J. Phys. Chem. 1983, 87, 3007-3009
and 0 = 0.63-0.71 for Pt(l10).34 Therefore, we conclude that the Raman signals due to the adsorbed chlorine are surface-enhanced. The enhancement factor was estimated on the order of 104-105, with larger enhancements at shorter wavelength^.^^ A common optical property of the group 1B metals is the possibility of having surface plasmon resonances in the visible. However, no such possibility exists for Pt because Pt does not have the right dielectric function to support oscillation of surface polarition fields in the visible region. We believe the electronic mechanism3 dominates in the present case, and charge transfer between the metal substrate and the adsorbate probably occurs through active sites on the Pt surface, such as Pt ad atom^.^^
Conclusion SERS has been observed from C1, adsorbed on Pt with 488-, 496.5, and 514.5-nm Ar+ and 647.1- and 676.4-nm (34) W. Erley, Surf. Sci. 114, 47 (1982). (35) To estimate the Raman enhancement factor, we compare the SERS signals of Clz on Pt with that of pyridine on Ag under the same experimental conditions. (36) (a) J. Billman, G. Kovacs, and A. Otto, Surf. Sci., 92, 153 (1980); (b) J. Timper, J. Billman, A. Otto, and I. Pockrand, ibid.,101,348 (1980); (c) A. Otto, J. Billman, and I. Pockrand, Chem. Phys. Lett., 45 46 (1980).
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Kr+ laser excitation wavelengths. Two broad surface vibrational features at 509 and 541 cm-l are ascribed to the linear and bridged molecular chlorine species adsorbed on Pt, respectively. We conclude that the electronic mechanism is the most important mechanism for the observed SERS. Chlorine is one of the promoters routinely used to improve the activity, selectivity, and stability of metal catalysts in chemical industries. However, knowledge of the influence of promoter-metal charge transfer on the bonding of adsorbates (e.g., hydrocarbons) is still la~king.~'The SERS technique could provide a valuable in situ vibrational spectroscopic tool for this type of study.
Acknowledgment. The author thanks Dr. George Pimente1 for a helpful discussion, Dr. A. Otto for a preprint of ref 3e, and Dr. William Bischel for technical assistance. Part of this work was completed at the Molecular Physics Laboratory, SRI International. Registry No. Chlorine, 7782-50-5; platinum, 7440-06-4. (37) (a) T. Edmonds and J. J. McCarrol, "Topics in Surface Chemistry", Plenum Press, New York, 1978, p 261. (b) G. Broden, G. Gafner, and H. P. Bonzel, Surf. Sci., 84,295 (1979). (c) G.Ertl, M. Weiss, and S. B. Lee, Chem. Phys. Lett., 60, 391 (1979).
Mode-Selective Cooling of Vibrational Energy during Supersonic Expansion of Pyrimidine: Evidence for Noncommunicating Sets of Vibrational Manifold A. Kelth Jameson, Department of Chemistry, Loyola Unlversky of Chicago, Chlcago, Illinols 60626
H. Salgusa, and E. C. Llm" Department of Chemistry?Wayne State Unlverslty, Detfoff, Mlchlgan 48202 (Received: April 12, 1983; In Final Form: June 16, 1983)
We present here evidence for mode-selective cooling of vibrational energy during supersonic expansion of pyrimidine. The 16a: sequence band is strongly cooled in the expansion while the 16bi sequence band is cooled very little. Within the context of a unified physical model for intramolecular and intermolecular energy flow, the observation of mode-selective vibrational cooling implies that intramolecular vibrational energy redistribution may be incomplete even for molecules with large excess vibrational energies.
In recent years it has become possible to deposit energy into a single molecular resonance (vibronic level) and then probe the disposition of the energy as time pr0gresses.l Much of the impetus for the study of vibrational energy redistribution arises from the possibility (asyet unfulfilled) that mode-specific chemistry may be accomplished. If vibrational energy can be localized into one mode to the exclusion of another then specific reactions, unattainable under equilibrium conditions, could be made to occur. The majority of the past studies on vibrational energy redistribution has centered on two classes of molecules: (1) relatively small polyatomic molecules (less than about 6 atoms) which are vibrationally excited in their ground electronic state and (2) more complex molecules which are (1) See, for recent reviews, "Photoselective Chemistry, Advances in Chemical Physics", Vol. 47, J. Jortner, R. D. Levine, and S. A. Rice, Ed., Wiley-Interscience, New York, 1981. 0022-3654l8312087-3007$0 1.5010
vibrationally excited in their lowest excited singlet state. At low vibrational excitation energies (e.g., one or two quanta) class (1)molecules show no evidence of intramolecular vibrational redistribution (IVR) on the time scale of the radiative lifetime, which may be s or longer. Collision of the excited molecule with a monatomic buffer gas can cause V-V transfer in about 10 collisions, while V-T/R energy transfer occurs on a relatively slow time scale (>lo0 collisions for the loss of one quantum of vibrational energy from the lowest frequency mode). For class (2) molecules with relatively modest excess vibrational energies, the bulk of the most direct evidence (although not all) is consistent with a picture of IVR taking place on a time scale of lo-" to s . ~ Collisionally induced V-V energy transfer processes in these larger molecules (2) See, for example, R. E. Smalley, J. Phys. Chem., 86, 3504 (1982); C. S.Paramenter, ibid., 86, 1735 (1982).
0 1983 American Chemical Society
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Letters
The Journal of Physical Chemistry, Vol. 87,No. 16, 1983
1
D
'T
0
O-340.
-240.
-140.
-40.
60. ENERGY I Cn-'l
160,
260.
360.
460.
Flgure 1. Fluorescence excitation spectrum of jet-cooled pyrimidine taken with a YAGpumped dye laser (DCM) which is frequency doubled. Excitation wavelengths cover 318.0-325.2 nm. The 0-0 band of So S, excitation (31 072.7 cm-') is the origin of the energy. The jetcooled pyrimidine was prepared by mixing pyrimidine vapor (parthi pressure 1-2 torr) with 200 torr of Ar carrier gas, and subjecting the mixed gases into a continuous supersonic expansion through a 200-~mdiameterpinhole. Experimentaldetails can be found in ref 9.
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often occur very nearly on every collision. If one considers the collision "complex" to be a supermolecule with a single very low-frequency mode during the lifetime of the complex then it is relatively easy to obtain a physical picture of collisional energy transfer as creation of a transient molecule with normal modes which have been perturbed from those of the original molecule. Suppose we count only those fragments of time during which a collision occurs. Then, although 10 collisions occur over a time span dependent on density, the transient collision complex, or supermolecule, exists for an average of ten times the duration of a collision. An upper limit to the collision duration can be estimated from the vibrational frequency for van der Waal's complexes (1-10 cm-' or 3 X 1O'O-3 X 10" s-l). Then ten collisions results in a total "existence time" of ten times the duration of an average collision and yields effective redistribution times in the vicinity of 0.1 ns, consistent with IVR rates in larger electronically excited molecules. A more realistic, shorter, collision duration results in shorter effective lifetimes for this hypothetical molecule, perhaps by 1-2 orders of magnitude. Even though this physical picture argues strongly against long time retention of a large amount of vibrational energy in electronically excited molecules the question of the vibrational manifold possibly consisting of noncommunicating sets is unanswered. Mode-specific collisionally induced vibrational redistribution has been observed in both ground and electronically excited state^.^ Of particular interest to the observations presented here are groundstate energy transfer experiments. There is evidence in small polyatomic molecules for some vibrational modes being very poorly coupled to other modes and also to the collision partner during the collision process, independent of buffer gas. In the naive physical picture of a supermolecule this would mean slow to nonexistent IVR rates involving these sets of vibrational modes. The classic example of molecules exhibiting this behavior is SO2 in which the bending mode is nearly independent of the stretching modes in terms of collisionally induced energy t r a n ~ f e r . ~ Indeed, the possibility for mode-specific (3) For a recent review for electronically excited molecules see S. A. Rice in ref 1. (4) D. Siebert and G. Flynn, J. Chem. Phys., 62, 1212 (1975); R. C. Slater and G. W. Flynn, ibid.,65, 425 (1976).
0-L. 0
-290.
-240.
-I%. EmRcrl,44h-i, ;
-90.
-40.
IO.
Bo. *
Flgure 2. Fluorescence under effusive conditions (upper spectrum) and under jet-cooled conditions (lower spectrum). Conditions are identical with those of Figure 1 except that the excitation wavelength range is 321.3-325.0 nm. Those features in effusion which disappear upon jet-cooling are marked with "c".
I
D
1
-1
I
I1
C
II ~
1
-40,
90.
140.
190.
240. ENERGY I Cn-'l
290.
340.
390.
440.
Figure 3. Higher energy (318.0-32 1.3 nm) fluorescence excitation spectra of pyrimidine under effusive (upper trace) and jetcooled (lower trace) conditions which are the same as in Figure 1. Four small features which disappear under jetcooled conditions are marked with "C".
chemistry in this molecule has been explored. Another example is CH3C1 in which collisionally induced intramolecular V-V transfer between V6 and u3 are substantially slower than V-V transfer between yg and other vibrational modes.5 These are however small molecules and one might question whether more complex molecules can show a similar behavior. We have found a phenomenon in pyrimidine (l,&diazabenzene) which we interpret in just this manner. Figure 1 presents fluorescence excitation spectrum of jet-cooled sample covering the wavelength region of 318.0-325.2 nm and centered about the 0-0 band of So SI absorption. In this spectral range we see a number of features (see Table I). These are expected to be hot bands and sequence bands, most of which have not been assigned.6 Figures 2 and 3 compare fluorescence excitation
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(5) J. T. Knudtson and G. Flynn, J. Chem. Phys.,58, 2684 (1973). (6) K. K. Innes, H. D. McSwiney, Jr., J. D. Simmons, and S. G. Tilford, J . Mol. Spectrosc., 31, 76 (1969).
The Journal of Physical Chemistty, Vol. 87, No. 16, 1983
Letters
TABLE I: Energies and Assignments of Features Appearing in Figure 1 energy,a cm-’ - 318 - 308 - 276 -272 -180 - 174 -156.5 - 88 - 69 -61 - 39
0 t8 22 32 65 108 163 194 225 240 285 291 307 328 333 337 339 364 37 2 372.5
relative intensityb effusive 20 2 1 7 1 1 58 1
-800 -50 11 0 2
4 1 3 1 1 1 2 2 3
jet-cooled
assignmentd
65 1C
16a:
4 c 2 C
4c 3 2 3 1 e -10 68 3 2 15 0.5 2 5 1 8 9 2
16a:
0-0
16b:
12y6aA16b:
C
C
c 2 3 3 c
6af8ai
Energies relative t o the 0-0 band at 31 072.7 em-’. Relative intensities are t o be compared only within each column, not between columns. Vibronic bands which are strongly cooled in the jet expansion, All assignments are due t o Innes (ref 5) except 16a: which is established here. e The intensities of these bands relative t o the very strong 0-0 band cannot be established with certainty, a
spectra taken under jet-cooled and effusive conditions for enlarged sections. The most spectacular difference between the two conditions in Figure 2 is the near disappearance of 16ai band under jet-cooled conditions. By contrast, 16b: band and the feature at -318 cm-’ are intransigent to cooling. Closer inspection of the weaker features in Figures 2 and 3 indicates that some of them cool efficiently while others do not. 16a and 16b are the two lowest frequency vibrational modes known in So (394 and 344 cm-l, respectively). It is therefore remarkable that the higher frequency of these two out-of-plane bending modes is cooled so dramatically while the somewhat lower frequency mode is cooled very little. We estimate the vibrational “temperature” of these features to be over 200 K for 16b: and less than 100 K for 16a: with a backing pressure of 200 torr of Ar. We have varied the backing pressure and found that to achieve the cooling attained at 200-500 torr of Ar for the “intransigent” features requires only about 25 torr of Ar for 16a:. Making the debatable assumption that the effective collision number for a similar degree of cooling is related to the backing pressure
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we find that the 16a: feature has an effective collision number of about a factor of 10 less than the intransigent features. In the supersonic jet molecules proceed in perhaps 10-6-10-5 s from an equilibrium situation to an isolated molecule status. The quasiisentropic expansion is strongly nonequilibrium and nonisotropic, and cannot easily be modeled. The collisionally induced energy transfer process is therefore ill-defined. Nevertheless, it is quite clear that, on a time scale of 1-10 ps, 16a: is strongly cooled while 16b: is hardly cooled at all. Collisionally induced V-V energy transfer between 16a and 16b is therefore slow on this time scale and IVR between the two modes is essentially nonexistent. We suspect that other features which disappear upon jet-cooling involve 16a or vibrations strongly coupled to 16a. We can, for example, assign 16ai to the weak irregularity (-308 cm-l) on the R branch of the strong feature near -318 cm-l. This leaves the assignment of the major feature at -318 cm-’ in doubt (possible vibrational assignments of various vibronic bands in the 3200-A absorption system of pyrimidine will be presented elsewhere). The mode-selective vibrational cooling which has been observed in pyrimidine sharply contrasts with the results on S-tetrazine in which 16a and 16b modes are found to cool almost equally efficiently.’ Vibrational predissociation of van der Waal’s complexes has been predicted by Ewing to be an effective mechanism for vibrational cooling.8 If the complex has a binding energy between 394 and 344 cm-’ then especially efficient vibrational energy transfer from 16al could occur. If the binding energy of the complex lies lower in energy than both low-frequency modes in S-tetrazine then this mechanism could be effective in cooling both modes. But whatever the means by which 16al is preferentially cooled there is remarkably ineffective intramolecular communication between 16a and many other vibrational modes in pyrimidine, resulting in no apparent intermodal energy transfer on the time scale of the expansion process. As experiments on specific molecular systems are devised and executed a general picture of IVR may emerge. Presently, very rapid IVR seems commonplace for the large molecules with high excitation energies, while in small molecules with low excitation energies collisions couple most or all vibrations. However, the existence of effectively noncommunicating parts of the vibrational manifold under collision in some small molecules and now a 10 atom molecule may hold out hope for slow IVR at moderate excitation energies in some of the more complex molecules.
Acknowledgment. We wish to thank a referee for pointing out the possibility of vibrational predissociation giving differential cooling. This work was supported by a grant (CHE-8119202)from the National Science Foundation. Registry No. Pyrimidine, 289-95-2. (7) D. H. Levy, private communication, P. R. R. Langridge-Smith,D. V. Brumbaugh,C. A. Haynam, and D. H. Levy, J. Phys. Chem., 85,3742 (1981). ( 8 ) G . Ewing, Chem. Phys., 29, 253 (1978). (9) H. Saigusa and E. C. Lim, J. Chem. Phys., 78, 91 (1983).
