J. Phys. Chem.
1983,87,3003-3007
spectroscopic differences between adsorbed and free Ru(bpy),2+are quite small, we infer that no ligand dissociation has taken place during the chemisorption process. This inference is based on our studies of SERS/SERRS and RRS of Ru(bpy),Cl, which shows easily discernible spectral changes relative to Ru(bpy)gP+. Given that Ru(bpy),2+is not dissociatively chemisorbed on Ag/GaAs, it is possible to understand why the observed spectral differences between the adsorbed and free molecules are small. Ru( b ~ y ) ~is , +a very large, high-symmetry molecule (viz., D, point group) possessing many ligand-centered, strongly coupled normal modes capable of distributing an adsorption induced structure change so that the resulting frequency shifts are very small. The surface coverage of Ru(bpy)SB+on GaAs in these experiments is believed to be less than 1monolayer based on the following indirect evidence: (1) in the experiments reported here, both resonance and surface enhancement were required in order to observe high SIN spectra from adsorbed Ru(bpy)?; (2) submonolayers of strongly resonance Raman active molecules on surfaces yield high SIN spectra even without surface e n h a n ~ e m e n t and ; ~ ~ (3) in experiments to be reported separately,4lwe have observed high S / N SERRS from Ru(bpy),,+ chemisorbed on Ag/ p-GaAs surfaces where the maximum possible surface coverage of the RR probe molecule was restricted to one monolayer by exposing a set of p-GaAs wafers of known (40)M. Barr, A. M. Stacy, and R. P. Van Duyne, manuscript in preparation. (41)R. P. Van Duyne and J. Haushalter, manuscript in preparation.
3003
surface area to an ACN solution volume containing the number of Ru(bpy),2+molecules equivalent to one monolayer. It is not certain that the Ru(bpy)gl+detected by SERRS is, in fact, chemisorbed on the GaAs surface after the electrochemical deposition of Ag. However, since Ru( b ~ y ) ~SERR ,+ spectra of comparable intensity are obtained independent of whether Ag is vacuum deposited, photochemically deposited, or photoelectrochemically deposited on n- or p-GaAs (which have very different Ag deposition potentials), it is very probable that R ~ ( b p y ) ~ , + remains adsorbed to GaAs.
Conclusions In this Letter we have demonstrated the first application of surface-enhanced Raman spectroscopy to the molecular surface characterization of a semiconductor photoelectrode. We have observed SERR spectra for a monolayer or less of an intensionally adsorbed sensitizer, Ru(bpy)gl+,as well as for surface carbon contamination. These results have been achieved by a surface modification process that induces surface enhancement on GaAs while minimally perturbing its photoelectrochemical properties. We believe that this strategy opens up a new methodology for the in-situ molecular surface characterization of SC photoelectrodes. Acknowledgment. The authors acknowledge the generous support of this research by the Office of Naval Research (Contract No. N00014-794-0369) and Dr. Maria Janik-Czachor for the SEM photograph in Figure 3A.
Surface-Enhanced Raman Spectroscopy of Platinum.’ 2. Enhanced Light Scattering of Chlorine Adsorbed on Platinum B. H. Loo Department of Chemistry, The University of Alabama in Huntsviiie, Huntsville, Alabama 35899 (Received: April 4, 1983)
Surface-enhancedRaman scattering has been observed from electrogenerated chlorine adsorbed on a platinum electrode with 488-, 496.5-, and 514.5-nm Ar+ and 647.1- and 676.4-nm Kr+ laser excitation wavelengths. Two vibrational features at 509 and 541 cm-I with an approximate intensity ratio of 1:2 are assigned to the surface vibrations of bridged and linear chlorine on platinum, respectively. This is the first report of a surface vibrational spectrum of molecular chlorine adsorbed on a metal surface.
Introduction The group 1B metals (Cu, Ag, and Au) were first shown to exhibit unambiguous surface-enhanced Raman scattering (SERS), and several theoretical models (electrodynamic models) based on the optical properties of these metals have been proposed.2 These metals have the common optical property that their plasmons are weakly damped in the visible region where laser excitation occurs. The second group of theories (electronic models) involves
the electronic interaction between the metal and the adorba ate.^ This group of theories requires a favorable configuration of electronic levels in the molecule; however, there is no general requirement on the optical properties of the substrate. In principle, SERS may occur on any metal if the bonding between the metal substrate and the adsorbate were compatible. Platinum is one of the best catalysts known, thus extending the analytical capability of the SERS technique to the study of adsorption on Pt is of great interest in both
(1)B. H.Loo, Solid State Commun., 43, 349 (1982). (2)(a) S.S.Jha, J. R. Kirtley, and J. C. Tsang, Phys. Rev. B , 22,3973 (1980);(b) S. L. McCall, P. M. Platzman, and P. A. Wolff, Phys. Lett. A , 77,381 (1980);(c) D.S.Wang, M. Kerker, and H. Chew, Appl. Opt., 19,4159 (1980);(d) J. L.Gersten and A. Nitzan, J. Chem. Phys., 73,3023 (1980);(e) F.J. Adrian, Chem. Phys. Lett., 78,45 (1981).
(3)(a) J. I. Gersten, R. L.Birke, and J. R. Lombardi, Phys. Reu. Lett., 43, 147 (1979);(b) E.Burstein, Y . Chen, C. Chen, S. Lundquist, and E. Tossati, Solid State Commun.,29,565(1979);(c) J. Billman, G. Kovacs, and A. Otto, Surf. Sci., 92,153 (1980);(d) F.J. Adrian, J . Chem. Phys., 7 7 , 5302 (1982);(e) A. Otto, J . Electron Spectrosc., to be published.
0022-3654/83/2087-3003$0 1.50/0
0 1983 American Chemical Society
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Letters
The Journal of phvsical Chemlstry, Vol. 87,No. 16, 1983
electrocatalysis and heterogeneous catalysis. SERS has only been reported from a few molecules adsorbed on Pt surfaces; they are Iz and 1,- on a Pt electrode,l pyridine on vacuum-deposited Pt films: and benzene on small Pt cluster^.^ In the present work we use Raman spectroscopy to study the evolution of chlorine at a Pt electrode. We report here the observation of SERS from chlorine adsorbed on Pt. This is the first report of a surface vibrational spectrum of molecular chlorine adsorbed on a metal surface. No surface vibrational spectrum of adsorbed chlorine has been reported previously with Raman spectroscopy, infrared reflection-absorption spectroscopy (IRAS), or high-resolution electron energy loss spectroscopy (HREELS). The present work also shows that the electronic mechanism is the predominant mechanism for the observed Raman enhancement since Pt does not have dielectric functions like those of the group 1B metals to support electromagnetic resonances on surfaces.
Experimental Section The in situ Raman experiments were made on a Pt surface in an electrochemical cell which consisted of a polycrystalline Pt working electrode, a Pt counterelectrode, and a saturated calomel reference electrode (SCE). Because of its high electrocatalytic activities, the preparation of a clean Pt surface was emphasized. The Pt electrode was polished with 1-pm Alto3 slurry, degreased with acetone, and then treated with hot chromic acid? It was then thoroughly rinsed with distilled deionized water. The electrode was then introduced into the electrochemicalcell at a cathodic potential and the surface-enhanced Raman (SER) spectra were recorded during the anodic scan. The oxidation-reduction cycling treatment of the electrode surface was not necessary for the observation of the SERS signals. The potential of the Pt working electrode was controlled with a potentiostat (Pine Instrument RDE3) and, unless otherwise noted, all potentials are reported with respect to SCE. Reagent grade potassium chloride (J. T. Baker) was used in preparing 1 M KCl solutions. The electrolyte was deoxygenated with nitrogen for about 30 min before the experiment. The 647.1- and 676.4-nm lines of a Kr+ laser (Spectra Physics 171) and the 488-; 496.5-, and 514.5-nm lines of an Ar+ laser (Spectra Physics 171) were used as excitation sources. The experiments were done in backscattering ge~metry,~ with p-polarized light and an angle of incidence of about 60'. The laser power used was typically 100-200 mW. The Raman scattered light was collected with an elliptical collector mirror (Spex 1459 UVISIR illuminator) and analyzed with a double monochromator (Spex 1402) with 1200 lines/" gratings, blazed at 5000 A. In conjunction, a cooled photomultiplier (RCA C31034A) and a standard photon-counting system were used. The emission lines of a pen Hg lamp were used for the wavelength calibration. Results and Discussion
The electrochemical production of chlorine by electrolysis of brine is one of the major processes in the chemical industry. The fundamental and applied aspects of anodic chlorine evolution were recently reviewed by Novak et aL8 (4) (a) H. Yamada and Y. Yamamoto, Chem. Phys. Lett., 77, 5020 (1982); (b) H. Yamada, Y. Yamamoto, and N. Tani, ibid., 86,397 (1982). (5) W. Krasser and A. J. Renouprez, Solid State Commun., 41, 231 (1982). (6)D. T. Sawyer, and J. L. Roberta, Jr., "Experimental Electrochemistry for Chemists", Wiley, New York, 1974, p 78. (7) R. P. Van Duyne in "Chemical and Biochemical Applications of Lasers", Vol. 4, C. B. Moore, Ed., Academic Press, New York, 1978, Chapter 4.
-&
(a'
:
l'zv
(cl 1 . o v
\
(bl 1 ' 3 v
WAVELENGTH inml
Flgure 1. Surfacwnhanced Raman spectra of Ft' in 1 M KCI solution for a sequence of applied potentials: (a) 1.2, (b) 1.3, (c) 1.0 .V, X, = 647.1 nm Kr' line. Laser power 150 mW.
''
514
5.&
'
514
A2
'
518
'
5g2
SI4
'
528
!532
WAVELENGTH Inml
Figure 2. Surface-enhanced Raman spectra of Pt in 1 M KCI solution at different applied potentials: (a) 1.0, (b) 1.2, (c) 1.3 .V, X, = 514.5 nm Ar+ line. Laser power 175 mW.
It is one of the simplest gas evolution reactions, and its kinetics and mechanisms are very similar to those for the cathodic hydrogen evolution reaction. The anodic C12 evolution reaction has a Coulombic yields of about 96% except in dilute solutions where some oxygen and chlorine oxides may be formed. The thermodynamics of the C12-C1reaction at equilibrium 2e
+ C12 s 2C1-
are well e~tablished.~ This reaction is relatively reversible and can be established as a practical reversible electrode at platinized Pt electrodes or plantinized-iridized surfaces in a way similar to that employed for Hz-H+ reversible electrodesq8 The recommended standard reversible potential is 1.35828 VH (Stockholm convention)10 or 1.116 VSCE*
With this brief description of the electrochemistryof the anodic evolution of chlorine at a platinum electrode, we proceed now to the analysis of the surface Raman results. More electrochemistry and surface chemistry will be discussed in the later sections where appropriate. Figure 1 shows the SER spectra of Pt in 1 M KC1 solution at three different applied potentials, with the 647.1-nm Kr+ line excitation wavelength. The Pt electrode was scanned from a cathodic potential at 0.3 Vf min to an anodic potential of 1.2 VSCE, where weak Raman signals were observed (Figure la). The Raman signals became more prominent at 1.3 V (Figure lb), and two peaks are apparently visible, one at 509 and another at 541 cm-'. A t this potential, (8) D. M. Novak, B. V. Tilak, and B. E. Conway in "Modern Aspects of Electrochemistry", Vol. 14, J. O'M. Bockris, B. E. Conway, and R. E. White, Ed., Plenum Press, New York, 1982, Chapter 4. (9) D. J. Ives and G . I. Janz, Ed., 'Reference Electrodes", Academic Press, New York, 1961, p 111. (10) A. J. Bard, Ed., "Encyclopedia of Electrochemistry of the Elements", Vol. 1, Marcel Dekker, New York, 1973.
The Journal of Physical Chemistry, Vol. 87,
Letters
No. 16,
1983
3005
TABLE I: Vibrational Frequencies (in c m - ' ) for Chlorine in Gaseous, Liquid, and Solid States, and Adsorbed on the Platinum Electrode solidC
37
c1,
5C137C1
gaseousa
liquidb
18 K
54 0
533.4
522
5 23
547
540.9
530
531
adsorbed ond Pt electrode
79 K
1
509 (bridged) 541 (linear)
5 54
3 5 ~ 1 ,
Reference 11.
Reference 12.
iE;
548.4 Reference 13.
Present work.
WAVE LENGTH Inml
Flgwe 3. Surfaceahanced Raman spectra of Pt in 1 M KCI solution at different applied potentials: (a) 1.3, (b) 1.2, (c) 1.0 . ,V X, = 496.5 nm Ar+ line. Laser power 175 mW.
evolution of C12at the Pt electrode was visible and some bubbles were formed on the electrode surface. The formation of bubbles did not interfere with our Raman measurements. However, at higher potentials (>1.4 V), vigorous bubbling of C12gas at the Pt electrode made the Raman measurements difficult. Upon returning the electrode potential to 1.0 V (Figure IC)where there was no C12 evolution, the SERS signals disappeared. The Raman signals reappeared when the electrode potential was scanned back to a more anodic potential at which C12 evolution resumed. The SERS signals at 509 and 541 cm-l were also observed when 676.4-nm Kr+ and 488-, 496.5-, and 514.5-nm Ar+ excitation wavelengths were employed. Figure 2 shows the SER spectra of Pt in 1M KC1 solution with 175-mW, 514.5-nm Ar+ line excitation for a sequence of applied electrode potentials, 1.0, 1.2, and 1.3 VScE. Figure 3 shows the SER spectra with 175-mW, 496.5-nm Ar+ line excitation for a sequence of electrode potentials, 1.3,1.2, and 1.0 VSCD In both cases, the Pt electrode was also introduced at a cathodic potential and then scanned anodically at 0.3 V/min to the required anodic potentials. The oxidationreduction cycling (ORC) treatment of the electrode surface was not necessary for the observation of the SERS signals. Two salient features associated with the SER spectra are, first, the intensity ratio of the observed bands I[v(509)]:I[v(541)] 1:2 for all the excitation wavelengths used and, second, after correcting for the wavelength dependence of the photoelectric response of the spectrometer system (the photoelectric response of the spectrometer system at different wavelengths was performed by Dr. William K. Bischel), it was found that the enhancement in the Raman intensity is greater at shorter wavelengths. A similar wavelength dependency of the Raman enhancements was observed for 13-adsorbed a Pt electrodel and benzene on small Pt cluster^.^ This observation is different from those observed from molecules or ions ad-
-
sorbed on Cu, Ag, and Au substrates. In the latter cases, the Raman enhancements are greater at longer wavelengths. The frequencies and assignments of the observed SERS signals for Pt in 1M KC1 solution are summarized in Table I along with the reported vibrational frequencies for gaseous,ll liquid,12and solid ch10rine.l~ Raman spectra of molecular chlorine are well characterized in all three phases,11-16 thus providing a good basis for comparison. Molecular chlorine exists in three different isotopic species 35C135C1,35C137C1,and 37C137C1 with natural abundance of 9:6:1. The Raman spectra of chlorine in gaseous, liquid, and solid states exhibit three peaks in the fundamental region which correspond to the three isotopic species (Table I). The intensity distribution of these peaks reflects the natural isotopic abundance, and the frequency differences also agree with isotopic predictions. In the solid state,13-16the strongest fundamental band, ~ ( ~ ~ csplits l,), into two components, one corresponding to the two molecules stretching in phase, the A, mode, and the other to out-of-phase motion, the B3gmode. The observed SERS signals exhibit two relatively broad peaks (Figures 1-3), one at 509 and another at 541 cm-'. Because these two peaks are broad and overlapped with each other, vibrational bands due to different isotopic molecular chlorine species cannot be resolved. The stronger band at 541 cm-l is closer to the solid-state value of 540 cm-l for ~ ( ~ ~ c l ~ ) . We assign this band to the adsorbed molecular chlorine with the linear structure given by I. The 509-cm-l band Cl
I Cl I pt I
CI
I
?\
Pt
Pt
Pt
/cI-cI
P 't
I11
I1
is assigned to the adsorbed molecular chlorine with bridged structures I1 or 111. If all of the surface platinum atoms are assumed to have an equal probability of bonding with the chlorine atoms, then structure I will occur twice as often as structures I1 or 111. This is because, for each adsorbed molecular chlorine, one surface platinum atom is needed for structure I whereas two surface platinum atoms are needed for structures I1 or 111. This would explain the observed intensity ratio I[v(509)]:I[v(541)] 1:2. Bridged metal-halide complexes are known for many metals including p1atinum.l' Both the bridging and
-
(11)W.Holzer, W.F. Murphy, and H. J. Bernstein, J . Chem. Phys., 52, 399 (1970). (12)H.Stammreich and R. Forneris, Spectrochim. Acta, 17, 775 (1961). (13)A. Anderson and T. S. Sun, Chem. Phys. Lett., 6 , 611 (1970). (14).M. Suzuki, T.Yokohama, and M. Ito, J . Chem. Phys., 50,3392 (1969). (15)J. E. Cahill and G . E. Leroi, J. Chem. Phys., 51, 4514 (1969). (16)G. Dumas, F. Vovelle, and J-P. Viennot, Mol. Phys., 28, 1345 (1974).
T A B L E 11: Vibrational Frequencies (in cm-I)'
species C13- (solid) C10 (matrix)
c1,o
C10,- (solid) C10,' (solution) c10,PtCl, (solid)b PtC1,2a
Reference 21.
vibrational frequencies 268 ( u , ) , 1 6 5 ( v 2 ) , 242 ( v i ) 970 630.7 ( u l ) , 296.4 ( u 2 ) , 670.8 ( u ? ) 790 ( V I ) , 400 ( u 2 L 840 ( v 3 ) 933 ( u l ) , 608 ( u 2 ) , 977 ( u 3 ) , 477 ( v 4 ) 928 ( v l ) , 459 ( u 2 ) , 1 1 1 9 (v,), 625 ( u 4 ) 318 ( Y P t - C l ) 330 ( v i )! 1 7 1 ( p , ) , 1 4 7 ( u i ) , 312 ( ~ 4 1 , 313 ( ~ ),6 1 6 5 ( ~ 7 )
Reference 22.
TABLE 111: Vibrational Frequencies (in cm-l ) of Dioxygen and Atomic Oxygen Species on Platinum' dioxygen atomic oxygen, system u(0-0) u(Pt-0,) v(Pt-0) 0,(9) 0 20,zO,/Pt( 111) O/Pt( 111) a
Letters
The Journal of Physical Chemisv, Vol. 87,No. 16, 1983
3006
1555 1145 84 2 870
390 490
Reference 18.
nonbridging metal-halide vibrations have been well established. Generally the metal-halide stretching vibration for a nonbridging halogen ligand occurs at a higher frequency than the corresponding bridged metal-halide vibration. Although no surface vibrational spectroscopic studies of chlorine on metal surfaces have been reported before, diatomic molecules such as CO and O2 adsorbed on Pt surfaces have been well studied by HREELS and IRAS.'sJg Bridged structures have been found for the surface adsorbates for the CO/Pt and 02/Pt systems. HREELS data of CO on Pt(lll)lgb"have shown that changes in the CO bonding site from linear (C,,, similar in structure to I) to bridged (C2",similar in structure to 11)cause significant down shifts of the v(C-0) frequency from 2100 to 1850 cm-'. In the case of O2 on P t ( l l l ) , HREELS data show that the oxygen is coordinated in a side-on configuration with the 0-0 axis parallel to the Pt surface (similar in structure to 111). The side-on bonding is necessary to explain the low value of the 40-0) frequency of 870 cm-l from the free gas value of 1555 cm-l.18 As we have mentioned earlier the anodic chlorine evolution reaction has a Coulombic yields of about 96% except in dilute solutions where some oxygen and chlorineexkks may be formed. And at high anodic potentials, hypochlorite, chlorite, chlorate, and perchlorate ions may also be formed. Other possible products include C1y and PtCld2-ions.20 The possibility that the observed SERS signals may be due to the presence of these possible products can be ruled out through the examination of the vibrational frequencies tabulated in Tables I1 and 111. One interesting note here is that although AuC1,- is the predominant product of Au in chloride solutions at high anodic potentials and its SERS has been reported recently,23 (17)J. R. Ferraro, "Low Frequency Vibrations of Inorganic and Coordination Compounds", Plenum Press, New York, 1971, Chapter 5. (18)B. A. Sexton, A p p l . Phys., A26, 1 (1981). (19)(a) R. A. Shigeishi and D. A. King, Surf. Sci., 58,379 (1976);(b) H. Froitzheim, H. Hopster, H. Ibach, and S. Lehwald, AppE. Phys., 13, 147 (1977);(c) H. Steininger, S. Lehwald, and H. Ibach, Surf. Sci., 123, 264 (1982). (20)W. M. Latimer, "Oxidation Potentials", Prentice-Hall, Englewood Cliffs, NJ, 1952,2nd ed. (21)K. Nakamoto, "Infrared and Raman Spectra of Inorganic and Coordination Compounds", Wiley, New York, 1978,3rd ed. (22)D. M. Adams, M. Goldstein, and E. F. Mooney, Trans. Faraday SOC.,59,2228 (1963). (23)B. H. Loo, J . Phys. Chem., 86,433 (1982).
we observed no similar behavior for Pt in chloride solutions at high anodic potentials. We now elaborate on the oxygen evolution reaction and the state of the platinum surface at high anodic potentials. Thermodynamically oxygen, which has the standard potential of 1.230 VH or 0.988 VSCE,should be evolved first before chlorine. However, the exchange current densities for C12evolution are generally much greater than those for O2 evolution on platinum metals, so that C12evolution is usually the preferred anodic process in the electrolysis of aqueous C1- solutions. HicklingZ5has shown that the predominant process was C12evolution after the addition of C1- ions to a solution from which O2 was being evolved anodically at Pt electrodes. No SERS signals due to atomic or molecular oxygen adsorbed on Pt (with reference to the HREELS results of Table 111) have been observed from Pt in 1M KC1 solution. Negative SERS results were also obtained from Pt in 0.5 M H2S04,in which case the O2 evolution reaction is the only possible anodic process. This observation rules out the possibility that the observed 509- and 541-cm-' bands were due to the adsorbed atomic or molecular oxygen species on Pt surfaces. At high anodic potentials, most metal electrodes will be covered or partially covered with a surface oxide film. However, C1- ion adsorption has a major effect on the development of the surface oxide film. On any bare metal sites, C1- is normally strongly chemisorbed. As a result, the formation of monolayer surface oxides especially at Pt or Rh is strongly inhibited. Conway et a1.26investigated the effects of the C1- ion over a wide range of concentrations on the blocking of surface oxide at Pt. They found that, up to potentials where Clz evolution commences significantly, the quantity of surface oxide generated at Pt electrodes decreases to -10% of a monolayer as the C1- ion concentration is increased to 1.0 M. Therefore, it is reasonable to conclude that the Pt surface is near oxide-free under our present experimental conditions. In the absorption spectrum of C12,27a weak continuum occurs in the region 250-450 nm which represents the transition 'nu X lZg+. The maximum absorption is at 330 nm with an absorption coefficient tmar = 66 L mol-' cm-l. This continuum is preceded by a much weaker band in the region 478-600 nm, corresponding to the transition B 311(0,+) X 'Eg+,where most of the laser excitations occurred. Because of the feebleness of this absorption, no resonance Raman effect of C12has been reported with laser excitation wavelength as low as 476.5 nm." In fact, the relative normalized differential Raman cross section (Zj) for the 554-cm-' band of gaseous C12 is only about twice those for Q branches of vibrational bands of N2,02, or CO, measured at the exciting lines 488 and 514.5 nm.28 SERS has been reported from CO on Ag and Aump3Oand from O2 on Ag and Al.31932The saturation coverage of chlorine on platinum is less than a monolayer; 6 = 0.51 for Pt(lll),33
-
+-
-
(24)B. E. Conway, 'Electrochemical Data", Elsevier, Amsterdam, 1952. (25)A. Hickling, Trans. Faraday Soc., 41,333 (1945). (26)B. E.Conway and D. M. Novak, J. Chem. SOC.,Faraday Trans., 75, 2454 (1979). (27)H. Okabe, "Photochemistry of Small Molecules", Wiley, New York, 1978. (28)H. W. Schrotter and H. W. Klockner in "Raman Spectroscopy of Gases and Liquids", A. Weber, Ed., Springer-Verlag, Berlin, 1979. (29)T. H. Wood, M. V. Klein, and D. A. Zwemer, Surf. Sci., 107,625 (1981). (30)H. Seki, Solid State Commun., 42,695 (1982). (31)A. Otto, I. Pockrand, J. Billman, and C. Pettenkofer in "Surface Enhanced Raman Scattering", R. K. Chang and T. E. Furtak, Ed., Plenum, New York, 1982. (32)T. Lopez-Rios, C. Pettenkofer, I. Pockrand, and A. Otto, Surf. Sci., 121, L541 (1982). (33)W. Erley, Surf. Sci., 94,281 (1980).
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).
3007
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
