J . Phys. Chem. 1990, Y4. 6858-6864
6858
metastable liquidlike chain packing. Although the observed boundary possibly represents a fractionation of the naphthalene/phospholipid binary fluids, it is difficult to say from the diffraction data what molecular moiety is involved in the (presumed) ordering process. Calculation of the temperature depression with the Schroder equation above does not result in a good match with the experimental curve. Attempts to account for this nonideal behavior with Bragg-Williams theoryz5 (1 Po
= - -T
1
(2)
AHA
leads to a rather large positive interaction term, po = 1690 cal/mol. This interaction, in face of the somewhat small transition enthalpy for DHPE, AHA = 1901 cal/mol, produces a sigmoidal curve (not shown) which still does not match well the observed temperature falloff with increasing naphthalene concentration. In our study of nonideal paraffin/benzoic acid interaction^,'^ neither Bragg-Williams theory25nor a lattice model based on Flory-Huggins theory2* produced an adequate fit to the experimental transition. Thus, this transition line may indicate states where hydrogen-bonding interactions in the phospholipid are important (by analogy to the known benzoic acid interaction in our earlier study). This intermolecular interaction also may be of (25) Lee, A. G. Biochim. Biophys. Acta 1977, 472. 285
significance to the study of fluorescent probes in model phospholipid bilayer systems.26 Since the alkyl chain packing is quite fluid at higher temperatures, the nonideal transition curve just discussed may also indicate a nonrandom distribution of the aromatic in the fluidized chains when they are cosolubilized. It is. nevertheless, surprising to find a nonideal phase boundary at a temperature range higher than the nearly ideal phase boundary discussed above. To summarize these results, the epitaxial orientation of phospholipids on for example aromatic substrates is a trivial example of a lamellar eutectic solid. Only when the concentrations are favorable for primary crystallization of the aromatic can specific epitaxial relationships be established between its major face and the lateral chain packing of the lipid which separates as the secondary solid. The solid is thus somewhat analogous to an eutectic alloy2’ which also includes epitaxial interactions at layer interfaces. Acknowledgment. Research was supported by a grant from the Manufacturers and Traders Trust Co. Registry No. DHPE, 61423-61-8; naphthalene, 91-20-3. (26) Radda, G. K. In Methods in Membrane Biology; Vol. 4, Biophysical Approaches; Korn, E. D., Ed.; Plenum: New York, 1975; pp 97-188. ( 2 7 ) Kerr, H. W.; Lewis, M. H. In Advances in Epitaxy and Endotaxy; Schneider, H. G.,Ruth, V., Eds.; VEB Deutscher Verlag fur Grundstoffindustrie: Leipzig, 1971; pp 147-164.
A Test of Surface Selection Rules for Surface-Enhanced Raman Scattering: The Orientation of Adsorbed Benzene and Monosubstituted Benzenes on Gold Xiaoping Gao, John P. Davies, and Michael J . Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: February 27, 1990)
Surface-enhanced Raman (SER) spectra for benzene, toluene, and benzonitrile adsorbed at gold-aqueous interfaces and on gold in vacuum at 20 K are analyzed in terms of previously proposed surface selection rules. The electrochemical adsorbate systems are of particular interest in this regard since independent information on the surface binding geometries have been obtained previously from alterations in the band frequency and band shape upon adsorption and from dipole-dipole coupling, thereby providing a bona fide test of the surface selection rules. In each case, the adsorbate orientation as deduced from the surface selection rules is uniformly consistent with that obtained on the basis of the latter information. For benzene, the flat orientation deduced earlier from the frequency downshift and broadening of the u , ring mode is also consistent with the essential absence of the u2 (C-H stretching) mode, since the polarizability tensor normal to the surface for the latter should be small under these conditions. The observed greater SERS intensities of the elgrelative to the e2%symmetry modes are also as expected on this basis. Electrosorbed toluene also exhibits a barely detectable ring C-H vibration, while the out-of-plane methyl C-H band is relatively intense. The flat absorbate orientation, inferred again from band frequency shifts and band shapes, is also consistent with the observed greater surface enhancement factors (SEF values) for the a2 relative to b2 symmetry modes. Electrosorbed benzonitrile, in contrast, yields a relatively intense C-H stretching band, with larger SEF values for the b2 relative to a2 modes. The inference of a “vertical”, or tilted, adsorbate geometry from the surface selection rules is in harmony with our earlier deduction that benzonitrile is bound via the cyano group based primarily on SERS band frequencies and band shapes. The orientation of benzene and toluene adsorbed on gold in vacuum, however, is less clear-cut, as deduced from both the band frequency and selection rule analyses.
A fundamental issue in surface electrochemistry, as for metal interfacial chemistry in general, concerns the evaluation of adsorbate orientation and hence the surface binding geometry. Such information can in principle be obtained from surface vibrational band intensities by utilizing so-called “surface selection rules”.’x2 ( 1 ) (a) Moskovits, M. J . Chem. Phys. 1982, 77, 4408. (b) Also see: Moskovits. M. Rev. Mod. Phys. 1985, 57, 783. (2) For recent explanative reviews, see: (a) Creighton, J. A. In Spectroscopy of Surfaces; Advances in Infrared and Raman Spectroscopy 16; Wiley: New York. 1988; Chapter 2. (b) Bradshaw, A. M.; Schweizer, E. ref 2 3 . Chapter 8
0022-3654/90/2094-6858$02.50/0
Such selection rules are most straightforward for surface infrared spectroscopy in that the observation of adsorption bands requires that the relevant transition dipole has a component normal to the metal While this simplicity is an obvious virtue, the consequent inability to observe vibrational motions parallel to the interface together with the paucity of bands typically detected in surface infrared spectroscopy constitute significant restrictions of this technique. Despite its well-documented limitations, surface-enhanced Raman scattering (SERS) has the virtue of providing spectra containing typically a large fraction of the anticipated adsorbate C 1990 American Chemical Society
Surface Selection Rules for S E R S vibrational modes. This spectral "richness" arises not only from the inherent sensitivity of S E R S but also from the anticipated ability of the technique often to detect surface vibrations predominantly parallel, as well as perpendicular, to the metal surface.'-4 This latter characteristic arises from the more subtle and involved nature of the surface selection rules predicted for SERS.'qZa While vibrational modes possessing polarizability tensors in the direction of the surface normal should commonly experience the greatest intensity enhancement, modes where the bond axis is largely parallel as well as perpendicular to the surface often contain substantial polarizability components of this type. The more complex nature of Raman selection rules, however, tends to cloud their practical applicability to the elucidation of adsorbate orientation. Nevertheless, several such applications of SERS have been reported recently, chiefly for adsorbed aromatic species in electrochemical and colloidal surface environment^.^,^ These studies utilize the anticipated sensitivity of certain vibrational modes, specifically C-H vibrations and some out-of-plane and in-plane ring modes, to the adsorbate o r i e n t a t i ~ n . ~The . ~ * aromatic ~ C-H vibrations appear to provide a relatively unambiguous monitor of adsorbate orientation since the polarizability tensor in this case lies predominantly along the bond axis.4w While the situation is commonly not as straightforward for the aromatic ring modes, substantial increases in the intensities of some in-plane relative to out-of-plane modes (specifically b2 relative to a2 modes for molecules having C ,symmetry) are expected if the adsorbate ring orientation is altered from flat to ~ e r t i c a l . ~ Applications of these selection-rule predictions to a number of aromatic molecules on silver surfaces has yielded largely consistent results, with inferred adsorbate orientations that are intuitively r e a ~ o n a b l e . ~In . ~ order to privide more persuasive tests of the Raman intensity selection rules, however, it is desirable to select systems for which independent evidence regarding the adsorbate orientation is available from spectroscopic or other sources. A suitable series of systems is provided by monosubstituted benzenes adsorbed at gold electrodes in aqueous s o l ~ t i o n s . ~For monoalkylbenzenes such as toluene, as well as for benzene itself, strong evidence for an essentially flat adsorbate orientation on gold was obtained from the significant (ca. 10-30 cm-l) S E R frequency downshifts and band broadening of ring breathing modes upon adsorption, indicative of ring-surface A orbital ~ v e r l a p . ~In contrast, for molecules such as benzonitrile and nitrobenzene the ring mode frequencies and band shapes are essentially unaltered upon adsorption, whereas the substituent vibrations exhibit increased intensities along with shifted frequencies and larger bandwidths in the adsorbed state.5 This latter behavior is consistent with surface attachment entirely via the substituent, the benzene ring being pendant and therefore at least tilted to the gold surface. The extent of adsorbate vibrational coupling from D/H mixed isotope SERS measurements for these systems is also in harmony with these inferred orientations.6 The present report contains a further investigation of these systems aimed at ascertaining to what extent the Raman intensity selection rules for the C-H and suitable ring vibrations are consistent with this independent, relatively reliable, information on adsorbate orientation. In addition to detailed analyses for benzene, toluene, and benzonitrile, data are included for benzoic acid on gold; this adsorbate provides an example of potential-dependent orientation. Some corresponding SERS results are also included for adsorption of benzene and toluene on gold in a low-temperature-vacuum environment. Taken together, the results indicate that the S E R S intensity selection rules, especially for C-H vibrations, can indeed constitute a reliable as well as sensitive means of deducing the adsorbate orientation. ( 3 ) Creighton, J . A . Surf. Sci. 1983, 124. 209. (4) (a) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984,88, 5526. (b) Suh, J . S.: Moskovits. M. J . Am. Chem. SOC.1986. 108. 4711. ( c ) Moskovits. M.: DiLella, D. P.; Maynard, K. J. Lungmuir 1988, 4, 67. (d) Moskovits, M.;Suh, J. S. J . Phys. Chem. 1988, 92, 6327. ( 5 ) Gao, P.; Weaver, M. J . J . Phys. Chem. 1985, 89, 5040. (6) Gao, P.; Weaver, M . J . J . Phys. Chem. 1989, 93, 6205.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6859
Experimental Section The electrochemical S E R S instrumentation was largely as described previously.' A conventional scanning spectrometer (SPEX Model 1403) was employed, with 647.1-nm excitation from a Spectra Physics Model 165 Kr+ laser. The incident laser beam (60-mW power, ca. 65" to the surface normal) was p-polarized, the scattered light being collected at 90" to the incoming beam. The spectrometer band-pass was 4 cm-I. A significant recent modification was the installation of a new photon counter system (EG&G Model 1109) which resulted in 3-5-fold higher signalto-noise (S/N) ratio than attained with the original SPEX system. This together with subtle improvements in the electrochemical roughening procedure enabled substantially superior S / N to be obtained for the present electrochemical systems than reported previou~ly,~ allowing additional spectral features to be discerned (vide infra). For the bulk- (liquid-) phase Raman measurements, the sample was contained in a small vial, the scattered light again being collected at 90" to the incident beam. In this case the incident light was s-polarized (Le., with the electric field vector normal to the spectrometer axis). The apparatus for SERS measurements in a cryogenic vacuum environment is described in detail in ref 8. Briefly, this consists of a closed-cycle helium cryogenic system (Air Products Displex Model CSW-202) mounted in a vacuum chamber evacuated by means of a diffusion pump (Edwards MK- 100) backed by a rough pump (Alkatel Model M20/2). The sample stage was attached to the expander tip of the cryostat. The sample (the gold substrate placed on a copper sample holder) was introduced into an interlocking chamber (held at Torr) and subsequently into the main chamber (base pressure ca. Torr). The sample stage could be rotated to face the gas-dosing port (consisting of a high vacuum leak valve; Granville-Phillips Model 606) or the incoming laser beam through an optical port. The angle of incidence of the laser beam on the metal surface was variable from 0" to 54". Scattered light was collected normal to the surface using a 5"diameter fl.72 aspheric lens, which was focused onto the spectrometer entrance slit by means of a 5-cm-diameterfi lens. The gold surfaces used in both the electrochemical and vacuum SERS measurements were roughened by means of prior oxidation-reduction cycles in 0.1 M KCI essentially as described in ref 9. The electrodes were 4-mm-diameter rods sheathed in Teflon, whereas the vacuum SERS surfaces were polished gold disks that underwent roughening after mounting in a Teflon holder. In both cases, the surfaces were rinsed thoroughly with water after roughening to remove excess electrolyte; the latter surface was blown dry with N2 immediately prior to insertion into the vacuum system. Benzene and the substituted benzenes were obtained (mostly Gold Label grade) from Aldrich Chemical Co. Water was purified by means of a Milli Q system (Millipore Corp.). All electrode potentials are quoted versus the saturated calomel electrode (SCE), and all electrochemical measurements were made at room temperature, 23 f 1 OC. Results and Discussion Benzene. It is convenient initially to consider adsorbed benzene in view of its structural simplicity, although the high (&) symmetry of the free molecule yields rather different spectral features from the other adsorbates considered here. The selection rules for this and related adsorbates have been discussed eruditely by Moskovits and c o - ~ o r k e r s . ' . ~ Table I summarizes band frequencies and approximate relative intensities (the latter in parentheses) for liquid benzene and for benzene adsorbed in three different environments: (a) on electrochemically roughened gold at -0.1 V vs S C E in aqueous 0.5 M H2S0,; (b) on the same surface under vacuum at 20 K; and (7) Tadayyoni, M. A,; Farquharson, S.;Li, T. T.-T.; Weaver, M. J. J . Phys. Chem. 1984, 88, 4701. (8) (a) Davies, J. P. Ph.D. Thesis, Purdue University, 1988. (b) Davies, J. P.: Weaver. M. J. Manuscriot in oreoaration. (9) Gao, P:; Goztola, D.; Leing, L:-W. H.; Weaver, M. J . J . ElecfroanaL Chem. 1987, 233, 21 1 ,
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Gao et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, I990
TABLE I: Raman Frequencies (em-') and Relative Intensities for Benzene in Liquid Phase and Adsorbed Electrochemical and Vacuum Environments bulk liquidC electrochem SERS on Aud vacuum SERS on Au' vacuum SERS on Ag' u , cm-' U, cm-I U , cm-' modea symb u, cm-I 973 ( I .O) 992 (1 .O) 982 (1.0) "I a18 992 (1.0) 3061 (0.04) 3060 (0.02) 3063 (1.06) (IOO-fold) to which the intensity of the to the vi mode, 4v2)/1(vi), is attenuated upon benzene adsorption is perhaps surprising; a smaller effect is predicted on the basis of azzestimates obtained from depolarization ratio data.4b Weak u2 features are nonetheless detected for benzene adsorbed on both silver and gold under vacuum (Table I). These larger I ( u 2 ) / f ( u I )intensity ratios are suggestive of a slightly tilted benzene orientation on these surfaces or, more likely, from a more fluxional structure yielding a distribution of orientations. At least the latter interpretation is consistent with the observed milder v1 frequency perturbations upon adsorption, AuI, in the vacuum environment. Indeed, the negligibly small Avl value for benzene on gold under vacuum contrasts with the 20-cm-I downshift observed in the corresponding electrochemical environment (Table 1). Rather than reflecting an intrinsic difference in the surface binding, however, we believe that the former behavior arises in part from benzene adsorption onto a contaminant layer present at the gold-vacuum interface, thereby preventing benzene chemisorption in the same fashion as in the electrochemical environment.8 The other Raman-allowed modes also exhibit intensities that are consistent with a flat benzene orientation. In this case, e!, modes (having cyyz, azxpolarizability components) should exhibit some SERS intensity, yet the eZgmodes [scattering only via (axx - a,: axy)]should yield weak or undetectable SER bands. Inspection of Table I shows that these predictions are largely borne out. Thus the I ( v l o ) / I ( u l ) ratio is markedly larger in all three surface environments than in the bulk phase (uIo has elg symmetry), yet the corresponding intensity ratios for the e2gvibrations are for the most part smaller at the surface than in the liquid. The typically more intense e2s bands seen at the gold-vacuum interface than for the other two surfaces (Table I) is again suggestive of a less flat (i.e., more random) adsorbate orientation in the former case. Turning briefly t o the modes that are Raman-forbidden in the bulk phase, the situation is less clear-cut. Relatively few such modes were observed at either gold electrochemical or vacuum interfaces. For the former surface, this is restricted to one b2" mode ( u 1 4 ) and an e," mode (q6); only a single e,, mode ( u l , ) is
.u2 relative
observed at the latter surface (Table I). Of course, the observation of such inherently weak modes can easily be obscured by contaminants and by interferences with other bands. Perhaps surprising, nonetheless, is the absence of a detectable a2, mode (most prominently the vI1 band) on either gold surface. This band has been observed at the silver-vacuum and also in related surface-unenhanced Raman spectra.I3 Its presence has been invoked in arguments by Moskovits et al. favoring local surface field effects rather than molecule symmetry lowering as the underlying reason for this appearance of symmetry-forbidden bands in surface Raman spectra." Toluene. The lower symmetry (C2u)of monosubstituted benzenes provides a larger number of Raman-active vibrations that in principle can be utilized to deduce the adsorbate orientation. Of the many possible adsorbates, toluene (C6HSCH3)provides an interesting starting point since the methyl group is unlikely to interact strongly with the metal surface. Evidence that toluene is indeed adsorbed on gold electrodes in an essentially flat orientation has been obtained from the significant (5-10 cm-I) frequency downshifts and the band broadening observed for several ring modes, especially u12.5 Some D / H mixed isotope vibrational coupling data for this system are also consistent with this deductior6 Summarized in Table I1 are band frequencies and relative intensities for toluene in the bulk liquid and adsorbed at a gold electrode at 0.2 V from 5 mM toluene in 0.5 M H2S0, and at gold under vacuum at 20 K. The format is largely as in Table I; the ring vibrations are organized in the symmetry classes a l , a2, b,, and b2. The normal Raman spectrum of liquid toluene in the frequency regions 300-1300 and 2850-3100 cm-l (A) is shown along with a typical electrochemical SER spectrum on gold (B) in Figure 2. (13) Hallmark, V. M.; Campion, A. Chem. Phys. Lett. 1984, 110, 561; J . Chem. Phys. 1986,84, 2933. (14) Fuson, N.; Garrigou-Lagran$e, C.; Josien, M. L. Specrrochim. Acta 1960, 16, 106. (1 5 ) Varsanyi, G. Assignments for Vibrational Spectra for 700 Benzene Deriuatiues; Wiley: New York, 1974.
6862 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
Gao et al. TABLE 111: Raman Frequencies (cm-') and Relative Intensities for Benzonitrile in Liquid Phase Compared with Adsorbed Electrochemical Environments
electrochem SERS on mode" VI
461 (0.25) 770 (0.2)
VI2
1001 (1.0)
Y6a
"IBa u9a
v13 "lga "8,
0-13
12b0
'
'
'
800 I
'
'
'
'
400
'
Ramon Shift, Cm-'
Figure 2. Comparison of ( A ) normal Raman spectrum of liquid toluene with (B) SER spectrum for toluene adsorbed at gold electrode at 0.2 V from 5 m M toluene i n 0.5 M H2S04. Other conditions as in Figure I .
Table I1 and Figure 2 contain two major pieces of information that can be used to deduce the adsorbate orientation by means of Raman intensity selection rules. The first involves a comparison of the intensities of modes belonging to the a2, b,, and b2 symmetry classes. Creighton has shown that for aromatic molecules having Cz, symmetry a flat adsorbate orientation is predicted via the selection rules to yield markedly different surface enhancement factors for these different symmetry type^.^^,^ Specifically, the surface enhancement factors for the a2, b,, and b2 vibrations are predicted to be approximately in the ratio 4:l:l for "flat" ad~ sorption and 1:4:4 for edge-on (or "vertical") a d ~ o r p t i o n .This large change in the relative enhancement factors for the a2 and b2 ring vibrations can be understood qualitatively from the "out-of-plane" and "in-plane" nature, respectively, of these two symmetry types. For a flat ring orientation, therefore, the a2 vibrations will contain a larger polarizability tensor component normal to the surface than for b2 vibrations, whereas the opposite should be the case for a vertical adsorption geometry. For convenience, Table I1 contains values of the relative surface enhancement factor, SEF, for each vibration normalized to that for the u I mode. (These values are simply ratios of the relative integrated S E R S intensities to those for the corresponding bulk-phase Raman bands.) Inspection of the SEF values for toluene adsorbed in the electrochemical environment shows them to be clearly consistent with the above selection-rule predictions if the molecule is indeed oriented largely parallel to the metal surface. Thus the S E F values for the a2 modes are larger than those for the b, and especially the b2 vibrations, even though the enhancement factors (not surprisingly) vary within each symmetry class. Such a definite deduction, however, cannot be made from the corresponding data for toluene adsorbed a t the gold-vacuum interface (Table 11). The observation of b2 S E F values in this environment that are larger than a t the electrochemical surface is certainly suggestive of adsorption geometries for the former that encompass at least tilted ring orientations. While no a2 or b, modes could be detected at the gold-vacuum interface, the relatively high limiting S E F values so derived are larger than the observed values for the b2 vibrations, thereby vitiating the clear-cut application of the above selection rule. Examination of the C-H stretching mode region of the electrochemical S E R S spectrum also uncovers intensity patterns consistent with an essentially flat orientation of toluene on the gold electrode. Thus the aromatic C-H stretch (v2; ca. 3060 cm-I) is barely detectable for adsorbed toluene (cf. adsorbed benzene), the S E F value for u2 being 20-fold smaller than for the u , band and 50-fold less than for the major (uI2) ring mode (see Table l l and Figure 2). An interesting contrast is provided by the methyl symmetric C-H stretching band ( u s ) centered at 2920 cm-I, which exhibits a comparable intensity (and S E F value) to the major ring modes. The latter finding can be understood simply from the
symb
bulk liquid' Y, cm-I
u2 "loa "16a u17a VIS Y16b *4
VI I "17b 05
1026 (0.15) 1176 (0.25) 1192 (0.25) 1492 (0.004) 1598 (0.60) 3065 (1.2)
Au
cm-'
Y , ~
490 (0.70) 780 (0.3) 1001 (1.0) 1026 (0.15) 1178 (0.65) 1203 (0.65) 1480 (0.08) 1597 (1.6) 3068 (0.35)
848 (0.005) 401 (0.002) 978 (0.04)
f
383 547 690 755 925 987
377 548 690 762 922 988
(0.05) (0.09) (0.002) (0.18) (0.02) (0.04)
( IO-fold) more intense relative to the other a, modes than is the case for toluene. This finding provides straightforward evidence for a vertical, or at least tilted, orientation for adsorbed benzonitrile. As noted in our earlier study5, the v2 band intensity increases relative to that for the other a l modes toward more positive potentials. This is consistent with the occurrence of a more vertical benzonitrile orientation under these conditions, as anticipated since the double-layer field should act to align the dipolar benzonitrile to a greater extent at more positive electrode charges. A related example of a potential-dependent adsorbate orientation is provided by benzoic acid (C6H5COOH)on gold. Figure 4 shows SER spectra for benzoic acid adsorbed on gold from a I O mM solution in 0.1 M NaCIO., at four electrode potentials, -1.0 (A), -0.5 (B), -0.2 (C), and 0.4 V (D). (Similar spectra were obtained in strongly acidic media.) Several related changes in the SER spectra are seen as the potential is made more positive, which are indicative of an alteration of the adsorbate geometry from essentially flat to a tilted orientation. At the most negative potentials, the vIz band is downshifted in frequency by 8-10 cm-I
*Go
1200 I Raman Shift, U T - ' 1
1
'
, j 800
Figure 4. SER spectra for benzoic acid adsorbed at gold electrode from a 10 mM solution in 0.1 M NaCIO, at (A) -1.0, (B) -0.5, (C) -0.2, and (D) 0.4 V. Other conditions as in Figure 1.
and broadened compared with its normal Raman counterpart (cf. ref 5). This finding combined with the absence of a detectable C-H stretching ( v 2 ) mode, expected at 3065 cm-l, provides good evidence for a largely flat benzoic acid orientation. For potentials positive of ca. -0.5 V, however, the v 1 2mode sharpens and shifts to 1002 cm-l, essentially identical with the bulk-phase band. The concomitant appearance of the u2 feature (Figure 4) is again also indicative of the release of the aromatic ring from direct surface binding. Additional information on the binding geometry can be gleaned from the pair of bands at 1281 and 1370 cm-I. The latter feature is assigned to a symmetric carboxylate ( C 0 2 - )stretching mode. A similar band is observed in the infrared spectrum for benzoate adsorbed on gold at more positive potentials and is indicative of surface binding involving both carboxylate oxygen^.'*^'^ The 128 1-cm-' band observed preferentially at more negative potentials is consistent with a carboxylate C - 0 stretch, indicating that a single oxygen is coordinated. This oxygen may well be coordinated to a proton (i.e., the adsorbed benzoic acid remains undissociated).l* Similar potential-dependent SER spectra were obtained for solutions at higher pH, where benzoate is the predominant species, although as expected the 1281-cm-' feature is rather weak. Unfortunately, however, most of the a2, b,, and b2 modes are ill-defined or too weak to enable a meaningful application of the above selection rule to this system. This results from the markedly lower intensity of the present S E R spectra for benzoic acid in comparison with those for the other adsorbates discussed here.
Concluding Remarks The three adsorbate systems discussed in detail here-benzene, toluene, and benzonitrile on gold-together provide strong evidence that S E R S selection rules can yield reliable, albeit somewhat qualitative, information regarding adsorbate orientation on metal surfaces. Particularly encouraging is the uniform consistency (18) Corrigan, D. S . ; Weaver, M. J. Langmuir 1988, 4, 599. (19) A similar observation has also been made in the surface infrared spectrum for acetic acid on gold and platinum.20 (20) Corrigan, D.S.; Krauskopf, E. K.; Rice, L. M.; Wiekowski, A.; Weaver, M . J. J . Phys. Chem. 1988, 92, 1596.
J . Phys. Chem. 1990. 94, 6864-6875
6864
found between the adsorbate binding geometry as deduced from alterations in band shapes and band frequencies upon adsorption and from the relative intensities by utilizing the selection rules. As inferred also from earlier ~ ~ r k , the ~ examination ~ - ~ ~ - of ~ C-H . ~ stretching modes is particularly insightful in this regard. The selection rule for C , adsorbates based on comparison of ring modes of a,, b,, and b, symmetry, as introduced by Creighton for pyridine a d ~ o r p t i o nhas , ~ also established its value for the present class of adsorbates. A practical limitation to its applicability, however, is brought about by the relatively low intensity of ring modes in these symmetry classes. The clear-cut qualitative obeyance to the S E R S selection rules for the present systems, especially the very marked dependence of the aromatic C-H stretching intensities to the adsorbate orientation, might be considered surprising. This is because such a sharp demarcation is anticipated only for Raman excitation and scattering wavelengths that are distinctly to the red of the surface plasmon resonance frequencies.'.2a (This is because only then is the electric field vector normal to the surface anticipated to be much greater than that parallel to the interface.) For the present case, however, the surface plasmon resonances for roughened gold are expected to lie at frequencies comparable to that of the excitation wavelength h (647 nm).21 It would clearly be interesting to examine the extent to which selection rules are manifested as X is altered,22although this will be restricted to X Z 600 nm for gold due to the presence of an interband transition at shorter wavelengths. Of course, the electromagnetic theory upon which (21) For example, see: Wang, D. S.; Kerker, M. Phys. Rec. B 1981. 24,
1777; 1982, 25, 2433.
(22) Birke. R . L.; Lombardi. J. R. In Soectroelectrochemisrrv: Gale, R. J., Ed.: Plenum: New York. 1988; Chapter 6. (23) For example, see: (a) Gao, P.; Patterson, M. L.; Tadayyoni, M. A,; Weaver, M. J. Langmuir 1985, I , 173. (b) Weaver, M . J.; Corrigan, D. S.; Gao, P.; Gosztola, D.; Leung, L.-W. H. J . Electron Spectrosc. Relat. Phenom 1987. 45, 291,
the selection-rule predictions are based predicts that the plasmon resonance is dependent upon the geometry of the surface roughness.ib,21The theoretical predictions noted herein assume that the surface roughness takes the form of equal-sized Even though the predictions are independent of the spherical radius r (if r
