2+ ’ and subsequently to some other organic cations’ which we found earlier to decay radia- tively, and coincidence measurements.The latter depend on monitoring emitted photons at the same time as energy-selected electrons’ or mass-selected ions,l0 after formation of the cations by photoionisation. In this article the results obtained for chlorodiacetylene and dichlorodiacetylene cation using the emission, laser-induced excitation and photoelectron-photon coin- cidence spectroscopic techniques are presented. The emission spectra of these two182 OPEN-SHELL ORGANIC CATIONS open-shell cations, which lie in the 500-700 nm wavelength region, correspond to the following electronic transitions: 1 1 , 1 2 and the states characterized by R = 3/2 lie energetically below R = l/2.I3 The study of these transitions by the above-mentioned methods yields the vibrational frequencies of most of the totally symmetric fundamentals for these cations in their 2 and 2 states as well as their lifetimes and fluorescence quantum yields in selected levels of their 2 states.As the apparative details have already been presented for the three techniques in earlier articles, only the salient features of the measurements are sum- marized below in the sections dealing with each approach in turn. EMISSION SPECTRA The emission spectra of chlorodiacetylene and dichlorodiacetylene were recorded with a crossed electron-sample beam apparat~s.'~ The excited cations were produced by a 20-40 eV electron beam impinging on an effusive jet of the sample and the result- ing photons were dispersed by a 1.26 m monochromator, e.g.The spectra were recorded on-line with an LSI 11/03 micro-computer using single- photon counting electronics. The emission spectra covering the whole band systems were recorded with optical resolutions of 0.16 nm. The spectra were presented and vibrationally analysed pre- viously;l'~l2 they are nevertheless reproduced in fig. 1 and 2 for comparison with the laser-induced excitation spectra (uide infra) and with the high-resolution scans of the 0; bands (insets of fig. 1 and 2). For the latter measurements a resolution of 0.008 nm was achieved using sample pressures in the collision region around Torr and electron currents of 1-2 mA.It was concluded on the basis of the emission and photoelectron spectra that the R = 3/2 t-) 3/2 and R = 1/2 t) 1/2 components of the J2n, ---f X2n* band systems of chlorodiacetylene and dichlorodiacetylene cation lie energetically so close together that each emission band contains both As in the photoelectron spectra the spin-orbit splittings (0.020 & 0.005 eV) are resolved only for the bands corresponding to the ionization to the ~ ' l 3 ~ states, the implication is that the spin- orbit splittings are also of this magnitude for the f 2 h states. The high-resolution recordings of the " 0; band " show that it is composed of several peaks (fig.1 and 2). A possible interpretation is that it shows that the R = 3/2 and R = 1/2 transitions are separated by ca. 1 and 2 cm-I in the case of chlorodi- acetylene and dichlorodiacetylene cations, respectively. The remaining vibrational structure is due to sequence bands. More definite assignment should be possible when some of the other bands are also recorded with a corresponding resolution. The vibrational frequencies of five fundamentals of chlorodiacetylene and of theJ . P . MAIER, 0. MARTHALER, L . MISEV AND F . THOMMEN 183 I - 19720 19730 1 .n 1 I 1 I 1 I I 1 I 1 1 I 1 1 1 I I I 1 I I I I I I I 17- 14000 16000 18000 20000 C /cm- FIG. 1,-The zzII, -+X2II* emission system of chlorodiacetylene cation. The whole spectrum was recorded with an optical resolution of 0.16 nm, whereas the region around the 0: band was scanned with 0.008 nm (f.w.h.m.), A vibrational assignment of some of the prominent bands is indicated.three C 1; modes of dichlorodiacetylene could be inferred from the low-resolution spectrum to an accuracy of 5 10 cm-'. The assignments are indicated in fig. 1 and 2 and the frequencies are given in table 1 where the molecular values 15*16 are also given. In view of the laser-induced excitation spectra to be discussed in the next section, several of the sequence transitions can no,w be identified in both sets of spectra. In addition, the frequencies inferred for the A2n* state from the hot bands apparent to higher energy of the 0; bands (fig. 1 and 2) are consistent with the interpretation of the excitation spectra.I' 19090 19100184 OPEN-SHELL ORGANIC CATIONS TABLE 1 .-VIBRATIONAL FREQUENCIES (cm- I) OF THE TOTALLY SYMMETRIC FUNDAMENTALS OF CHLORODIACETYLENE (c ') AND DICHLORODIACETYLENE (c;) CATIONS IN THEIR GROUND AND FIRST EXCITED ELECTRONIC STATES INFERRED FROM THE EMISSION (em.) AND LASER EXCITATION (exc.) SPECTRA (fig. 1-4). Uncertainty of all values i 10 cm-'. Values for v 8 of z symmetry are deduced from 2 v8. The ground molecular state values are taken from ref. ( I 5) and (1 6), respectively. v1:v, v2:v, (C-H) (C=C) CI-C=C-C=C-H X'T, + 3327 2252 CI-C-C-C=C-H+ f211a em. 2190 X2na em. exc. 2150 v1:vs v2:v, cI-c~c-c=c-cI X'C; 2245 1202 C1-- C-C-C-C-Cl+ em. 2210 1315 A213a,l, em. 1170 exc. 1165 (C=C) (C-C) v3:va v4:v, v5:v, v8:(sym. (C-C) (C-C) (C-X) bend) 2071 1093 525 335 1910 1180 540 305 550 305 1080 530 3 10 1085 520 310 v3 : v, 330 3 90 3 90 365 (C-W LASER-INDUCED EXCITATION SPECTRA Laser-induced excitation spectra of chlorodiacetylene and dichlorodiacetylene are shown in fig.3 and 4. These were obtained by producing the cations in their ground states by Penning ionization using helium or argon met as table^'.^ and by pumping the electronic transition with a tunable, pulsed, dye laser while the undispersed fluorescence was sampled, e.g. t HeW c IfC= c+* c I The signals were accumulated by a transient digitizer interfaced to an LSI 11/03 micro-computer which also steers the 1 a ~ e r . I ~ The spectra shown have been recorded with a laser bandwidth of 0.02 nni and have been corrected for the laser intensity.The spectra were actually put together from three scans using the appropriate dye so- lutions. The wavelengths were calibrated using atomic emission lines produced by excitation of the helium and argon metastables. Vibrational assignments of some of the prominent bands in the xzlln t f213n laser excitation spectra are indicated in fig. 3 and 4. Also apparent are a few bands due to laser excitation of the C2 Swan system. This fragment is produced by the Penning excitation processes. In table 1 are collected the vibrational frequencies inferred from the spectra, which correspond mainly to the totally symmetric, C + , or C,+ , funda- mentals in the J2& state of these cations. The few hot bands evident to lower energy of the 0; bands yield vibrational frequencies for the f211a state, in agreement with the emission spectra. Some of the sequence transitions are also apparent, especially to lower energy of the bands assigned to the progressions and combinations of the fundamentals (fig.3 and 4).J . P. MAIER, 0. MARTHALER, L. MISEV AND F. THOMMEN 185 c2 i 19000 20000 21000 22000 V /cm-' FIG. 3.-Laser excitation spectrum of the X'IIQ t ~ T I Q transition of chlorodiacetylene cation re- corded with 0.02 nm bandwidth. A vibrational assignment of some of the bands is indicated. The maxima of the 0: bands are found at 19 721 & 3 cm-' and 19 092 3 cm-l in the laser excitation spectra of theA2& t ~'IIQ transitions of chlorodiacetylene and dichlorodiacetylene, respectively. This is in good agreement with the recordings of the 0; emission band with the higher resolution where the further structure is resolved (fig.1 and 2). Finally, it is seen that the excitation spectra cover an energy range of 19000 20000 i~ /cm-l FIG. 4.-Laser excitation spectrum of the Al'II,,, -+ X'IIn,, transition of dichlorodiacetylene cation recorded with 0.02 nni bandwidth. A vibrational assignment of some of the bands is indicated. only about a third of that of the emission spectra. could be detected. photoelectron-photon-coincidence measurements are considered. Outside the shown range, no bands The reason for this becomes apparent when the results of the PHOTOELECTRON- P H 0 TON- C 0 1 N C I D EN C E S P E C TROS COPY The photoelectron-photon-coincidence apparatus consists of two parts; one for the detection of energy-selected electrons and one for any emitted photons, following186 OPEN-SHELL ORGANIC CATIONS photoioni~ation.~+’~ The individual events in each channel are then sampled in delayed coincidence.True coincidences are obtained only if the ejected electron and emitted photon originate from the same ion which was produced in the state defined by the kinetic energy of the photoelectron, t?K.E., This technique has been applied to chlorodiacetylene and dichlorodiacetylene, to show that vibrationally excited cations in the .,@IIQ states decay radiatively, and from the quantitative measurement to determine the fluorescence quantum yields, qF(u’), and lifetimes, ~ ( u ’ ) , of the selected levels u’. True coincidences were detected for chlorodiacetylene and dichlorodiacetylene cations at the internal energy within their X2n, states as indicated above the photo- electron bands in fig.5 and 6. These energy locations correspond to slices of 100 meV centred on the vibrational peak maxima, which are due to the progressions of the totally symmetric C-C stretching fundamentals, v4 and v 2 (see table 1) for chloro- and COINCIDENCES ; 0 I H 9.0 10.0 11 0 12.0 13.0 14.0 15.0 IE /eV FIG. 5.-Photoelectron-photon-coincidence curve for the 0’ level of the A”*IT, state of chlorodiacety- lene cation: N,, 290 Hz; N r , 0.68 Hz; accumulation time, 17 h. The He(1a) photoelectron spec- trum, recorded under the coincidence conditions, shows the internal energies selected, dichloro-diacetylene, respectively. The results show that the radiative channel also depletes the 4“ and 2“ IZ = 1-3 vibrational levels of their A2l7* states.Thus in the emission and excitation spectra population of vibrational levels up to ca. 3000 cm-l within the z211n states should be evident. In the emission spectra only a few weak bands are apparent on the high-energy side of the 0; bands (fig. 1, 2) and therefore the intensity of the radiative transitions especi- ally from the more highly excited vibrational levels is concentrated in their sequence transitions. This is as suggested by the resolved structure in the higher-resolution recording of the 0; emission bands and the assignment of some of the stronger bands to sequence transitions in the emission and excitation spectra (cfi fig. 1-4).On the otherJ . P . MAIER, 0. MARTHALER, L . MISEV AND F . THOMMEN 187 0 COINCIDENCES 600 TIME 9.0 10.0 11.0 12.0 130 14.0 15.0 IE lev FIG. 6.-Photoelectron-photon-coincidence curve for the 0’ level of the A”zIIn,u state of dichlorodi- acetylene cation; N,, 280 Hz; NT, 0.39 Hz; accumulation time, 20 h. The He(1cr) photoelectron spectrum, recorded under the coincidence conditions, shows the internal energies studied. hand in the excitation spectra bands lying more than ca. 2000 cm-’ above the 0: bands are too weak to be detected. The reason for this becomes apparent when the fluorescence quantum yields and lifetimes are inspected. The ~ ~ ( 0 ’ ) values were obtained by measurements of the rates of detection of true electrons, N,, and of true coincidences, NT, because it can be shown that NTINe = f n V V F ( v ’ ) and the collection efficiency for photons,fn,, has been absolutely calibrated in the 200- 900 nm wavelength range.” In order to attain the 10% accuracy for the pF(v’) values ca.lo7 true electrons have to be counted. Typical accumulation times and count rates under the coincidence measurements are given in the legends to fig. 5 and 6. In table 2 are presented the determined ~ ~ ( 0 ’ ) values as well as the cascade-free lifetimes which were extracted by a weighted least-squares linear fit to a semi-logarith- mic plot of the decay part of the coincidence curves (cf. fig. 5 and 6). For comparison, TABLE 2.-FLUORESCENCE QUANTUM YIELDS, VF(U’). AND LIFETIMES Z(V’) OF CHLORODIACETY- LENE AND DICHLORODIACETYLENE CATIONS IN THEIR A STATES CORRESPONDING TO THE INDICATED POSITIONS IN FIG.5 AND 6 The lifetimes given in the last column were obtained using electron impact excitation. 11,12 cation state V&’) z/ns r/ns Cl-C=C-C=C-H+ L2n, 0’ @) 0.79 f 0.08 41 f 2 41 j = 2 5l 36 i 2 s 37 k 2 4’ @ 0.37 -f 0.04 33 &- 3 42 @ 0.18 f 0.02 30 1 3 43 @ 0.06 f 0.01 2l @ 0.14 f 0.02 1 7 f 3 19&3 22 @ 0.035 i 0.007 Cl-C=C-C=C-CI+ A”zII*,, 0’ @ 0.47 f 0.05 2 1 1 2 2 1 f 3188 OPEN-SHELL ORGANIC CATIONS the lifetimes measured by means of a pulsed electron beam (ca. 20 eV) excitation are included.11*12 There is good agreement between the two sets of measurements. From the data given in table 2, the radiative and non-radiative rate constants as func- tion of internal energy can be directly obtained.It is seen that the qF(u’) values fall off with increasing internal energy within the A211a states of chloro- and dichloro-diacetylene cations (table 2). In the laser- excitation spectra the intensity of the bands is proportional to the yF(v‘) value of the emitting level U‘ and to the probability of populating that level initially. Relative to the zeroth vibrational level this may be judged by the Franck-Condon profile of the relevant photoelectron band. Thus, for example in the case of chlorodiacetylene cation the intensity of the 4; band in the excitation spectrum is expected to be about an order of magnitude less than that of the 0: band. This is approximately the ob- served ratio (fig. 3). On the other hand, the q+(u’) data for dichlorodiacetylene cation indicate that the 2; band should be even weaker relative to the 0: band and further- more the absolute values are also expected to be a factor of two weaker than for chlorodiacetylene cation (table 2).In fact the 2; band could not be discerned above the noise in the excitation spectrum of dichlorodiacetylene. CONCLUDING REMARKS It has been shown how the application of the emission, laser-induced excitation and photoelectron-photon-coincidence spectroscopic techniques provide detailed, and complementary, information on the structure and decay behaviour of open-shell organic cations in the gas phase. These methods can be applied to all those cations whose radiative decay is manifested1*’ and in this article such studies on chlorodiacety- lene and dichlorodiacetylene cations in their X’n, and Z’IIa states have been described.The emission spectra yield the vibrational frequencies, of mainly the totally sym- metric fundamentals for the ground cationic states, and with the improvements in resolution, at present down to 0.004 nm, further detail and in some of the smaller cations rotational structure becomes apparent.2 In addition the lifetimes of the cations in the lowest vibrational levels of the excited state can be measured. The laser-induced excitation spectra provide the corresponding vibrational frequency data for the excited electronic state and the presently employed resolution of 0.02 rim can be increased by a factor of ten. The photoelectron-photon-coincidence measure- ments can be used first of all to prove that selected levels of an excited state decay radiatively and the wavelength range of the emitted photons can also be established.18 This can provide valuable information in the rationalization and in the investigations of the cations using the emission and laser techniques.Furthermore, the absolute fluorescence quantum yields and cascade-free lifetimes can be determined. These data enable one in turn to discuss the radiationless decay of excited open-shell cations as function of the internal energy/vibrational excitation because the radiative and non-radiative rate constants can be derived. This work has been supported by fhe Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung (Project No. 2.212.0-79, E 35). Ciba-Geigy SA, Sandoz SA and F.Hoffmann-La Roche & Cie. SA, Base1 are thanked for financial support.J . P . MAIER, 0. MARTHALER, L . MISEV A N D F. THOMMEN 189 ‘See J. P. Maier in Kinetics of Ion-Molecule Reactions, ed. P. Ausloos (Plenum Press, New York, 1979); J. P. Maier, Chimia, 1980, 34, 219 for reviews of this field. J. P. Maier, 0. Marthaler, L. Misev and F. Thommen, in MoIecuIar Ions, ed. J. Berkowitz (Plenum Press, New York, 1981). ’ See G. Herzberg, Molecular Spectra and Molecular Structure (van Nostrand, New York), vol I, 1950 and vol. 111, 1966, and references therein. G. Herzberg, Quart. Rev. Chem. SOC., 1971,25,201; S. Leach in The Spectroscopy of the Excited State (Plenum Press, New York, 1976). J. H. Callomon, Can. J. Phys., 1956, 34, 1046. C. Cossart-Magos, D. Cossart and S. Leach, Mol. Phys., 1979, 37, 793; C. 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