3252
J . Phys. Chem. 1985,89, 3252-3260
Nonradiative Transltions and Unimolecular Dissociation of Excited Electronic States of CICN', BrCN', and ICN' 0. Braitbart,+ E. Castellucci,t G. Dujardin, and S. Leach* Laboratoire de Photophysique MoMculaire du C.N.R.S.,Bdtiment 21 3, UniversitO Paris-Sud, 91 405 Orsay Ci?dex, France (Received: January 2, 1985)
Radiative and nonradiative relaxation processes of electronic excited states of ClCN', BrCN', and ICN' formed by He I and Ne I lamp excitation have been studied by photoelectron-photoion and photoion-fluorescence photon (PIFCO) coincidence techniques. The B211 states of these ions are found to fluoresce with an intrinsic biexponential decay. The results are interpreted in terms of the small molecule, resonance limit model of radiationless transitions. Average values of the interelectronic state mixing-coefficients andjoupling matrix elements are deterniined. Morse function sections of the potential energy strfaces of the X211, A%+, and B211states of the representative ion CICN' are calculated and are shown to be propitious for B state relaxation via interelectronic interactions. Dissociative ionization processes of the XCN' species are also studied, in particular fragmentation into the three channels X' + CN, CX' + N, and CN' + X. Electronically excited CN radicals are detected in He I excited dissociative ionization of BrCN and ICN.
I. Introduction The nondissociative relaxation of excited electronic states of isolated molecules or molecular ions can occur by radiative and/or nonradiative channels. In linear triatomic gas-phase cations, nonradiative transitions should correspond to the small molecule resonant, or possibly to the intermediate, case within the general classification of radiationless transitions defined in current models.',* This follows from their small number of vibrational modes (two stretching, one doubly degenerate bending), 50 that these species should exhibit rather low density p(X,) of ground-state vibrational levels, X , @energetic with the first one or two electronic excited states A, B. If interaction between the lowest excited states and the 2,levels is sufficiently small, or the level spacings are such that only relatively few rovibronic levels undergo a significant interelectronic interaction, the measured fluorescence quantum yields could be unity and the decay pattern monoexponential. Such behavior has been observed, for-example, in N20+for the nonpredissyiated u, = 0 level of the A 2 n state.3 On the other hand, if B, X, and A, gginterelectronic interactions are sufficiently strong or frequent, the X, levels can act as effective dilution pathways (Douglas cffe$t4) fof the electronic oscillator strengths of the zero-order A-X and B-X transition; (we assume for the moment no interaction between A and B states). Possible outcomes include fluorescence quantum yields less than unity. This would correspond to the case where the mo_lecula_r(ion) statts formed by interaction between isoenergetic A and X, or B and X, levels have such long lifetimes that these excited levels are deactivated by collisional or infrared (vibrational) emission processes before they can effectively radiate electronically to the ground state. An even more sensitive test (than diminished fluorescence quantum yields) for interelectronic state interactions is the appearance of multiexponential fluorescence decay behavior.',2 This has been observed in the small molecule resonant case as fluorescence efficiencies (and, by implication, lifetimes) which vary with individually resolved rovibronic levelss and, in the intermediate case, where p ( X , ) is of the order of 104/~m-',236by biexponential decay behavior6,' of the emitting levels probed-in coarse-grained fashion. At higher densities of X,, e.g. when p ( X , ) k 107/cm-', one often encounters the statistical limit where the coarse-grained averaging implied by the detection scheme reduces the observed decay to monoexponentiaL2 The present study is concerned with the radiative and nonradiative behavior of three XCN' cations: ClCN', BrCN', and ICN'. These are linear species whose emissions were first dis'Permanent address: Racah Institute of Physics, Hebrew University of Jerusalem, Israel. 'Permanent address: Dipartimento di Chimica, Universita di Firenze, Italy.
0022-3654185 12089-3252SO1.SO10
TABLE I: Electronic State Energies and Lowest Dissociation Limits of XCN' Ions energv, eV electronic state ClCN' BrCN' ICN' ...u2*4&3 Pn 12.37 11.85 10.91 13.55 13.17 ...u2*4au4 A22+ 13.82 ...U2*3ll2*4 B 2 n 15.16 14.16 13.35 ...u1r4fJ2A4 e z + 19.0 18.07 16.69
-
lowest dissociation limit
-
17.2
15.52
13.62
covered independently by Eland et aL3 and by Allan and Maier.8 The energies of the relevant electronic states of these species are given in Table I as determined from photoelectrongJOand optical8 spectroscopies. Using a mass-selected photoion-fluorescence photon coincidence (PIFCO) technique, Eland et al. measured fluroescence quantum yields less than unity for the XCN' cation^.^ Allan and Maier used an electron-impact, crossed-beam technique to icvestigate the XCN+ fluorescence spectra and reported that the B211state of ClCN' exhibited biexponential fluorescence decay.8 Multiexponential decay behavior was also found for the B211 states of BrCN' and ICN'." However, in the electron-impact technique the observed double exponential decay is not necessarily intrinsic to the XCN' B state, since it could result from cascade processes or from the simultaneous detection of neutral or ionic decomposition product emissions. Thus one of our objectives in the present study was to demonstrate by a coincidence technique whether the B state emission of the XCN' ions is indeed intrinsically biexponential. Furthermore, the improved PIFCO apparatus used here was expected to enable us to refine the fluorescence quantum yield measurements and so provide better information for interpreting interelectronic coupling and radiationless transitions. Since this work was completed, Maier et a1.I2 have very (1) P. Avouris, W. M. Gelbart, and M. A. El Sayed, Chem. Reu., 77, 793 (1977). ( 2 ) S. Leach, G. Dujardin, and G. Taieb, J . Chim. Phys., 77, 705 (1980). (3) J. H. D. Eland, M. Devoret, and S. Leach, Chem. Phys. Left.,43, 97 (1976). (4) A. E. Douglas, J. Chem. Phys., 45, 1007 (1966). (5) (a) H. J. Vedder, M. Schwarz, H. J. Foth, and W. Demtriider, J . Mol. Specfrosc.,97,92 (1983); (b) K. H. Fung and D. A. Ramsay, J . Phys. Chem., 88, 395 (1984). (6) G. Dujardin, S. Leach, G. Taieb, J. P. Maier, and W. M. Gelbart, J . Chem. Phys., 73, 4987 (1980). (7) F. Lahmani, A. Tramer, and C. Tric, J. Chem. Phys., 60,4431 (1974). (8) M. Allan and J. P. Maier, Chem. Phys. Letf.,41, 231 (1976). (9) E. Heilbronner, V. Hornung, and K. A. Muszkat, Helo. Chim. Acfa, 53, 347 (1970). (10) R. F. Lake and H. W. Thompson, Proc. R. SOC. London, Ser. A , 317, 187 (1970). ( 1 1) M. Allan, Ph.D. Thesis, University of Basel, 1976.
0 1985 American Chemical Societv
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3253
Relaxation of XCN+ Excited States recently reported photoelectron-fluorescence photon coincidence (PEFCO) measurements of quantu-m yields and lifetimes of energy-selected levels of the ClCN+ B211state. Similar measurements are in progressI3 on energy-selected levels of the BrCN+ ion, using the threshold photoelectron-fluorescence photon coincidence (T-PEFCO) technique. Some of the results of the TPEFCO study will be reported here. The XCN+ cations are studied here under collision-free conditions with a view to identifying the mechanisms of relaxation of their electronic excited states. A brief preliminary presentation of the nondissociative aspects of this work has been given elsewhere.14 The present report also includes an investigation of dissociation processes occurring in electronic excited states of XCN+ ions formed by H e I and N e I excitation. This part of the work has also been briefly presented e1~ewhere.l~ 11. XCN+ Electronic Manifolds and Transitions
ClCN+, BrCN+, and ICN+ are 15-valence-electron systems whose ground-state molecular orbital configuration is 1$ 2u2 3u2 ld 4 2 2 2 . The electronic states involved in this study are similar for each cation and differ only in energy scaling and in the size of the spin-orbit splitting of corresponding 211 states. We remark that the ground and first three excited states of the XCN+ ions are presumed linear, on the basis of the Walsh-Mulliken rules16 conc_erning 15-valence-electron triatomic species. The origins of the AZZ+and *II states lie below the lowest-observed dissociation thresholdsI7 for each ion (Table I). The CzZ+ state lies above the dissociation threshold in each case and is therefore not expected to relax by photon emission. Selection rules for electric dipole transitions between the grou_nd and excited states of the cations lead us to expect that the_B211 and A2Z+ states can each be radiatively coupled to the XzII ground state as well as to one another. We note that with the rare gas excitation sources used in the present study, H e I (21.22 eV) enables all four electronic states of each cation to be focmed on photoionization, whereas N e I (16.85 eV) excludes the C2Z+state of ClCN+ and BrCN+. 111. Experimental Section The present PIFCO experimental setup has been described in detail elsewhere.18 ClCN, BrCN, or I C N molecules are photoionized in a gas jet using an He I or Ne I rare gas dc lamp source mounted on a VUV monochromator whose band-pass is -50 A at 584 A. In all experiments the pressure of halocyanide molecules is about torr in the gas jet. Photoions and photoelectrons are accelerated away from the region of formation by an electric field of 20 or 150 V/cm and are detected by microchannel plates and a channeltron, respectively. The fluorescence photons emitted from excited species are detected at right angles by an EM1 9635 QB (160-600 nm) or a Hamamatsu R943 (160-900 nm) cooled photomultiplier. In some experiments optical filters were used in front of the photomultiplier to select specific spectral regions of fluorescence. Total fluorescence was detected within the transmission range of the particular photomultiplier-optical filter combination. The emission spectra of the XCN+ ions have been reported by Allan and Maier.8J1 From their published spectra the spectral Langes of the vaGous transitions are as follows:- ClC-N+: A X, 84_5-89_5 nm; B X, 435-715 cmm;BfCN': A X, 690-870 nm; B, X, 455-625 nm; ICN': A X, 690-77_0 a_nd 525~562 X, 560-670 and 475-525 nm. The A-X and B-X nm; B transitions of ClCN+ and BrCN' fall into separate spectral regions. The i&% transition of ClCN+ was spectrally isolated by
--
-
-
- -
(1 2) J. P.Maier, M. Ochsner, and F. Thommen, Faraday Discuss.Chem. Soc., 75, 77 (1983). (13) E. Castellucci, G. Dujardin, and S.Leach, unpublished results. (14) E. Castellucci, 0. Braitbart, G. Dujardin, and S. Leach, Faraday Discuss. Chem. SOC.,75, 90 (1983). (15) 0. Braitbart, E. Castellucci, G. Dujardin, and S. Leach in "Photophysics and Photochemistry above 6 eV", F. Lahmani, Ed., Elsevier, Amsterdam, 1985, p 141. (16) A. D. Walsh, J. Chem. SOC.,2266 (1953). (17) V. H.Dibeler and S. K. Liston, J . Chem. Phys., 47, 4548 (1967). (18) G. Dujardin, S. Leach, and G. Taieb, Chem. Phys., 46,407 (1980).
using the Hamamatsu R943 photomultiplier in conjuction with a Kodak Wratten 92+94 filter comkinatipn which transmits X > 780 nm. Spectral isolation_of the A X transition of BrCN+ was not attempted. The B-X transitions of ClCN+ and BrCN+ were detected by \he EM1 9635 QB-photomultiplier alone. For ICN+, where the A X and B X transitions overlap, photon detection using the Hamamatsu R943 photomultiplier included both simultaneously. Photoion-photoelectron coincidences are first recorded and used to give a time-of-flight (TOF) mass analysis of photoions (parent ions and fragment ions).3,18 After this analysis, mass-selected photoion-fluorescence photon coincidences are counted as a function of delay time, together with the total number of photoions. This technique enables us to distinguish fluorescence of parent ions from fluorescence associated with the appearance of fragment ions. The particular electronic state j involved in the detected fluorescence is not given directly by the PIFCO signal, but can be inferred from data obtained with optical filters used to isolate emission spectral regions, in conjunction with other data such as photoelectron spectra which define ion state energies. The PIFCO measurements provide fluorescence decay curves and efficiencies averaged over all vibrational levels of the emitting jth state which are accessible by the photoionization process. Conversion of the fluorescence efficiency qF to the vibrationally averaged quantum yield $F(j)for a specific electronic state requires a knowledge of the electronic branching ratiofj for forming the j t h state of the emitting ion. The branching ratios are usually determined from photoelectron spectroscopy data obtained with the same excitation source as in the particular PIFCO experiment. For these quantities to be determined with precision from photoelectron spectra, the latter must be corrected for instrumental functions and for the angular distribution of the photoelectrons. When this information is not available, as is currently the case for the halocyanine cations, fj can be estimated (on a single configuration basis) as the fraction of electrons in the molecular orbital from which ionization has occurred to form the j t h state, with respect to the total number of electrons in all orbitals accessible with the photoionization source.1g Each coincidence event between an electron or a fluorescence photon and the corresponding ion was counted in a time-to-amplitude converter and accumulated in a multichannel analyzer. Fluorescence quantum yields were determined from the following $F(j) = vF/ffhv = ( T / o / f f h vwhere T (true coincidences), Z (total number of ions counted), and f h v (photon detection efficiency) are all experimentally determined quantities. fhv is obtained from a measure of emission of N2+ (B22,+ X2Zg+)which has unit quantum yields3 In practice, N2+photoelectron-photoion and photoion-fluorescence photon coincidence measurements were carried out before and after each XCN measurement, with nothing altered in the experimental parameters. The N2+ and parent ion masses are used as reference masses for identification of ionic fragments in the photoelectron-photoion coincidence experiment which gives a T O F mass spectrum. Assignments to specific ions were made on the basis of these masses and by using known appearance potentialsi7 as additional relevant information. Peak intensities of T O F mass spectra reported here have not been corrected for ion detection efficiency (which varies little with ion mass except for small masses), nor for the variation of electron detection with the kinetic energy of the latter. Electron kinetic energy distributions corresponding to ion formation differ for parent and fragment ions, and also for the same ion produced in different states of internal excitation. Our discussion requires qualitative rather than precise quantitative data on the fragmentation pattern, so that peak intensity corrections have not been carried out. Because of the limited acceptance angle of the photon detectors, part of any long-lived fluorescence (>1.5 ps) could escape detection when the photoions are accelerated with a field of 150 V/cm. We therefore measured any long-time fluorescence components with
- -
-
-
(19) 0. Braitbart, E. Castellucci, G. Dujardin, and S. Leach, J . Phys. Chem., 81, 4799 (1983).
3254 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
Braitbart et al.
TABLE II: Fluorescence Efficiencies, qF, Electronic Branching Ratios, /r Fluorescence Quantum Yields, &, Fluorescence Decay Component Lifetimes, il and tj,and Relative Amplitudes, A ,/A 2, for Electronic Excited States of XCN' Cations
cation CICN+
state A2L+
BrCN+ ICN+
0.30 0.10 0.28
f; 0.4 0.2 0.4
0.40
0.5
nF
B2n B2n
B2n+ A2Z+
.rl /ns
JJF
0.76
170 f 20 very long
>0.50
0.70
320 f 40
0.80
390 f 40
A, / A ,
840 f 80
=2
=2
2300
270 900
B2na
A2Z+a
T,/ns 970 f 80
15
"See text.
A
ION-hVF
50 BrCN*-
4000
e-- ION TOF- MS
y3000
q W v)
0
5
I/
$2000
0
5
U
0
HzO'
1000.
';'
O IH :
I 0
c I'
200
a:
I
4
Q
IL 400 600 CHANNELS
800
Figure 2. He I excitation. Coincidence curves for ionized BrCN: (a) TOF-MS; (b) PIFCO spectrum. The time scale is 4.88 ns/channel.
a low draw-out field of about 20 V/cm. Short-lived fluorescence components, which are somewhat smeared out in measurements with low fields, were determined exclusively with a 150 V/cm ion draw-out field. All reported fluorescence efficiencies and quantum yields were corrected for the following: (i) the ratio of the photomultiplier ?Zg+) emission with efficiency for detection of N2+ (BZZ,+ respect to the particular XCN+ emission studied, using information on spectral distribution provided by the known fluorescence spectra of the cation$ (ii) that part of the emission which was undetected because of decrease, in certain higher wavelength regions, of the photomultiplier quantum efficiency; ( 5 ) optical filter transmission. For very long-lived emission a correction has been applied for loss of fluorescence due to escape of ions out of the acceptance angle of the collecting optics, by analytical integration of the decay curve to t = *. When these corrections are taken into account, error limits are estimated to be within 15-20% for lifetime measurements and 20-30% for fluorescence efficiencies.
la. The two isotopic parent ions 35C1CN+and 37C1CN+appear, as well as fragment ions CN+, C1+, and CC1'. The COz+ peak results from a 1% carbon dioxide impurity in the ClCN sample gas. Figure Ib gives the corresponding TOF-MS obtained with Ne I excitation. No fragment ions are observed in this case. The COz+ impurity peak is much reduced in intensity, relative to the H e I case, since the photoabsorption cross section and the photoionization yield of COz at 16.85 eV are both significantly less than at 21.22 eveZo The photoion-fluorescence coincidence measurements carried out on ClCN excited by He I showed ClCN' emission and also emission from the COz+ impurity. In Figure I C is given the photoion-fluorescence photon coincidence curve for ClCN+ ions corm+ by Ne I excitation. Photon detection was limited to the BZII-X211 transition. The decay curve clearly does not follow a single exponential. With a fitting procedure described elsewhere: the curve was decomposed into two exponential components corresponding to two very different time constants T~ and tZ.In this procedure, the semilog plot of the longer component was first determined and then subtracted from the total fluorescence to give tl,the prompt fluorescence decay component. The values of 'il and T~ and the relative amplitudes of the two components, A,/A2, are given in Table 11. This table also contains the fluorescence efficjencies qF and estimated electronic-branching ratio, fb, for the BzII state of CICN'. The Az2+ XzII emission in delayed coincidence with ClCN' ion detection wa_s also_studied and the data obtained are given in Table 11. The A X emission decay appeared to be monoexponential, in agreement with the electron-impact study.*J1 Its lifetime was too long (>4 ps) for a
IV. Results A. CZCN'. The time-of-flight mass spectrum (TOF-MS) of photoions produced by He I excitation of ClCN is given in Figure
(20) J. Berkowitz, "Photoabsorption, Photoionization and Photoelectron Spectroscopy", Academic, New York, 1979.
a 0
200 CHANNELS
400
1
Figure 1. Coincidences curves for ionized CICN: (a) He I excitation, electron-ion coincidences (TOF mass spectrum); (b) Ne I excitation, TOF-MS; (c) Ne I excitation, photon-ion coincidences (PIFCO) for CICN+ B211 - k211emission. The time scale is 2.44 ns per channel.
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-
-
The Journal of Physical Chemistry, Vof.89, No. 15, 1985 3255
Relaxation of XCN' Excited States 250 0 . Ncl EXC TATION
w
5zns
v)
l
ION-hvF
2000, W
% 1500,
!l0O0 E u 500 u
I
t
-
ION hVF
b: Unfi/tered c : H 3 2 0 a filter
b C
IL
b
--I
KN+-
1500 v)
W
V
f 0 V
1
e--ION TOF-MS
e-- ION TOF- M S
200
tI
0
ClCN*
HzO+ 200
I
E;
a
400
600
800
800
600
1000
1000
CHANNELS
CHANNELS
Figure 3. N e I excitation. Coincidence c_urves for ionized BrCN: (a) TOF-MS; (b) PIFCO spectrum for B211-X211 emission of BrCN'. The time scale is 4.88 ns/channel.
Figure 4. H e I excitation. Coincidence curves for ionized ICN: (a) TOF-MS; (b) PIFCO spectrum; (c) PIFCO spectrum obtained with an M.T.O. H 320a optical filter in photon detection path. The time scale is 9.76 ns/channel.
precise value to be determined under our experimental conditions. B. B r C w . The TOF-MS obtained on He I excitation of BrCN (Figure 2a) contains peaks corresponding to the parent ion BrCN' and the fragment ions CBr', Br+, and CN'. Some impurity ions are also observed: H 2 0 + ,OH' (from H,O), ClCN', and C12'. The photoion-fluorescence photon coincidence spectrum (Figure 2b) shows that, in addition to fluorescence from the parent ion, emission also occurs in coincidence with the Br' ion. With Ne I excitation, apart from ClCN' and HzO' impurities, only the BrCN' and Br' ions appear in the TOF-MS (Figure 3), while only BrCN+ emission is observed in the PIFCO spectrum. The B211 state fluorescence exhibits biexponential decay. The time constants and relative amplitudes of the two components, 5s well as the measured ion fluoresence efficiency and estimated B21T state electronic branching ratio, are reported in Table 11. C. I C W . He I excitation of I C N gives rise to the following ions in the T O F mass spectrum (Figure 4a): ICN', CI', I+, and CN+. Impurity ions observed are C12+,ClCN', H20', and OH'. The photoion-fluorescence photon coincidence spectrum (Figure 4b) shows that emission occurs not only in coincidence with the ICN' ion, but also with the I' ion. The PIFCO spectrum (Figure 4c) was obtained with an M.T.O. H320a optical filter in conjuction with the Hamamatsu R943 photomultiplier in the photon detection channel. This filter transmits light between 220 and 420 nm only and thus cuts out ICN' emission. The emission in coincidence with I' remains detected. In the Ne I excited TOF-MS (Figure 5a), apart from elz+, N2+, and HzO+ impurity peaks, only ICN' and I' are observed. The PIFCO spectrum (Figure 5b) shows that emission occurs only in coincidence with the ICN' ion. The ICN' data reported in T_able I1 are for N e I excitation. The total emission decay curve (A 0 X transitions) could be fitted with an apparent single exponential decay constant 7 = 390 ns, but could also be well fitted with a triexponential function (see section VB).
products in their ground states were taken from photoionization'' and electron-impactZ1mass spectrometric studies. Excited-state product energies were then derived from known spectroscopic e n e r g i e ~ ? ~ -Correlations ~~ between products and parent ion states are given in Cmusymmetry and were derived with the usual correlation rules;25discontinuities in the correlations indicate the possibility of avoided crossings. We see that the ground-state products correlate with the XzII Earent ion ground state except for CCl+(XIZ+) N(4S,). The A22' state correlates with the
V. Discussion A . Dissociative Ionization of XCN. The T O F mass spectra obtained, and their modifications in going from He I to N e I excitation, can be interpreted in terms of the appearance potentials of dissociation products in their ground states as well as the estimated energies of the known states of excited products. CfCN. Figure 6 gives an energy diagram showing the ClCN' electronic states and the limits for fragmentation into the three channels C1' + CN, CCl' + N, and C N + C1. Energies of
(21) J. T. Herron and V. H. Dibeler, J. Am. Chem. Soc., 82, 1555 (1960). (22) K. P. Huber and G. Herzberg, "Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules", Van Nostrand-Reinhold, New York, 1979. (23) Ch. Moore, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No.35 (1958, 1971). (24) M.Tsuji, T. Mizuguchi, K. Shinohara, and Y. Nishimura, Can. J. Phys., 61, 838 (1983). (25) G. Herzberg, "Molecular Spectra and Molecular Structure. 111. Electronic Spectra of Polyatodc Molecules", Van Nostrand, New York, 1966.
+
-
-
+
EXCITATION ION-hV,
1
H,O*
N;
ci;
Q
h
200
0
400
600
800
1000
CHANNELS
Figure 5. N e I excitation. Coincidence curves for ionized ICN: (a) TOF-MS; (b) PIFCO spectrum. The time scale is 9.76 ns/channel.
+
3256
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
I4 !-
13L
12 L
Figure 6. ClCN+. Correlation diagram for electronic states of parent
ion and dissociation product species. C, symmetry is assumed throughout. Discontinuities in the correlations indicate possible avoided crossing regions.
+
ground-state products CN+(X'Z+) C1(2P,) but with excitedstate products for the other two dissociation channels. For the B211 state the three dissociation channels should correspond to excited-state products. The specific dissociation limits and energies for each electronic state of ClCN+ will be of particular importance in our discussion of the excited-state relaxation processes. The absence of fragment ions in the Ne I excitation TOF-MS is not surprising since the lowest fragmentation channel energy lies above 16.85 eV. In the H e I case, with 21.22-eV excitation energy available, the diatomic products C N , CCI', and CN+ in the three dissociation channels could be formed not only in their ground but also in the following excited states: CN(B2Z+,A211), CCl+(a311),CN+(A'II,a311). N o emission from these species was detected. This does not prove that the diatomic excited states are not formed since their detection would not be expected under our experimental conditions. The CN+ A'II X'Z+ transition would fall into the near infrared, around 1.2 p m , a 6 and the a311 X'Z+ at still longer both transitions W i g out of the range of our photomultipliers, while CCI+ a311 X I Z + emission has a very long lifetime (>>4 p) and would thus have a low transition probability." C N emission would be detected in coincidence with the C1+ ion, its companion species in the C1+ C N dissociative channel. However, a (weak) PIFCO signal due to C N would have been difficult to detect in our He I excitation experiment because of the existence, in the C1+ peak region, of the emission decay signal from the C02+impurity. Dibeler and Liston17have studied the photoionization spectrum of CICN, detecting CICN+, C1+, and CN+. The thresholds for appearance of the C1+ (17.32 eV) and CN+ (18.50 eV) fragments (as well as the threshold for CCI', 17.2 eV, observed in electron-impact ionization*' occur in energy regions where the He I photoelectron spectrum of ClCN shows no f e a t ~ r e s .The ~ fragments must therefore be formed at threshold by dissociative aut~ionization.~~ It would be interesting to identify the superexcited
-
-
-+
+
Braitbart et al. autoionizing levels. However, there exists no VUV absorption spectra or threshold photoelectron spectra (TPES) for CIN in these energy regions, from which one could perhaps observe Rydberg bands. Nevertheless, there are a number of discrete features in the photoionization spectra that indicate that some Rydberg bands of ClCN exist in the 17-21-eV region.I7 The C1+, CCl+, and CN+ fragment ions observed in our H e I excitation experiment must be formed by dissociative autoionization processes at 21.22 eV and/or by a direct ionization process involving an as yet unknown dissociative state of ClCN+ close to, but below, 21.22 eV. We remark that, in the photoionization spectrum of ClCN, there are a number of fairly narrow, quite intense peaks in the ClCN+ curve between 18.7 and 21 eV, the feature at 20.6 eV being particularly strong.17 -Thus it is not inconceivable that a CICN+ state, e.g., the 2 6 ' D2Z+ state, occurs at 21 f 1 eV. BrCN. The dissociation of BrCN+ can be discussed in a fashion similar to ClCN+. (Analogous correlation diagrams have been made but are not presented here.) With Ne I excitation, only the Br+ + C N dissociation channel is energetically accessible, its threshold being at 15.52 f 0.02 eV.I7 The threshold energies for the channels CBr+ + N (1 7.4 i 0.2 eV2') and C N + + Br (1 8.3 f 0.1 eV21) make these additional channels accessible with H e I excitation. These thresholds are in energy regions of negligible or very small He I PES signal9 so that dissociative autoionization is the likely process for their formation at threshold in the photoionization spectrum experiment of Dibeler and Liston.17 The photoionization spectra of BrCN obtained by these authors do not extend above 17.71 eV, and VUV absorption spectra have not been measured in this region, so that it is not possible to say if there are any prospective autoionizing levels in the region of the CBr+ and CN+ production thresholds. The photoionization spectrum of BrCN in the 15.5-eV region (Br+ onset) is insufficiently resolved for detection of sharp Rydberg features. The emission associated with the Br+ peak in our PIFCO experiment is probably not due to Br+ itself, but rather to its companion fragment, CN, formed in the B2Z+ excited state (see later the analogous case of C N emission in ICN+ dissociation). The Br+ CN(B2Z+) is threshold for the channel BrCN hv estimated to be at 18.72 f 0.02 eV. The BrCN' c 2 Z + state is populated only between 17.7 and 18.5 eV with H e I excitat i ~ n Thus . ~ the CN+ fragment could be formed via predissociation of the C2Z+state, but the excited C N fragment, as well as the CBr+ ion formed with He I excitation, must be produced by direct ionization involving a new BrCN' state or by dissociative autoionization processes at that energy. In this respect it is significant that we observed a broad feature over the range 20-22 eV in the TPES of BrCN.I3 Emission from other energetically possible excited fragments CN+(A111,a311),which would be outside the detection spectral range,22 and CBr+(a311),28which could be masked by the parent ion PIFCO signal, are not observed in the coincidence experiments. E N . The TOF mass spectra are consistent with the known parent ion and fragment channel energies. The threshold energies for the three channels observed are as follows: I' + CN, 13.62 f 0 . 0 2 e V ; 1 7 C I + + N , 17.6f0.3eV;21CN++I,18.1f 0 . 1 e V . 2 1 The only fragment ion observed with Ne I excitation is I+ as expected on energetic grounds. It can unioubtedly be formed by dissociative ionization involving the ICN+ B211state, which extends from 13.35 to -14 eV.9 The I+(3P) + CN(B2Z+) channel threshold is estimated to be at 16.82 eV. We do not observe any C N emission in coincidence with I+, using 16.85-eV Ne I excitation. This shows that the partial cross section for this channel is small at 16.85 eV and may indicate that its true threshold lies higher than 16.82 eV, which would be the case if, for example, the kinetic energy release for the excited-state channel is greater than for the unexcited state channel I+(3P)+ CN(X2Z+). The I+('D) CN(B2Z+) and I+('S) CN(B2Z+) channel energies are 18.52 and 20.87 eV, respectively (assuming the same kinetic
-
+
+
-
-
+
+
(27) R. S. Berry and S. Leach, Adu. Electron. Electron Phys., 57, 1 (26) P. J. Bruna, S. D. Peyerimhoff, and R. J. Buenker, J. Chem. Phys., 72, 5437 (1980).
(1981). (28) M. Tsuji, K. Shinohara, Phys., 61, 251 (1983).
T.Mizuguchi, and Y . Nishimura,
Can. J .
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3251
Relaxation of XCN' Excited States
+
energy release as for the I+(3P) CN(X2Z+) channel). The emission observed in coincidence with the I+ peak in the He I excitation experiments is undoubtedly due to the C N B2Z' X2Z+transition since this is the only assignment consistent with our energetic and spectral range observations. We note that emission associated with the I+ ion was detected by Eland et al.3 in earlier PIFCO experiments on ICN. N o definite assignment could be given by these authors. The thresholds for the channels leading to formation of the CI' and C N + fragments occur in a "Franck-Condon gap" region of the He I PES of ICN9 and thus probably involve dissociative processes, although the absence of any information on superexcited states in the relevant energy regions makes it impossible to clarify the mechanism at present. Similarly we cannot specify the mechanism(s), direct and/or indirect dissociative ionization, responsible for fragment formation in our experiments carried out at an excitation energy of 21.22 eV. Finally, we remark that the He I excitation energy is sufficient to form CN+ in the A'I122 and a31126excited states but, as mentioned earlier, the transitions of this species are expected to be at wavelengths which are beyond our detection limits. Nothing is known about excited states of the CI+ ion. As stated earlier, no emission was detected in coincidence with either CN+ or CI+ in the H e I excitation PIFCO experiments on ICN. B. Radiative and Nonradiatiue Processes in the D2II and A2Z+ States of X C W . 1 . Lifetiyes and Quantum Yields. This study shows conclusively that the B211states of ClCN' and BrCN' have an intrinsic biexponential decay fluorescence. The values of the time constants tl= 170 f 20 ns and 72 = 970 f 80 ns for CICN' are in good agreement with those obtained by electron-impact measurements by Allan and Maier;," P, = 205 f 20 ns, P~ = 900 f 100 ns. In recent measurements using a photoelectronfluorescence photon coincidence (PEFCO) technique, Maier et a1.I2 have observe? that the values vary with internal energy E, of the ClCN+ B211state. They measured a set of values ~'(u') = 280-90 ns over the range E, = 0-1530 cm-' and T2(U') = 400-780 ns over the range E, = 480-2580 cm-l. Our vibrationally averaged values are compatible with these results for the short, but not the long, component. We remark that our ratio Al/A2 = 2 is closer to the range of PEFCO values AI/A2 = 1.1-1.612 than the electron-impact u' = 0 value Al/A2 = 10." For BrCN', the electron-impact measurements gave t1= 270 f 30 ns.8 This value and our vibrationally averaged PIFCO value tl= 320 f 40 ns are compatible with the u'dependent values rl(u') = 20(>-400 ns which we have measured for a range of u' values using the T-PEFCO t e ~ h n i q u e . ' ~N o electron-impac: values have been published for the long component of BrCN+ B state emission but the value appears to be t 22 1 ps.1'329T-PEFCO measurements of T 2 ( 0 ' ) give values of the order of 1 ps. The amplitude ratio A , / A 2 = 2 is much smaller than the u ' = 0 value Al/A2 15 observed by Allan1',29with electron-impact excitation. A 1 / A 2in our T-PEFCO study was found to depend on E, and had values between =5 and ~ 1 5 . As menticned eaflier, it was not possible to separate the A2Z+ - W Z I I and B211- X211emissions in ICN'. The total emission decay curve time constant T = 390 ns is intermediate in value between the decay constants measured for the two electronic stat" in the electron impact experiments:8 7(A2Z+) = 1200 ns, tl(B211) = 300 ns. In the latter a long component t2(B211)= 2300 ns was also o b ~ e r v e d . ~ In ' . ~order ~ to see whether our resuIts are compatible with those of Allan and Maier,8,1'*29 we simulated our experimental deca_yturve as follows. Using the relative intensities of the A-X and B-X emissions observed in electron-impact excitation* we were able to fit the observed N e I excitation PIFCO c u y e of ICN' with a @exponential function whose constants are 7(AZZ+)= 900 ns, tl(B211) = 270 ns, t2(B211) = 2300 ns, Al/A2 = 15. These values are qualitatively consistent with the known photophysics of ICN', and they are in reasonable quantitative accord with the electron impact observations of Allan and Maier
-
-
We remark that, for ClCN+ and BrCN+, the larger A1/Az ratios in the electron impact as compared with the PIFCO experiment probably result from the fact that the electron impact fluorescence decays were measured over one or two more decades than in the photon impact studies. Because of the fact that 7,
Braitbart et al. TABLE III: Resonance Limit Radiationless Transition Model for XCN+ Ions" CICN+ BrCN' ICN+ C2 0.85 0.72 0.89 -
d2 p , states/cm-'
I
8, cm-I OB,,
Figure 7. Resonance limit model of radiationless transitions: zero-order
and molecular eigenstates. represented in Figure 7. The zero-order (Le. unperturbed, Born-Oppenheimer) electronic excited state B211rovibronic level IB) is coupled radiatively to the ground state IX). State lZ) is isoenergetic with /B) and corresponds to a rovibronic level of either the zero-order A22+or the XzII state. In the case of the XCN+ X oscillator strength is one or more orders of ions, the B magnitude greater than for the Z-X (i.e., A-X or X> - X*pi) transitions, as is evident from the long lifetimes of the h22+state (>1 ps)l' and of the high vibrational levels of the X211 state (expected to be in the millisecond to second range for vibrational u - 1). transitions u The IB) level is coupled nonradiatively, via matrix elements of the nuclear kinetic energy uBz, to one or more of the set of rovibronic levels lZ) quasi-isoenergetic wiLh IB). The resulting (perturbed) molecular (ion) states are JBb),IBa), and IX) as shown in Figure 7. A more complex situation arises if more than two zero-order levels interact.32 Because of the relative sparseness of the levels in the case of XCN+ we restrict our discussion to two-level systems. The wave functions of the B b and B, levels, in terms of_zeroorder functions, and after normalization, are respectively IBb) = clB) + dlZ) and IB,) = dlB) - clZ). We neglect phase factors which would occur in these expressions in the case where uBZis complex.33 If they exist, they would not modify our conclusions. The eigenvector coefficients are given by
-
-
where 6 = EB - EZis the energy separation between states B and Z. We note that c2 d2 = 1. Because the {Z)levels are sparse, each linear combination of interacting B and Z levels will correspond to different energy intervals and have a distinct ugZ value, so that each molecular eigenstate will emit with its own characteristic decay time. In the resonance limit, the PoincarE recurrence time will generally be short so that the observed individual level decay rates will each have exponential character. We expect the fluorescence lifetimes of the perturbed rovibronic levels to be intermediate between the zero-order lifetimes T(B) and ~ ( 2 ) For . Z = A22+,the zero-order radiative lifetimes r(A22+)should be close to those of the A2B+ u = 0 level, Le., 24.4 p s , 3 3 p s , and 31.2 p s for ClCN', BrCN', and ICN+, r e ~ p e c t i v e l y . ~As~ ~stated ~ earlier, for Z = X211, the zero-order radiative lifetimes of the high vibrational levels would be those for pure vibrational (infrared) emission, Le., of the order of lo-' s-1 s. In our PIFCO experiments the measured lifeti_mes correspond to averages over occupied rovibronic levels of the B2n state &e., both {&) and levels). The two components of the apparent biexpnential decay can be interpreted as being due to averages of (Bb]and @,), respectively. We will now show how the average mixing coefficients and zero-order intervals 6 can be determined from our experimental results, on the basis of the resonance limit model.
+
{sa)
(32) T. Bergeman and D. Cossart, J . Mol. Specrrosc., 87, 119 (1981). (33) G. H. Dieke, P h p . Reu., 60, 523 (1941). (34) J. P.Maier, 0. Marthaler, L. Misev, and F. Thommen in 'Molecular Ions", J. Berkowitz and K. 0. Groeneveld, Ed., Plenum, New York, 1983, p 125.
0.28 14 0.036 1102
0.15
14.5 0.034 520
MHz
0.1 1 18 0.028 337
Derived parameters: 2 and 3 are values of the average eigenvector coefficients squared; p is the density of vibrational levels of lower lying electronic states (Z) isoenergetic with a weighted average of occupied vibrational levels of the 8*II state; 8 is the average separation between interacting rovibronic levels of the zero-order B and Z states; BBz is the average matrix element for coupling B and Z rovibronic levels.
4 . Determination of Average Interacting State Mixing Coefficients and Energy Leuels. The intensities of the zero-order X and B X are proportional to the square of transitions Z the respective probability amplitudes R and to the fourth power and IBx of the emitted radiation frequencr v: Iw 0: IR~x12v~4 a IRBx12vsx4. For the transitions Bb X and Ba X involving the perturbed levels, the corresponding intensities are given by
-
-
-
-
rbg
rag
a
IRbi((Zvbi(4 = [C2(R~x12 + d21RzxI2 2 Re lcR~xdRzXllvbR4 (2) ~
~
~
=
~
[4RBX12
1
2
~
~
~
4
+ c21Rzx12- 2 Re IcRBxdRzxII vag4
(3)
The square brackets on the right-hand side of eq 2 and 3 contain interference effects between the two zero-order probability amplitudes. In practice, the energy separation Eb- E, is negligible with respect to the transition energy Ea,b- Ei( so that the u4 dependence of emission intensities can be neglected. As stated earlier, we consider that IR=l2
20
IO
0
I
u 2.6
r(C-CI )/A ,Figure 8. ClCN+.Morse function potential energy of the g2n,A2Z+, and B2nelectronic states, as a function of the C-CI coordinate. Curves 1 and 2 correspond to two possible values of ro(C-Cl) for the A22+ state.'O
combination band lA3;. In contrast, the A and B states in th_e PES show only progressions in v3, particularly marked for the B state.30 The interaction force constant concerned with v l and v3 is indeed expected to be small and was considered as zero in the FCF analysis.30 Thus it is reasonable to separate the C-Cl and C-N motions as we have done in our potential energy calculations on ClCN'. The internuclear distances for the B states of BrCN' and ICN+ have not been determined.'O Our calculations were therefore restricted to the case of ClCN', which is, however, completely representative of the XCN+ species. In the excitation process the initial state is that of the neutral species linear ground state, whose internuclear distances are ro(C-Cl) = 1.631 A and ro(C-N) = 1.159 A.37 Bearing this in mind, we note that the Morse function potential curves calculated for the C-Cl and C-N oscillators (respectively Figures 8 and 9) are consistent with the vibrational structure which we have detailed for each band in the photoelectron spectrum of ClCN. This is only to be expected, since the major parameters involved in the potential energy surface calculations have been determined from analysis of the PES. The results in Figuce 8 show that in the C-Xl dimension the potential surface of the A state intersect? the B211 curve. Thus the propensity for interaction between the B211state and lower lying stat_es is confirmed by these calculations. It is of interest that the B, A "potential crossing'! corresponds to an excitation energy of E,,, = 15.58 eV, which is the energy region where the short component disappears and only the T~ component was measured in the PEFCO experiments on C1CN+.I2 We remark, however, that quantitative estimations of the region of maximum interelectronic interaction is subject to uncertainties resulting from those in the ro determinations. Finally we remark that if =, A2Z+or X211the perturbed levels could emit not only in the B-X spectral region but 5lso as redshifted fluorescence (RSF),3spossibly in the A2Z+ X211region. This would occur if there were appreciable Franck-Condon factors
-
~~~~
(35) K.F. Freed and S. H.Lin, Chem. Phys., 11, 409 (1975). (36)G. Herzberg, "Molecular Spectra and Molecular Structure. 11. Spectra of Diatomic Molecules", 2nd cd, Van Nostrand, New York, 1950.
(37) J. K.Tyler and J. Sheridan, Trans. Faraday SOC.,59,2661 (1963). (38) M. A. Johnson, R. N. Zare, J. Rostas, and S. Leach, J. Chem. Phys., 80,2407 (1984).
3260 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
4c
r(C-N)/A
Figure 9. CICN'. Morse function potential energy of the R2II, A2.Z+, and D2II electronic states, as a function of the C-N coordin_ate. Curves 1 and 2 correspond to two possible values of r,(C-N) for the A22+state.%
for emission to excited vibrational lyels of the ground state from the vibrationally excited perturbed A%' or Xzll levels poplated by the interelectronic interaction process through the B*state accessible in the photoionization process. A red-shifted fluorescence of this type is analogous to that observed for the isosteric 15-valence-electron species C02+.38If RSF occurred it should show up not only in the spectra but_ also-as multiexponential X transition. Unforfluorescence _decay_lifetimes for the A tunately, the A X transitions of XCN' hiye not been spectrally analyzed in sufficient detail nor have the A state lifetimes been : We are therefore unable measured, except for the 0 at present to demonstrate RSF due to B, A interelectronic coupling in XCN+.
-
-
VI. Conclusion This study of relaxation processes in excited electronic states of the XCN' ions concerns two principal types of processes: (i)
Braitbart et al. radiative and nonradiative relaxation of states below the lowest dissociation limit, (ii) fragmentation processes taking place at higher energies. Using the mass-selected photoion-fluorescence photon coincidence technique, we have shown that the B211 states of the ClCN', BrCN', and ICN' gas-phase cations emit photons with a biexponerilial decay pattern. The data for the emitting electronic state refleet processes that are averaged over rovibronic levels accessible via photoionization by H e I or Ne I excitation. The results cannot be interpreted in terms of the intermediate case model of radiationless transitions, often invoked when biexponential fluorescence decay is observed. Instead, our observations can be understood by application of_the small molecule, resonance limit model. The propensity for B state relaxation by interelectronic state interactions has been investigated and average mixing coefficients and coupling matrix elements have been derived. To go beyond this coarse-grained experimental approach, work is in progress on vibrationally resolved electronic states using the threshold photoelectron-fluorescence photon coincidence (TPEFCO) technique. Even higher resolution would require laser excitation studies of the XCN' but necessitate prior analysis of their complex electronic spectra, which is far from being achieved at present. Nevertheless, it would be of interest to know whether the interelectronic coupling gives rise to red-shifted fluorescence, as in the 15-valence-electron C02' ion which is isosteric with XCN'. This could be detected by the PEFCO or T-PEFCO techniques using suitable optical filters. The dissociative ionization of the three halocyanide species has been studied by using H e I and N e I excitation and measuring the time-of-flight mas? spectra following ionization. Further progress in this area requires several new studies. It is necessary to measure energy-resolved photoelectron-photoion coincidences in order to follow in greater detail the evolution of the fragmentation processes with internal energy of the systems. Interpretation of the results would probably require further studies on the photoionization spectra of these species, extending the excitation energy range and at higher resolution as compared with earlier work. It will also necessitate extensive new studies on the vacuum-ultraviolet absorption spectra of the halocyanide species, especially in the region above 12 eV (Le., at wavelengths below the LiF cutoff). The present investigation has therefore clarified radiationless transitions involving the B211state of the XCN' ions and revealed new aspects of the dissociative ionization processes taking place at higher energies. It opens up new areas for further fruitful work on the spectroscopy, structure, and dynamics of the halocyanide cations. Registry No. CN, 2074-87-5; ClCN+, 37612-72-9; BrCN', 3474977-4; E N + , 34749-78-5.
