Faraday Discuss. Chem. SOC., 1983, 75, 77-88 Intramolecular Decay of Some Open-shell Polyatomic Cations BY JOHN P. MAIER, MARTIN OCHSNER AND FRITZ TWOMMEN Physikalisch-Chemisches Institut der Universitat Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Received 16th December, 1982 Radiative and non-radiative relaxation processes of selected open-shell polyatomic cations in various vibrational levels of their lowest excited electronic states have been studied by two complementary techniques. Fluorescence quantum yields and lifetimes are obtained by photoelectron-photon coincidence measurements, and laser-excited fluorescence of rotation- ally cooled cations provides lifetime data at much higher resolution and-for further levels. The information forthc_oming on the intramolecular decay of isojated CO*, B2 Ei, ClCN+@rI, H-(-CEC-)~-H + A 211v and 1,2,4,5tetrafluorobenzene + B2Bl, is discussed.INTRODUCTION Investigations of the radiative and non-radiative relaxation behaviour of open- shell polyatomic cations in their lowest excited electronic states in a collision-free environment have been systematically carried out in recent years.x.2 The opportunity for such studies was provided by the detection of the radiative decay of certain types of open-shell organic cations from their 2z or 2B excited states to their ground states 22.3 As a consequence, several techniques have been applied to probe their spectro- scopic structure in the gas phase 4*5 and to follow the radiationless processes quan- ti tative1y.l~ For spectroscopic purposes, our most recent approaches involve the study of the emission spectra of rotationally cooled cations prepared by electron-impact excitation of seeded helium supersonic free jets, and of the laser excitation spectra of rotationally and vibrationally cooled cations.6 These techniques enable one to deduce vibrational frequencies of the cations in their ground and excited states to within 5 1 cm-l, or better.With the smaller cations the rotational structure can be probed using the laser excitation method with a resolution of 0.001 nm. In studies of radiationless processes the initial phase involved the measurements of the lifetimes of the cations in selected vibrational levels within the excited state using the appropriate emission bands.3 This was followed by quantitative determinations of the radiative and non-radiative rate constants for the pathways depleting state- selected cations from the absolute measurements of the fluorescence quantum yields and lifetimes by a photoelectron-photon coincidence technique.By this means the radiative, k,(u'), and non-radiative, knr(v'), rate constants of the selected level u' have been evaluated for most types of open-shell organic cations that decay radiatively. These studies and a summary of the findings have recently been reviewed.l In this contribution we present the results of such quantitative investi- gations of the radiationless processes of some selected cations using the photoelectron- photon coincidence apparatus with an improved electron energy resolution (ca. 2078 INTRAMOLECULAR DECAY OF POLYATOMIC CATIONS meV) as well as the laser excitation measurements of lifetimes at much higher resolu- tion (ca.0.1 meV). By combining the two sets of results the non-radiative rate dependence on the vibrational modes excited can be quantitatively established. To illustrate these aspects the triatomic cation CO t is considered, since non-unity fluorescence quantum yields are observed for the ’X: state even though the frag- mentation pathway is not accessible at these energies, and the ClCN+ species in the 21J state provides a further example of pronounced non-exponential decay which can be followed as a function of the vibrational excitation. The other cations chosen, of diacetylenes and of 1,2,4,5-tetrafluorobenzene, show characteristic decay in the statistical limit, and the non-radiative rates of the various modes could be determined using the two techniques in conjunction.EXPERIMENTAL The principle of the photoelectron-photon coincidence technique ’ is indicated by the scheme shown in fig. 1, where the experimental arrangement is schematically depicted. The apparatus has been described in detail elsewhere.’ An effusive beam of sample gas is ionised by a collimated beam of He(1a) photons. The resulting photoelectrons (eKeE.) are LENS SYSTEM TO PM He LAMP LIGHT BAFFLES ELECTRON LENSES H EM1 SPHERICAL ANALYSER IONISATJON REGION n n u u IM Fig. 1. Arrangement of the photoelectron-photon coincidence apparatus used for the measurements of fluorescence quantum yields and lifetimes of state selected cations.energy-selected by a 180” hemispherical analyser (5.5 cm diameter) and are then used as a time reference for the creation of cations in a specific excited state, e.g. 2J(u’). The time differences between these electrons and wavelength-undispersed photons originating from the ionisation region are measured and then stored in a multichannel analyser. If the excited cation in the selected state ‘A(u’) decays radiatively, the result is a curve of true coincidences superimposed on a uniform background of random coincidences. From such a curve the fluorescence quantum yield, qF(u’), can be evaluated using the relationship NTINe = VF(U’)h”(4J. P. MAIER, M. OCIISNER AND F. THOMMEN 79 @ where NT is the rate of true coincidences and IV, is the rate of photoelectrons defining the level v' of the excited cation.8 The wavelength-dependent probability of detecting photons, &(A), can be determined by coinc_idence measu5emen ts using cations with fluorescence quantum yields of unity [e.g.N f (B 'EL), CO: (A 211,)] and is found to be in the range of ( 3 4 ) x For the measurements presented in this article the electron band-pass has been reduced to 20-25 meV (for ca. 2-3 eV electrons) by means of an electron lens system as sketched in fig. 1. The probability of detecting energy-selected electrons, fe, remains ( 3 4 ) x lob3. Typical accumulation periods to attain sufficient coincidence statistics are 15-20 h with N, = 50-200 Hz, while the rate of undispersed photons is often between 5 and 10 kHz. The sample pressure is held in the range 10-4-10-3 Torr * to ensure collision-free con- ditions.The measurements of lifetimes of cations in selected levels of the excited state at much higher resolution were carried out by laser-excited fluoIescence using the arrangement sketched in fig. 2. This apparatus has been used previously for the recording of laser excitation spectra for 250 < R/nm < 600. DIGITIZER M I C R O - COMPUTER - INTER-- - FACE t-- 4 Ini H PENNING I IONISATION I Fig. 2. Scheme of the experimental arrangement for the lifetime measurements by laser excited fluorescence. of open-shell cations.l0 The cations are prepared in the ground states, 2z, vibrationally and rotationally cooled to liquid-nitrogen temperature by means of Penning ionisation with_argon metastables and collisional relaxation with the precooled rare-gas ~ a r r i e r .~ The 2 A ( ~ ' ) t 'f(u" = 0) excitation is produced by a dye-laser pumped by a nitrogen laser, operating with 0.02 nm bandwidth, typically at 30 Hz. The data accumulation was accomplished by a transient digitizer-microcomputer system as indicated in fig. 2. The number of individual decay curves added depended on the intensity of the respective excitation bands, and was typicalIy ca. 600. The lifetimes were extracted from a weighted least-squares fit to the decay curves and the overall time resolution was ca. 5 ns. Although the total pressure in the excitation region was ca. 0.2 Torr, for the short lifetimes (<lo0 ns) measu_red no significant differences were found to the values (e-g.for the zeroth levels of the ' A states) obtained * 1 Torr = 101 3251760 Pa.80 INTRAMOLECULAR DECAY OF POLYATOMIC CATIONS previously either by the photoelectron-photon coincidence or the electron-impact emission approaches operating at much lower pressures ( < 10- Torr). EXAMPLES AND DISCUSSION co; B2zc,' Of the triatomic open-shell cations decaying radiatively from their lowest excited electronic states, many have lifetimes in the microsecond range, i.e. CS t (2 211 .), H20+ (k 'la,>, H2S+ (2 2A1) and XCN+ (2 2Z+) with X = Cl,Br,I.3 These are consequently not suitable for lifetime or fluorescence quantum yield studies by either the photo- electron-photon coincidence or the outlined laser excitation techniques, because of the restricted time window.The cations with the shorter lifetimes are on the other hand used to calibrate absolutely the coincidence apparatus, since they often have fluorescence quantum yields of unity, e.g. C o t (J211U), N20+ [J2Z+ (v' = O)]. In two cases predissociation competes with the radiative relaxation and leads to less than unity yields, i.e. N20+ [2 'Z+ (u' # O)], COS+ [z 211 (u' In addition, fluorescence quantum yields less than unity were measured for C o t B" 2Ei l1 and l2 levels, although fragmentation channels are energetically inaccessible for these levels.8 The measurements have now been repeated using the coincidence apparatus with improved resolution. Compared to the previous determinations, the progressions of the vl vibration originating in the 2 211 state and in the 2ZC= state are now sufficiently discernible, so that cations formed initially in the 1 " n = 1-3 0)].8 14.0 17.0 18.0 I/eV Fig.3. He(1or) photoelectron spectrum of COz, recorded under coincidence conditions, with constant band-pass of ca. 20 meV.J. P. MAIER, M. OCHSNER AND P. THOMMEN 87 teveIs of the B' 2Xi state could be relatively cleanly selected. This can be seen in the photoelectron spectrum, shown in fig. 3, recorded under coincidence conditions. The measured photodectron-photon coincidence data are presented in table 1 and confirm the earlier results within the error limits. The lifetimes for the 1 p1 = 1-3 levels are the same as for the vibrational levels of the 2 TI state (ca,120 ns, 1 with n = &4), and reflect the radiationless coupling between the and A states.Table 1, Fluorescence quantum yields and lifetimes within the B' E: state of CO; O0 1.00 f 0.05 139 f 7 l 1 0.85 & 0.09 119 z t 12 l2 0.64 f 0.09 120 f 12 l 3 0.57 f 0.09 120A 12 This coupling phenomenon has been discussed in several articles.2b11*12 Most recently, it was estimated from branching-ratio measurements using optical and electron spectroscopies that 5&60% of the state population crosses over into the 2 state,I3 and a high-resolution study of the rotational structure yielded a further insight into the interaction of the The consequence of the coupling is that the emitted photons are distributed over the wavelength region. For the 8 2Et Oo Ievel it was shown that two thirds of the photons are emitted in the d < 330 nm region, one third in the 330-450 nrn region and no coincidences were detected for 3, > 450 nm.*J5 The fluorescence quantum yieIds for the 1" levels given in table 1 were evaluated assuming a similar wavelength intensity distribution of the emitted photons as for the 0' level and that they still all fall below 450 nm.That is not the case could now be shown by coincidence measure- ments where the photons were wavelength selected using filters. For the l2 Ievel, only 50% of the intensity was found in the region A < 330 nm and true coincidences were measured even for R > 450 nrn. Quantitative Coincidence determinations, which are extremely time consuming, yield the following qF(12) values in the indicated wavelength regions: 0.48 :& 0.05, 250-330 nm; 0.15 & 0.03, 330-400 nm; 0.11 0.02, 400-450 nm.Because the sensitivity of the photomultiplier falls off with in- creasing wavelength, and especialIy rapidly above 750 nm, only a lower limit qF( 12) z 0.08 for A > 450 nrn can be given. It therefore appears that with increasing vib- rational excitation (1" for JI = 1-3) progressively more intensity of the transition is red-shifted to regions beyond 450 nrn as a result of the coupling with the LT 211 11 state and leads to apparently decreasing fluorescence quantum yields (cf. table 1). ClCN+ B zn Non-exponential decay, under collision-free conditions, has now been established for a few of the smaller polyatomic open-shell cations, i.e. CS; (B %:), X-C=N+ (8 %) and X-C=C-Hf (A2n) with X = Cl,Br,X.16 This decay behaviour was appar- ent initially in the electron excitation approach and has been confirmed by the p h ot oelectron-phot on coincidence measurements, which exclude cascad ing processes.zfl states of X-C=C-H+ X = C1,Br have been discussed in recent articles in detail.lJ6 The main feature of the decay curves is that the short component decreases and the long component increases in time, and that the amplitude ratio of the short to Iong components The coincidence data for selected levels within the82 INTRAMOLECULAR DECAY OF POLYATOMIC CATIONS increases with the excess energy. - Corresponding measurements have now been carried out for CI-C=N+ in the B 211 state. Photoelectron-photon coincidences were accumulated for six levels within the 211 state (fig.4) and these correspond to the excitation of the v,, v(C-CI) stretching vibration. The non-exponential behaviour is evident in the decay curve shown in the inset of fig. 4. loo0 1 I 1 I I I 1 I 1 I 12.0 13.0 14.0 15.0 16.0 I/eV Fig. 4. He(1ct) photoelectron spectrum of ClCN and a semilogarithmic plot of a photoelectron- photon coincidence curve accumulated at location 0. The levels studied are indicated by the arrows above the spectrum and the bars show the energy band-pass (ca. 20 meV). The lifetimes, zi, and amplitudes, Ai, for the short (i = 1) and long (i = 2) com- ponents have been evaluated by fitting a model of two exponentials to the decay part of the coincidence curves. The fluorescence quantum yields, ( ~ l ~ , of the two com- ponents were then fitted to the relationship Ptot = Pl + v2 = A171 + &2.These results are summarized in table 2. At locations 0, @ and @ only mono- exponential decay curves could be observed. The reason for this becomes clear when the semilogarithmic plot of a coincidence curve accumulated at location 0 is con- sidered. Pronounced biexponential decay can only be observed with this method if the lifetimes of the two components differ at least by a factor of two. The uncertain- ties of qtot are &lo% and &20% for the estimated qi and zi values. Thus it is assumed that for location @ the lifetimes and amplitudes of the two components are nearly the same. However, for locations @ and @ the fluorescence quantum yield ql is expected to be too small to be observed. Nevertheless, the trend that with increasing vibrational excess energy the lifetimes z1 become shorter and z2 longer, respectively, is demonstrated (table 2).This decay behaviour can be rationalized using the " intermediate-case " descrip-J. P. MAIER, M. OCHSNER AND F. THOMMEN 83 tion of radiationless transition theory.” Isoenergetic vibronic levels of the lower states, 8 2n and 2Z+ provide both a quasi-continuum as in the statistical limit and discrete levels as in the resonance limit. Thus the prompt decay of the system, described by ql and zl, reflects the radiative decay to the ground state, &x, and the Table 2. Fluorescence quantum yields and lifetimes for the prompt (1) and long (2) decay components and their relative amplitude ratio, A1/A2, measured at selected ionisation energies I,within the n state of ClCN+. The labels correspond to the locations in fig.4. 0.99 280 (280) 0.84 0.31 210 0.53 400 1.1 0.63 0.18 140 0.45 510 1.5 0.55 0.1 1 90 0.44 590 1.6 0.49 0.49 650 0.41 0.41 780 15.16 0 15.22 0 15.29 0 15.35 Go 15.42 0 15.48 0 irreversible radiationless transition form the state levels to the quasi-continuum. The long component is the result of a resonant coupling of the initially populated level with discrete isoenergetic levels of the 2 and 8 states followed by radiative relaxation to the ground state. With increasing vibrational excess energy the width of the distribution of the oscillator strength of the state levels increases and therefore the lifetimes 22 become longer. This dilution of oscillator strength has already been observed for the halogenoacetylene ~ati0ns.l~ Although the lower-lying 2Z+ state carries oscillator strength to the x2n state and has a lifetime around 4.4ps,18 radiation- less or radiative decay from the to the 2 state levels followed by radiative decay to the 8 state would result in long lifetimes but not in the observed increase of z2 with excess energy. H-(-C=C-)-2H+ A nu The relaxation of diacetylene cations in the A 213u state has been investigated earlier using the photoelectron-photon coincidence approach, sampling energy slices of ca.600 cm-l centred on the bands due to the excitation of the v3, symmetric C-C stretching mode.19 Values of qF(t)’) of less than unity were measured {qF(Oo) = 0.72 and 0.80 for diacetylene and [2H2]diacetylene cations} in the 2 state, and these decrease with excess energy.The non-radiative pathway depleting the selected level is assumed to be internal conversion to the 8 state manifold. For both diacetylene cations the rate constants for the radiative decay are found to be constant over 2500 cm-’ excess energy, k,(u’) = 1 .O x lo7 s-l. The dependence of the non-radiative rates on the vibrational levels populated can be further investi- gated using the laser excitation technique. The excited level can be selected with a resolution of 1 cm-’ and from the lifetime data the rates, k,Ju’), for the radiationless transition can be inferred according to z(u‘)-’ = k,(d) + knr(v’). Such lifetime determinations have been carried out using the prominent bands in the 2 2n t 8 211, laser excitation spectrum of the two diacetylene cations.The narrowing of the vibronic bands by cooling the species to liquid-nitrogen temperature enables the transitions to be clearly selected. Lifetimes of the levels corresponding to the excitation of the three ZC,+ (Dmh symmetry classification) stretching vibrations,84 INTRAMOLECULAR DECAY OF POLYATOMIC CATIONS i.e. v1 (-.C-H), v, (CFC) and vj (C-C), as well as the overtone of the v7 bending mode could be measured. The evaluated data are presented in table 3 and are associated with uncertainties of *5%. The displacements, AC, of the levels relative to the zeroth level are taken from the gas-phase excitation spectrum and the assign- ments correspond to those made in the matrix study of this transition.20 Table 3.Lifetimes of H-CGC-CC-C-H+ and D-CIC--C=C-D+ in various vibrational levels of the 12rIn, state measured by laser excitation. The values in parentheses are from the earlier photoelectron-photon coincidence determination^.^' H-C=C-CEEC-H+ D-CsC-GC-D' level AF/crn - zH(u)'/ns AC/crn - rD(v')/ns O0 3l 72 32 3172 74 2l 3272 3174 l1 0 805 860 1 592 1 664 1 684 1 959 2 453 2 477 2 815 72 (72) 62 (62) 60 58 (59) 61 58 50 57 56 49 0 783 835 1551 1 625 1 650 1887 2 390 2 446 2 746 80 (79) 73 (72) 75 70 (66) 69 69 62 70 70 68 The lifetimes for the Oo band as well as for the 3n, n = 1,2 levels are in good agree- ment with the values obtained from the coincidence measurements l9 which are given in brackets. Overall, there_ is a decrease of the lifetimes with increasing vibrational excess energy within the A 21-Iu state.Nevertheless, the data show that the non- radiative rate not only depends on the internal energy but also on the vibrational mode excited. The shorter the lifetime the more active the excited mode for the radiation- less process and it seems that the high-frequency stretching modes v1 and v2 are the most active ones. However, in the case for the 1' levels the ratio knr(l1)/knr(O0) is significantly higher for the di hydro- than for the dideutero-diacetylene cation. This behaviour may reflect a deuterium effect or else is simply the result that less energy is required to excite the l 1 level in the dideuterodiacetylene cation. That the C-H stretching mode is the more active mode for the radiationless process than the C-D one had already been considered in the coincidence study.19 The ratios krr(u')/kFr(u') for the 0' and 3 ", n = 1-3 levels are ca.1.5, indicating such a deuterium effect. For the 3", 72n levels and their combination bands, which are coupled by Fermi resonance,21 similar lifetimes are expected. These are indeed equal for the levels with similar excess energy within the 2 211 states (table 3). 1,2,4,5-TETRAFLUOROBENZENE CATION: 'Blu Halogeno-substituted benzene cations in their excited states, B, also show decay features characteristic of the statistical limit; 22 monoexponential decay, qF(u') < 1 and k,,,(u') increase with excess energy. To gain further insight into the relaxation behaviour a higher-resolution coincidence and laser excitation study of 1,2,4,5- tetrafluorobenzene cation was undertaken.The latter species was chosen because of the relatively high symmetry (&) and that qF(B 2Blu Oo) < 1 *23J. P. MAIER, M. OCHSNER AND F. THOMMEN 85 In the coincidence experiment the fluorescence quantum yields and lifetimes of the levels associated with the totally symmetric v3 and v, vibrational modes, and their combinations, could be determined (cf. fig. 5) with a band-pass of ca. 160 crn-l. The results are given in table 4, and it is seen that a significant increase of the non- I 1 1 1 I I I I 1 9.0 10,o 11,o 12,o 13.0 IleV Fig. 5. Photoelectron-photon coincidence curve for the Oo level of the 2Blu state of 1,2,4,5- tctrafluorobenzene cation; N, = 110 Hz, NT = 0.2 Hz accumulated in 21 h and the corres- ponding He(1a) photoelectron spectrum showing the levels studied.radiative rates for the levels of the v3, vg vibrations is apparent. The radiative rate is found to be constant within the error limits; k,(v’) = 1.9 x lo7 s-l, allowing further non-radiative rates to be calculated from the z(u’) data obtained for various levels by laser excitation with ca. 1 cm-l bandwidth. The non-radiativc rate describes Table 4. Fluorescence quantum yields, pF(v’), lifetimes, ~ ( u ’ ) , and non-radiative rates, knr(v’), at selected ionisation energies, I, within the 281. state of 1,2,4,5-tetrafluorobenzene cation. The labels correspond to the numbering in fig. 5. f 2.35 0.63 f 0.05 33 & 3 1.1 + 0 . 3 0.53 0.05 29 & 2 1.7 f 0.4 12.44 12.52 0.48 i 0.05 26 + 3 2.1 i- 0.4 12.61 0.38 + 0.04 21 3 3 3.1 i O .6 12.70 0.34 f 0,04 19 2 3.7 i 0.7 12.79 0.24 i 0.03 t l 5 6.0 f 1.1 12.88 0.18 k 0.04 t15 8.7 f 2.4 0 Q 0 @ 0 8 0 the transition from the initially populated level u’ of the excited state to the quasi- continuum formed by isoenergetic levels of the 2 and 2 state^.^^.^^ The B” 2Blu t 2 2B2g laser excitation spectrum of rotationally cooled 1,2,4,5- tetrafluorobenzene cation is shown in fig. 6. The spectrum covers an excess-energy86 INTRAMOLECULAR DECAY OF POLYATOMIC CATIONS 51, I I I r I I I u I I I 365 370 375 380 385 390 405 410 A/nm Fig. 6. Laser excitation spectrum of the 2 B l , t ~ zBzg transition of 1,2,4,5-tetrafluorobenzene cation at liquid-nitrogen temperature in the gas phase (not corrected for laser intensity distribution).l;1 5 H I I I 0 1000 d o 0 3000 4600 V"/cm-' Fig. 7. Relative non-radiative rates, knr(u')/knr(Oo), of 1,2,4,5-te_trafluorobenzene cation plotted as a function of the vibrational excess energy within the B 'Blu state. The values from the laser experiment are marked by 0 and from the coincidence measurements by w.J. P. MAIER, M. OCHSNER AND F. THOMMEN 87 range of ca. 2000 cm-l; only very weak bands could be seen at higher energy. The detection of higher excited levels is difficult because the intensity of an excitation band not only depends on the pF(v’) value of the emitting level but also on the probability of populating this level in the absorption process from the cationic ground state. Lifetimes were measured using most of the prominent bands in the spectrum, and these correspond to levels of the A,, v, to v6, vibrations.The assignment is taken over from the excitation spectrum recorded in a neon matrix.25 The results and the evalu- ated non-radiative rates are presented in table 5 and are associated with uncertainties of &5-10%. Table 5. Lifetimes z ( d ) of 1,2,4,S-tetrafluorobenzene cation in various vibrational levels of the & 281. state measured by laser excitation. The non-radiative rate constants k,,(d) are calculated assuming k,(d) = 1.9 x lo7 s-l, obtained from the coincidence studies. level AF/cm - z(d)/ns k,,(v’)/107 s-l 0 272 462 545 733 926 1180 1 200 1388 1 528 1 662 1 852 32 32 28 27 28 26 21 25 27 20 23 25 1.2 1.2 1.7 1.8 1.7 2.0 2.9 2.1 1.8 3.1 2.5 2.1 In fig. 7 the relative non-radiative rates, log[kn,(v’)/kn,(OO)], from both experiments are plotted against the vibrational excess energy within the ’BlU state.By and large, the non-radiative rates increase monotonically. However, the differences found for the levels probed within the 1900 cm-l range, suggest a mode-specific internal con- version process. It seems that the higher-frequency modes v2, v., are more active than the low-frequency modes vs, v6. CONCLUDING REMARKS The decay behaviour of the open-shell cations discussed illustrate further know- ledge that could be gained either using photoelectron-photon coincidence measure- ments with improved resolution, such as for the triatomics, or from complementary studies of lifetimes using laser excitation of rotationally cooled cations.Although the excess energy accessible (ca. 2000-3000 cm-l) in the latter approach is less than in the photoionisation experiment, the much higher resolution enables more vibrational levels to be probed. However, the coincidence data provide the absolute calibration for the evaluation of the non-radiative rates and some mode specific trends are apparent. The studies described in this article have been supported by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung (project no. 2.2- 0.17-0.81). Ciba-Geigy SA, Sandoz SA and F. 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