J. Phys. Chem. 1988, 92, 5514-5517
5514
rotational energy transfer. The flow pattern, Le., ( AE) values in the vibrational-rotational energy plane, indicates a partial but not very substantial rotational heating during the stepwise energy-loss process of vibrationally highly excited molecules. Therefore, we compare our measured ( AE) values with average vibrational energy transfer in the calculations. The comparison shows that the measured (AE)values are markedly (more than 1 order of magnitude) smaller than the calculated results. Experiments and calculations agree in an only weak temperatures dependence of ( AE). However, whereas the measurements gave (AE)a ( E ) * ,the calculations rather give ( A E ) a ( E ) . The discrepancies, hence, are largest for the smallest energies. This suggests that the neglect of quantization in the classical trajectory calculations is one important reason for the discrepancy. The introduction of quantization into classical calculations of vibrational relaxation of diatomic molecules is well established; see, e.g., ref 30. The generalization of this procedure to polyatomic molecules apparently is not trivial at all, such that no a posteriori quantization of the SO2 Ar results from ref 15 was tried. One might think of neglecting all collisions in which less energy is transferred than the smallest vibrational quantum of 5 18 cm-’. This would cut out a major fraction of the collisions and bring the calculated energy transfer much closer to the experiments. However, there is the question how far the validity of such a “propensity rule” would reach up into the vibrational quasi-continuum. There exists some information from studies of large polyatomic molecule^^^^^^ about the extension of such a propensity rule up to energies which correspond to a fairly dense vibrational quasi-continuum. Similar effects in triatomic molecules should be expected to reach up to much higher energies. For the present case, the comparison of trajectory calculations and the present
measurements suggests that such propensity rules extend up to the dissociation energy. The theory of collisional energy transfer in the relevant transition range from the lowest energy levels and from energies in the dense vibrational quasi-continuum is not well developed at this time. Whereas SSH-type theories can well describe single transitions, at higher densities of the levels probably the envelope of the collisional transition probability function becomes most important. Quantum calculations for the lowest discrete transitions in C3H6from ref 37 have confirmed a nearly exponential envelope of the energy transfer rate coefficients as a function of the energy difference before and after the collision. The width of this envelope function corresponds well to ( AE) measurements of highly excited large polyatomic molecules, whereas the averaged ( M )over the sparse manifold of actual energy levels at low energies is much therefore, possibly can smaller. The energy dependence of (AE), be rationalized in terms of several effects, the sparsity of the energy levels at low energies, the extension of propensity rules up to higher energies, and the properties of the envelope of a continuous collisional-energy-transfer probability. Classical trajectory calculations probably correspond only to the latter contribution. SSH-type quantum calculations should be extended to the range of the sparse vibrational quasi-continuum in order to provide a suitable treatment for the former contributions. Promising approaches in this direction are under way.35-37
(35) Krajnovich, D.J.; Parmenter, C.; Catlett, D. L. Chem. Rev. 1987,87, 237. (36)Thoman, J. W.;Kable, S. H.; Rock, A. B.; Knight, A. E. W. J . Chem. Phys. 1986,85,6234. Knight, A. E.W., manuscript in preparation. Kable, S. H.;Knight, A. E. W. J . Chem. Phys. 1987,86,4709.
335-57-9.
+
Acknowledgment. Financial support of this work by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich93 “Photochemie mit Lasern”) is gratefully acknowledged. Registry No. SO2, 7446-09-5;He, 7440-59-7;Ne, 7440-01-9;Ar, 7440-37-1; Kr, 7439-90-9; Xe, 7440-63-3;Hz,1333-74-0; Dz,7782-39-0; Nz, 7727-37-9;CO,630-08-0;HC1,7647-01-0;HzS, 7783-06-4;COz, 124-38-9; SF6,255 1-62-4; CH4,74-82-8;C2H6,74-84-0;C3H8,74-98-6; C7H1.5,142-82-5;CF4,75-73-0;CzF6,76-16-4;C,Fs, 76-19-7;C7F1.5, (37)Clary, D.C. J. Am. Chem. Soc. 1984,106,970; J. Phys. Chem. 1988, 92, 3173.
Spectroscopy of the no-3s Rydberg State of Isolated and Clustered Acetaldehyde D. J. Donaldson, Erik C. Richard, S . J. Strickler, and Veronica Vaida* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-021 5 (Received: December 29, 1987)
The direct absorption spectra of jet-cooled acetaldehyde-do, -dl, and -d4 are reported in the no-3s Rydberg electronic state. Cooling in a jet removes all vibrational hot bands, confirming that there is not a second, “intravalence” transition in this region. Some previously unreported vibrational features are assigned based on a normal-coordinate analysis of the excited state. Cluster-induced changes in the spectrum are observed. These changes are interpreted in light of previous work in which cluster-induced changes in the spectrum provide information about the dissociation dynamics.
Introduction The past decade has witnessed enormous advances in our understanding of molecular photodissociation dynamics.’ These have been spurred largely by improved experimental capabilities: more selective methods of initial-state preparation and new, highly sensitive techniques of product-state analysis. Despite these technical successes, a detailed description of photodissociation dynamics is still elusive for most polyatomic molecules. The major (1)Recent reviews include: Jackson, W. M; Okabe, H. Adv. Phorochem. 1985,13, 1. Simons, J. P.J. Phys. Chem. 1984,88, 1287. Bersohn, R.J. Phys. Chem. 1984,88,5145.Leone,S. R.Adv. Chem. Phys. 1982,50,255. Shapiro, M.;Bersohn, R. Annu. Rev. Phys. Chem. 1982,33, 409. 0022-3654/88/2092-5514$01.50/0
obstacle to a quantitative understanding of photodissociative molecules is an insufficient knowledge about the potential energy surface@) which control the dissociation process. Potential energy surfaces can be derived quite readily for bound systems, by use of high-resolution spectroscopic information. However, by definition, dissociating molecules are short-lived, and so the sensitive spectroscopic techniques appropriate for bound systems become useless. This has led us to developing the technique of direct absorption spectroscopy of molecules cooled in a free expansion.* This method has proven to be particularly powerful in the study of predissociating m o l e c ~ l e s . ~ (2) Vaida, V. Acc. Chem. Res. 1986,19, 114.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 5515
(no-3s) Rydberg State of Acetaldehyde We have very recently developed a method for investigating predissociative surfaces by probing changes in the absorption spectrum induced by the intermolecular interactions which result from cluster f ~ r m a t i o n . We ~ refer to this effect here and elsewhere4v5as “cluster-induced potential shifts” (CIPS). In this effect, the dissociative potential energy surface(s) responsible for the predissociation is stabilized differently by cluster formation than is the surface which is predissdated. This difference is manifested in the absorption spectrum of the clustered species. Specifically, activity is seen in vibronic modes which are strongly coupled to the dissociation channel. In the monomer, these modes are so broadened by lifetime considerations that they are not observed in the absorption spectrum. They appear in the spectrum of the clustered molecule because the position of the predissociative surface crossing has moved, due to the different behavior of the electronic potentials involved. We have reported at length on the CIPS effect and its implications to the dissociation dynamics of methyl iodide4v6and also of a ~ e t o n e .In ~ each case, the inferences made from the CIPS model are entirely consistent with those made from analysis of product energy disposal information. The purpose of the present work is to extend these ideas to a case in which there is as yet no product information available-the no-3s Rydberg state of acetaldehyde. The states and relative energies of acetaldehyde which are relevant to the present work are very similar to those of acetone (see ref 5). The first excited state, S1, with origin at 29771 crn-’, has been the subject of extensive spectroscopic and photochemical studies, most notably those of prof. E. K. C. Lee and co-worke r ~ . ’ - ~Because ~ acetaldehyde has C,, rather than C,, symmetry, pyramidal S1 So transition is allowed, though the planar weak. The major observed progression is in the out-of-plane bending mode, ~ 1 4 which , is significantly coupled to the torsional mode, ~ 1 5 . 9 A barrier to internal rotation of 653 cm-’ is determined from the spectroscopic dataegThe CCO bending mode, vl0, is also observed. Photodissociation of S1 occurs with unit quantum yield at wavelengths below 3 130 A.I2J6 The possible photoproduct channels are
-
CH3CHO*
+
CHjCHO* CH3CHO*
0 58.1d 321 599 657 762 90 1 1051 1156 1291 1491 1611 2452 2763
assignt
low
P
1.o 0.40
high
P
1.o
0.50
0.03 0.04
0.07 0.27 0.13 0.08 0.07 0.14 0.09
0.03 0.05 0.33 0.18 0.14 0.07 0.18 0.13
“ k 1 0 cm-I. bAverage of two determinations (k0.05).CAverageof two determinations (k0.02). From Crighton and Bell.”
CH4 + CO
(3)
H2 + CH2CO
(4)
Experimental Section
CH3 + C H O
(1)
+ CH3CO
(2)
H
+
+
cm-I
analogous to that believed to hold for acetone dissociation from S1.5910 The second excited singlet state, S2, has also been the object of considerable spectroscopic attention.1E-22 It is now quite well established that the transition is to the first Rydberg state, no-3s. The earlier suggestion22that an “intravalence” type2’ transition, no-u*, underlies the Rydberg transition has been shown to be in error.18 The spectrum has been measured by direct absorption’*J2 and by multiphoton ionization techniques.l*21 Short progressions are observed in about half of the symmetry-allowed vibronic modes. Most spectroscopic attention has been directed to the sequence bands in the torsional mode. The barrier to internal rotation in the S2state is found to be 880 cm-1,19,24about twice the ground-state value of 415 cm-I. In the following, we present absorption spectra of the S2 So transition of jet-cooled, isolated acetaldehyde-do, -dl, and -d4and those of the do and d4 isotopes taken under conditions favoring cluster formation. We observe some previously unreported peaks in the spectra and also changes in the relative peak intensities of some vibronic bands upon cluster formation. The “new” bands are assigned by using a normal-coordinate analysis. The cluster-induced differences are interpreted on the basis of the CIPS model outlined above.
-
CH3CHO*
TABLE I: Frequencies, Assignments, and Relative Intensities for Jet-Cooled CHJCHO in the S2Excited State re1 freq,“ re1 intensity
Horowitz and CalvertI6 report a monotonic increase in the quantum yield, a2,with decreasing excitation wavelength. It is believed that (1) is a result of decomposition on the first triplet surface, accessed via intersystem crossing from SI. Process 2 then arises from dissociation of the vibrationally excited S1 and (3) from dissociation of vibrationally excited So, following internal conversion to the ground state.lO*” This dissociation scheme is (3) Vaida, V. NATO A S I Ser. 1987, 200, 253. (4) Donaldson, D. J.; Vaida, V.; Naaman, R. J. Chem. Phys. 1987, 87, 2522; J. Phys. Chem. 1988, 92, 1204. (5) (a) Gaines, G. A.; Donaldson, D. J.; Strickler, S . J.; Vaida, V. J. Phys. Chem., in press. (b) Donaldson, D. J.; Gaines, G. A.; Vaida, V. J. Phys. Chem., in press. (6) Sapers, S. P.; Vaida, V.; Naaman, R. J . Chem. Phys., in press. (7) Stone, B. M.; Noble, M.; Lee, E. K. C. Chem. Phys. Lett. 1985,118, 83. (8) Noble, M.; Lee, E. K. C. J. Chem. Phys. 1984,81,1632; 1984,80,134. (9) Noble, M.; Lee, E. K. C. J . Chem. Phys. 1983, 78, 2219. (10) Lee, E. K. C.; Lewis, R. S. A d a Photochem. 1980, 12, 1. (11) Stoeckel, F.; Schuh, M. D.; Goldstein, N.; Atkinson, G. H. Chem. Phys. 1985, 95, 135 and references therein. (12) Horowitz, A. J . Phys. Chem. 1986, 90, 4393. (13) Ohta, N.; Baba, H. J . Phys. Chem. 1986, 90, 2654. (14) Malar, E. J. P.; Chandra, A. K. J . Phys. Chem. 1987, 91, 5111. (15) Yadev, J. S.; Goddard, J. D. J. Chem. Phys. 1986, 84, 2682. (16) Horowitz, A.; Calvert, J. G. J . Phys. Chem. 1982, 86, 3105. (17) Horowitz, A.; Kershner, C. J.; Calvert, J. G. J. Phys. Chem. 1982, 86, 3094.
-
The absorption measurements are performed using the apparatus and techniques which are described fully in several recent p u b l i ~ a t i o n s . ~ *A~ Jpulsed neat expansion of acetaldehyde is formed from a 1-mm Porsche fuel injector nozzle, operating at 100 Hz, with a pulse width of 1 ms. Stagnation pressures of 20 and 120 Torr are used in the low cluster and high cluster limits, respectively. The background pressure in the observation chamber is maintained at 5 1 mTorr by a cryobaffled 6-in. diffusion pump, with a rated pumping speed of 2500 L/s. No evidence of background contamination is observed in spectra taken under these conditions. The ultraviolet light from a high-brightness D2 lamp is passed in first order through a 1-m monochromator, outfitted with a holographic grating optimized for 1500 A. The slit settings used in these experiments provide an instrumental resolution of 18 cm-’ in the wavelength region of interest (1700-1800 A). The molecular jet is crossed 0.5 cm below the nozzle by the monochro-
-
(18) Crighton, J. S.; Bell, S. J . Mol. Spectrosc. 1985, 112, 304. (19) Eichelberger, T. S.; Fisanick, G. J. J. Chem. Phys. 1981, 74, 5962. (20) Fisanick, G. J.; Eichelberger, T. S.; Heath, B. A.; Robin, M. B. J. Chem. Phvs. 1980. 72. 5571. (21) Hiath, B.’A ;’Robin, M. B.; Kuebler, N. A,; Fisanick, G. J.; Eichelberger, T. S . J . Chem. Phys. 1980, 72, 5565. (22) Lucazeau, G.; Sandorfy, C. J . Mol. Spectrosc. 1970, 35, 214. (23) Mulliken, R. S. J. Chem. Phys. 1939, 7, 20. (24) Crighton, J. S.; Bell, S . J. Mol. Spectrosc. 1985, 112, 315.
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The Journal of Physical Chemistry, Vol. 92, No. 19, 1988
Donaldson et al.
TABLE 11: Frequencies, Assignments, and Relative Intensities for Jet-Cooled C H X M ) in the S, Excited State
re1 freq,a cm-' 0
56.3'
327 826
assinnt origin ("15' "10
- Y15")
1.oo
0.1 1
1185 1324
u6
0.21 0.07
1517
("6
+ "IO)
1 .oo
0.40
Vl
V5
R
re1 intensity hinh P
low pb
I
11
0.10
Oil0 crn-'. *f0.05. cFrom Crighton and Bell.24 TABLE 111: Frequencies, Assignments, and Relative Intensities for CDXDO in the S, Excited State re1 freq," re1 intensity cm-' assignt low pb high F 0 1.oo 1.oo 43.7d
300 645 807 875 994 1182 1297 1869 2211
0.43 0.13
0.13 0.1 1
0.54 0.10 0.07 0.29 0.29 0.08 0.07 0.15 0.12
58000
57000
56000
FREQUENCY
(cm " )
55000
Figure 1. Absorption spectrum of jet-cooled acetaldehyde-do. High cluster expansion conditions: 120 Torr of neat acetaldehyde stagnation pressure.
"110 cm-I. bAverageof two determinations (f0.07). CAverageof two determinations (i0.04). From Crighton and mator output; the transmitted light falls on a solar blind photomultiplier tube. A lock-in amplifier operated at the pulsed nozzle frequency measures the difference (Io - It). This result is normalized by division by Zo; the result is the relative transmitted intensity, (Io - Zt)/Zo, which is proportional to absorbance for low-absorbance values (
