Vibrational Frequencies of C2H4 and C2H6 Adsorbed on Potassium

1196

Langmuir 1996,11, 1196-1200

Vibrational Frequencies of C2H4 and C2H6 Adsorbed on Potassium, Indium, and Noble Metal Films W. Akemann"9t and A. Otto Lehrstuhl f i r Oberflachenwissenschaft (IPkM), Heinrich-Heine-Universitat Diisseldorf, 40225 Diisseldorf, Germany Received September 28, 1994. In Final Form: January 27, 1995@ Surface enhanced Raman spectroscopy (SERS) was applied to determine the vibrational frequencies of C2H4 and C2H6 adsorbed to cryocondensed metal films of potassium, indium, copper, silver, and gold at 40 K under ultrahigh vacuum conditions. In the case of indium and noble metals, downshifted (E) and unshifted (N) frequency bands with respect to the band position in condensed C2H4 or C2H6 films are resolved. The YE C-H stretch frequencies are downshifted by 10-30 cm-l for CzH4 and CzH6 commonly. C2H4 exhibits additional mode softening of the Y E C-C stretch and the BE CH2 scissor vibrations on noble metal surfaces, but not on indium, yielding nu-parameters for the E-species of 0.1 (Ag),0.2 (Cu), and 0.25 (Au). On potassium C2H4 reacts and is at least partially cleaved, whereas C2H6 is stable and forms a physisorbed layer featuring N-frequencies exclusively.

1. Introduction Vibrational spectroscopic data have proven extremly useful in assessing the molecular principles of hydrocarbon adsorption and reaction at metal surfaces,1z2for example in the case of C Z H ~The . ~ foremost observation is that the frequencies of the v(CC) stretch mode and the 6 (CH2) bending mode decrease when C2H4 is adsorbed on transition metal surfaces at low temperature, e.g. a t 80 K, and that the frequency shift is particularly substantial a t surfaces capable of promoting C2H4 dehydrogenation a t elevated temperature^.^ On the other hand, the shift should be small on metals supporting only weak chemisorption of C2H4. This case seems to be relevant to the hydrocarbon oxidation reaction where silver is used as a catalytic agent.s,6 Thus, to clarify these shifts we investigated C2H4 on potassium, indium, and the noble metals using surface-enhanced Raman spectroscopy (SERS) and compare these results with the case of C2H6. Utilizing SERS as a n experimental probe allows determination of the vibrational frequencies of molecular species with a resolution of a few wavenumbers and likewise accuracy easily. However, SERS provides high sensitivity only if the surface features either microscopic disorder or some sort of regular microcu~vature.~ The substrates used in this study are metal films grown by condensation of metal vapor on a low-temperature substrate according to a procedure commonly applied.* Experimental investigations revealed a polycrystalline structure with grains of average size below 10 nm,9 open grain boundaries,1° and a n extraordinary high surface area.ll Basically, the surface structure may be seen as t Present address: Institut fiir Energieverfahrenstechnik(IEV), Forschungszentrum Jiilich (KFA),52425 Julich, Germany. @Abstract published in Advance A C S Abstracts, April 1, 1995. (1)Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf. Sci. 1977, 1 , 1. (2) Sheppard, N. Ann. Rev. Phys. Chem. 19SS,39, 589. (3) Sheppard,N. J.Electron Spectrosc. Relat. Phenom. 1988,38,175. (4)Stuve, E. M.; Madix, R. J.; Brundle, C. R. Surf. Sci. 1986, 152/ 153, 532. (5) Force, E. L.; Bell, A. T. J. Catalysis 1975, 38, 440. (6)Bao, X.; Barth, J. V.; Lehmpfuhl, G.; Schuster, R.; Uchida, Y.; Schlogl, R.; Ertl, G. Surf Sci. 1993,284. 14.

(7)Otto, A.; Mrozek, I.; Grabhom, H.; Akemann, W. J . Phys.: Condens. Mat. 1992,4, 1143. (8) Wood, T. H.; Klein, M. V. Sol. State Commun. 1980, 37, 263. (9) Biilow, H.; Buckel, W. 2.Phys. 1958,138, 109. (10)Albano, E. V.; Daiser, S.; Miranda, R.; Wandelt, K. Surf. Sci.

1986,150,386. (11)Eickmans, J.;Otto, A.; Goldmann, A. Surf. Sci. 1988,171,415.

a n ensemble of small-sized facet planes of any orientation with an abundance of defects. In our previous work we already took advantage of this particular surface structure in demonstrating the structure sensitivity of COz and Nz reactions on noble metal surfaces.12 Obtaining the vibrational frequencies of C2H4 adsorbed to disordered metal surfaces seems to be of similar interest in view of all what is indicated about active sites being involved in hydrocarbon ~ata1ysis.l~ In the SERS literature there is already a large set of published work dealing with hydrocarbon adsorption to metal surfaces,14 especially for C2H6 on s i l ~ e r . l ~The -~~ work on silver in particular has already led to the discovery of two different adsorption states characterized by distinct frequencies of C6H6 skeleton vibrational modes.21 It appears that one set of frequencies is shifted and the other one unshifted with respect to the frequencies of the free molecular case. In ref 7 we have introduced the notion of referring to the shifted band by the letter E and to the unshifted bands by N. In the following we will demonstrate that E and N frequencies are resolvable in the spectra of C2H4, too. In order to work out some of the implications, we will compare the spectra with the spectra of adsorbed C2H6.

2. Experimental Procedure The samples were prepared and investigated under ultrahigh vacuum conditionsof 2 x 10-lombar base pressure. The vacuum system was equipped with various metal evaporation sources and a free movable sample holder which was flexibly attached to a double stage He refrigerator allowingthe sample to be cooled to 40 K. The substrate for deposition of the sample films was a finely polished copper plate precovered at room temperature with a silver film of 300 nm thickness. The noble metals were released by ohmic heating from a tantalum filament, indium from a molybdenum filament, and potassium from a SAES dispenser source. The sample films were grown with a rate of 0.1 n d s up to a final film thickness of 150nm while the substrate (12) Akemann, W.; Otto, A. S u g . Sci. 1992,272, 211. (13) Somorjai,G. A. Chemistry in TwoDimensions;Comell University Press: Ithaca, 1981. (14)Seki, H. J. Electron Spectrosc. Relat. Phenom. 1988, 39, 289. (15) Moskovits, M.; DiLella, D. P. Chem. Phys. Lett. 1980, 73,500. (16) Wood, T. H.; Zwemer, D. A. J . Vac. Sci. Technol. 1981,18,649. (17) Manzel, K.; Schulze, W.; Moskovits, M. Chem.Phys. Lett. 1982, 85, 183. (18) Pockrand, I. Springer Tracts Mod. Phys. 1984,104. (19) Gass, A. N.; Kapusta, 0. I. Sou. Phys. Sol. State 1985,27,1385. (20) Ertiirk, 0.;Otto, A. Surf. Sci. 1987,179, 163. (21) Mrozek, I.; Otto, A. Europhys. Lett. 1990, 11,243.

0743-746319512411-1196$09.00/00 1995 American Chemical Society

Vibrational Frequencies of C.& and C f i 6 I

In

C2H4

Langmuir, Vol. 11, No. 4,1995 1197

I

(El

C2H4 ( 8 L )

(1L)

1283

1332

'i3

3 O

30b0

2;oo

2obo

c-

lobo

1Sbo

b

5o;

R o m o n shift Icm-')

Figure 1. SERS spectrum of CzH4 (1langmuir) on indium at 40 K. The sample film was prepared by cryocondensation of indium at 40 K and then was exposed t o 1 langmuir of CzH4 at the same temperature.

$ 0 m

I

l

l

1

1

1

Au

1

1

1

1

1

1

1

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,

1618

l0I A

1

05

1

I

I

I

l

1

,

1

1

1

1

1

1

1

1

l

Figure 3. Part of the SERS spectra of CzH4 on copper and gold coveringthe modes VI, Y Z , and v3 after an exposure of 8 langmuir at 40 K. The labeling of the bands with E and N is explained in the text and in ref 25.

I

1316

Ag

1582

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2500

2000

1500

1000

500

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946

2979

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2d00

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Figure 2. SERS spectra of CzH4 on copper, silver, and gold films at 40 K after an exposure of 5 langmuir.

was kept at 40 K. For CZ& or C& dosage, the gas of purity 99.995% was admitted to the chamber through an adjustable leak valve and a constant partial pressure of about 10-7 mbar was maintained for the time of exposure. Quantitatively the exposure will be given by partial gas pressure times exposure Torr to produce s) time in units of langmuir (1langmuir = with the gauge reading corrected accordingto ref 22. To produce the Raman spectra an Ar+gas laser with 514.5nm radiation was used in the case of silver, potassium, or indium, and a Kr+ laser with 647.5 nm radiation was used in the case of copper and gold. The laser beam was slit focused onto the sample surface with p-polarization and with an angle of incidence of about 70"with respect to the surface normal. The scatteredlight was collected in the normal direction and imaged into the entrance slit of a double stage Raman spectrometer. The polarization of the scattered light was not further analyzed. The spectrometer provided a resolution of about 4 cm-l and the accuracy of the (22) Bartmess, J. E.; Georgiadis, R. M. Vacuum 1983, 33, 149.

(23)Mrozek, I.; Otto, A. J.Electron Spectrosc. Relat. Phenom. 1990, 54/55, 895.

Akemann and Otto

1198 Langmuir, Vol. 11, No. 4, 1995

Table 1. Vibrational Frequencies of C& Adsorbed to In,Ag, Cu, and Au Films As Rsvealed in This Work= In v (cm-1)

~ 7 1 ~ s w(CH2)

cu

Ag

log GR

v (cm-1)

log 6

v3

6(CHz)

938 1332

4.0 3.8

954 1319 (E)

5.3 5.0

v2

V(CC)

1612

4.0

5.3

v1

v(CH)

2984 (E) 2995 (N)

3.5

1583 (E) 1618 (N) 2971 (E)

3.9

v (cm-l)

902 1291 (E) 1339 (N) 1555 (E) 1618 (N) 2972 (E) 2995 (N)

Au log GR

5.5 4.9 5.0 3.0

v (cm-')

946 1273 (E) 1341 (N) 1541 (E) 1618 (N) 2969 (E) 2995 (N)

log GR

3.8 4.3 4.3 3.0

reference v (cm-l)

943 13331 1352* 1616 29991 3063*

"he frequencies, except in case of the wagging mode, are labeled according to the following scheme: E applies to a band shifted significantlyversus the reference frequency and N labels a second band that is observed in addition to E and is assigned to the same mode of vibration. E frequencies and unlabeled frequencies refer to 0.5langmuir exposure, N frequencies to an exposure of 5 langmuir. "he reference sample consistsof a thick film of condensed C2H4 on sapphire. * marks a factor group splitting (Davydov splitting)of the respective mode. All the frequencies are peak frequencies determined to an accuracy of f 3 cm-l. The quantity GR is a measure of the SERS enhancement (see the text) and is given as a base 10 exponent with an experimental error of f0.3.

"1

1L62

!

I

28:6

821

978

I

I

,

O

I

30b0

I

I

1

1000 c - Raman shift ( c m ' l ) 2000

I (

Figure 5. SERS spectra of CzHe on potassium, silver, and indium films at 40 K. The exposure is 1 langmuir for the potassium and indium spectrum and 5 K for silver.

modes of the adsorbed molecules by the mode numbers derived for the free moleculez4for reasons of simplicity. All mode shifts quoted will be referenced to the frequencies of a several micron thick bulk CzH4 layer condensed at 40 K on sapphire. Concerning v1 and v3, one has to realize that the modes are split into the Davydov doublets (factor group splitting) a t 299913063 cm-l and 133311351 cm-l, respectively. However,we will use the component oflower frequency corresponding to the in-phase motion as marks of reference. In the case of indium (Figure 11, the band intensities relative to each other, too, are in close resemblance to the reference spectrum leading to rather uniform values of GR as given in Table 1. Regarding the noble metal spectra given in Figure 2,the differences are easily realized. First of all the bands of v2 and v3 vibrations experience a substantial downward shift, increasing in (24)Shimanouchi, T.Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. (US.) 1972,3.9, 1.

the order silver, copper, and gold. Relative to vz and VQ, the VI band has rather low intensity and in consequence the enhancement GR of VI appears to be reduced by more than 1 order of magnitude; see Table 1. Furthermore, the spectra in Figure 2 reveal small features supplementary to the shifted Y Z and v3 bands. On silver this is only faintly seen for VZ, but the vz and v 3 supplementary bands are well resolved on copper and gold; see Figure 3. In addition, Figure 3 confirms a second feature also in the range of VI. The frequencies of these supplementary bands are nearly degenerate with the vl, vz,and V Q reference values and we will label these unshifted bands N, while shifted bands are labeled E; see Figure 3. The assignment of E- and N-bands will be given elsewhere.25 In the case of Cu, the N-bands ofC& are due to species adsorbed a t (111)facets, the E-bands are originating from species at "annealable sites". Most probably this conclusion also holds for silver and gold. Our conclusions below do not depend on this assignment. In the followingwe will use the position of the E-bands to infer the trends in bonding to the various metal surfaces. It is undisputed, that the E-bands originate from adsorbed m01ecules.l~We note in passing that, in addition to the bands given here, weaker bands due to other vibrations of CzH4 have been observed; see the compilation in Table 5 of ref 18. In addition t o what is summarized in Table 1, we have also probed C2H4 adsorbed to potassium; see Figure 4. Apparently, the spectrum indicates a profound distortion of the molecule, very unlikely to be the case for weak chemisorption on noble metals. In particular, the broad structure between 300 and 800 cm-l is striking. It is reasonable to assign the band to the motion versus the surface of either a molecular C2H4 species, e.g. di-a-bonded CzH4, or any decomposition species, e.g. ethylidyne or methylene. Such a strong band at about 500 cm-l also occurs in the EELS spectra of CzH4 on various transition metal surfaces.26-28 The band a t 1554 cm-l may be related to d(CHz), whereas 4CC) may be hidden in the structure between 1100 and 1300 cm-l. But the absence of v(CH)is striking in any case. Obviously,there is no conclusive assignment to the spectrum yet, but it is interesting to realize in the following the difference between the spectra of a n unsaturated and a saturated hydrocarbon on potassium. 3.2. SERS Spectra of Ethane. The SERS spectrum of on potassium, shown in Figure 5, reveals pure physisorption as to be judged from the match of the (25)Grewe, J.; Otto, A. Proceedings of the XNth International Conference on Raman Spectroscopy, Hong Kong, 1994,Yu, N.-T., Li, X.-Y., Eds.; p 614 and in preparation. (26)De Fresart, E.;Darville, J.;Gilles, J. M. Appl. Surf. Sci. 1982, 11112, 637. (27)Marinova, T.S.; Chakarov, D. V. Surf. Sci. 1987,192,275. (28)Gates, J. A.;Kesmodel, L. L. Suf. Sci. 1983,124, 68.

Vibrational Frequencies of Cf14 and Cf16

Langmuir, Vol. 11, No. 4, 1995 1199

Table 2. Vibrational Frequencies of C a s Adsorbed to Potassium, Indium,and Silver Films Together with the Quantity GRProviding the Experimental SEW EnhancemenP

K V(CC)

v3

vi a

993

d(CH3) 4CH)

VdV8

(FR)

In

log - GR-

v (cm-l)

v (cm-l)

4.0

992

4.7

1462 287612934

log - GR -.

v (cm-l)

985 (E) 995 (N) 1450 (E) 284612901 (E)

3.9

1457 286812923 (E) 287912958 (N)

3.9

reference

Ag

log- GR 3.6 3.0

v (cm-1)

4.5

996

4.3 4.3

1461 287912958

The frequenciesof reference correspond to a thick CzH6 film on a sapphire substrate. For all other details, see footnote a of Table 1.

t

30

I

. 3 VI

U

6

N

01

g 20

-

-

I

I

0

S

I

10 @(LI

X

-

I

15

10

1-1

Lj

4-

Raman shift Icm-'1

Figure 6. Part of the SERS spectrum of CzH6 on indium covering the band of v(CH) vibrations after an exposure of 4 langmuir at 40 K.

frequencies to free CZH6; see Table 2. Unfortunately, the vibrational spectra of CZH6 in general are liable to some ambiguity in assignment; see refs 28-30. It appears unambiguous, however, to assign the 821 cm-' band to CH3-bending(that is, presumably vg), the 993 cm-l band to CC-stretch (that is, vg), the 1462 cm-l band to CH3deformation (that is, vz or Vg), and the 287612934 cm-I band to a Fermi resonance between the CH-stretch mode (VI) and the second overtone of the deformation band a t 1462 ~ m - l . ~Furthermore l 1191 cm-I may correspond to v12 and 1368 cm-l to v6. Anyway, the physisorption of CZH6 on potassium is quite a particular case, when compared, for example, to silver or indium; see Figure 5. On silver all modes are downshifted, though the shifts observed are fairly small, e.g. 10 cm-l for v3 and 30 cm-l for VI. Further bands are v g a t 813 cm-1 and V 2 h 8 a t 1446 cm-l. The downshifts are even smaller on indium, amounting to 10 cm-' in the case of v1 and being nearly insignificant in the case of vdvg and v3. In spite of this only weak shift of v1 there are also N-bands related to the v1 resonance. This is to be seen from Figure 6, which displays a scan of the v(CH)band of C Z Hon ~ indium after exposure to 4 langmuir. There is a clearly resolved band at 2879 cm-I which did not appear in the spectrum of Figure 5 because the band is only detected for an exposure beyond 2 langmuir; see Figure 7. We thus assign the band a t 2879 cm-I together with the small feature a t 2958 ~

~

(29) Sverdlov, L.M.; Kovner, M. A.;Krainov, E . P.Vibrational Spectra o f Polyatomic Molecules; John Wiley: New York, 1974.

(30)Nakamoto, K. Infrared and Raman spectra of inorganic and coordination compounds; John Wiley: New York, 1986. (31)Herzberg, G. Infrared and Raman spectra of polyatomic molecules; V a n Nostrand: Princeton, 1964.

a-

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I

7z--

0

2879cm"

I

I

5

10 Q

(LI

-

I

1 I

IS

Figure 7. SERS intensities versus exposure for several modes of CzHsadsorbed t o potassium and indium. In cases where the assignment is unambiguous, the mode numbers of the free molecule are used.

cm-' to v1(N) and the doublet a t 287912923 cm-l to vl(E); see Table 2. Without displaying the spectra we like to mention one particular feature of the SERS spectra of CZH6 adsorbed to copper and gold. The spectra were of low intensity with v1 hardly detected at all, yet v3 was clearly seen. Interestingly, the v3 frequency on copper and gold appeared to be the same as on silver contrary to the trend observed for v(CC) in case of CzH4. 4. Discussion There is a n unambiguous trend to be realized in the way the E-vibrations of CzH4shift upon adsorption to the various metals; see the overview of experimental frequencies in Figure 8. Apparently, the modes v2 and v3 are afYected most and decrease in progressive order from silver to copper to gold, whereas for indium only v1 decreases significantly. The trend may be understood by reference to the principles of olefin coordination chemistry. The coordination of CzH4 to metals is usually described as a n-complex with a a-donation involving a a-bond between a metal acceptor orbital and the z-orbital of CzH4 and a n-backdonation between a metal donor orbital and the n* orbital of the CzH4 ligand.32 Anyway, the olefin bond is weakened upon coordination to the metal center and this (32) Chatt, J.; Duncanson, L.A. J. Chem. SOC.1963,2939.

1200 Langmuir, Vol. 11, No. 4, 1995

Akemann and Otto compared to the value of 4.1 eV for indium.39 A low work

v1

C,H, 1

1

1

NllE

In Ag

IE

I

l

Nl

NI

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Nl

Au

Nl

(E

NI

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ref

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~ 7 1 ~ s function allows for considerable penetration of metal

v3

v2

l

3070 3020 2970 1630 1580 1530 1360 1310 1260

I

980

I

l

l

930

880

decrease in C-C bond order should lead to a lowering of the v(CC) frequency. However, v2 and v g are coupled in CzH4 because the two modes obey common symmetry and are not separated in energy by too much. This coupling must even increase upon coordination as lowering of the C-C bond order would tend to make v(CC) and 6(CHz) more degenerate. In effect, the couplingof vz and vg results in two vibrational bands with mixed v(CC) and d(CH2) character, both shifted to lower frequencies and avoiding band crossing. This behavior is clearly seen in Figure 8. The combined frequency shift of the coupled modes v2 and v3 is conveniently expressed by Anu, the so-called nuparameter. Adopting the normalized version of the parameter introduced by Stuve and Madix in ref 33, we obtain 0.09 for C2H4 on silver, 0.19 on copper, and 0.25 on gold. The values are fairly below 0.38, which is the parameter value in the case of Zeise's salt which is referred to as a prototypical n - c ~ m p l e x .Thus ~ ~ we may conclude that the E-species of C2H4 are weakly n-bonded to the noble metals with only a small rehybridization of the molecular electronic structure. This is opposed for example tothecaseofCzH4onPt(lll), whereA,amounts to 0.92, indicating d i - a - b ~ n d i n g .The ~ ~ small differences in A,,, of the noble metals are likely to reflect the extend of d-band contribution to the coordination bond. This may be inferred from the binding energies, with respect to EF, of the upper d-states, namely 4 eV in silver35and 2 eV in copper36and gold.37 On the other hand, the d-states are 16 eV below EFin indium38and consequently we observe A,, as low as 0.01, indicating almost pure physisorption. The difference of the adsorption of CzH4 on indium (Anu = 0.01) and potassium (dissociation; see Figure 4) is assigned to the low work function of 2.3 eV of potassium (33) Stuve, E. M.; Madix, R. J. J.Phys. Chem. 1986,89,3183. (34) Raval. R.. Cheaters. M. A. Surf. Sci. 1989.219.L505. (35) Fuster, G:; Tyler, J.'M.; Nrene;, N. E., Callaway, J.; Bagayoko, D.Phys. Reu. 1990,B42,7322. (36) Kuppers, J.; Nitschke, F.; Wandelt, K.; Ertl, G.;Brundle, C. R. J . Chem. SOC.Farad. Trans. 1979,75,984. (37) Norton, P. R.; Rapping, R. L.; Goodale, J. W. Su$. Sci. 1978,72, 0"

JJ.

(38) Leveque, G.; Olson, C. G.; Lynch, D. W. Sol. State Commun.

1984,51,377.

electrons into the C-C antibonding n* bZgorbital of CzH4, which may trigger C-C bond breaking. This is not possible in the case of Cz& due to the lack of n* orbitals. We think that this is the reason why CzH6 remains intact on potassium. Inspite of the low An0 parameter, however, the VI frequency decreases on indium as well as on the noble metals. Since v1 is fairly decoupled from vz and vg, the observed shift must be due to a n interaction involving the local CH groups of the molecule in the first place. The kind of interaction might resemble what is known as agostic bond in coordination chemistry. It is understood that CH groups may act as ligands in certain coordination compounds, forming a covalent bond with the hydrogen atom shared between the carbon and the metal center.40 At surfaces such a n interaction obviously must be important for the mechanism of hydrocarbon dehydrog e n a t i ~ n . In ~ ~fact, , ~ a softening of v(CH) vibrations by 310 cm-l as a precursor to dehydrogenation was first observed for C6Hlz on Pt(lll).41In the case of indium and the noble metals the interaction certainly is too weak to cause CH bond breaking. This is reflected by the only small v1 downshifts observed in this work remaining below 30 cm-l altogether. The softening of the CH bonds is without reference to the olefin structure and therefore does likewise occur in the paraffine case. We observe the same degree of v(CH) mode frequency decrease for C2H6 on indium and silver as in the case of CZ&. On the other hand, the shifts of v(CC) and d(CH,) are not alike and the decrease is much smaller for CzH6 than for CzH4. None the less we resolve vdE) and vg(N)of CzH6 on silver, though the shift is only 10 cm-l (not displayed; see Table 2). On silver, the first layer sensitivity of SERS is wellconfirmed and was accounted for by a metal-electronmediated resonance Raman effect; see ref 7 and references therein. This mechanism particularly explains the chemical specificity and vibrational selectivity of the enhancement on silver substrates. An inspection of Tables 1and 2 reveals that these features are less established on indium. However, since Cz&is weaklymbonded on silver, but is physisorbed on indium, one may expect the n*(bzg) charge transfer orbital of CzH4to be higher in energy on cryocondensed indium compared to cryocondensed silver, where it is found a t 2.2 eV above EF.' Hence, the cross section of scattering of photoexcited metal electrons by adsorbed (22% might be quite off resonance, leading to similar yields of SERS from C2H4 and C2H6, in agreement with what is observed. LA9408300 (39) Holzl, J.;Schulte,F. K. Springer Tracts in modern Physics 1979,

85. (40) Brookhart, M.; Green, M. L. H. J. Organometal. Chem. 1983,

250,395.

(41) Demuth, J. E.;Ibach, H.; Lehwald, S. Phys. Reu. Lett. 1978,40, 1044.