SURFACE-ENHANCED RAMAN-SCATTERING AS A UBIQUITOUS VIBRATIONAL PROBE OF TRANSITION-METAL INTERFACES - BENZENE AND RELATED CHEMISORBATES ON PALLADIUM AND RHODIUM IN AQUEOUS-SOLUTION

A recently developed strategy for utilizing surface-enhanced Raman sca ttering (SERS) to obtain uniquely detailed vibrational information for a myriad of organic land other) adsorbates on transition metals in el ectrochemical and other ambient environments is illustrated for benzen e and a pair of monosubstituted benzenes, toluene and benzonitrile, on palladium and rhodium films in aqueous solution. The transition-metal layers, formed by constant-current deposition onto SERS-active gold s ubstrates, can be sufficiently thin (3-5 monolayers, ML) so to yield n ear-optimal Raman scattering intensities, yet are essentially devoid o f exposed ''pinhole'' sites, thereby eliminating spectral interference s from adsorption onto gold. Benzene was selected in view of the detai led vibrational information also available for this archetypical organ ic chemisorbate on transition metals in ultrahigh vacuum (UHV) by mean s of electron energy loss spectroscopy (EELS). Comparison of the spect ral information obtained by SERS and EELS provides an instructive asse ssment of how the different properties, including surface-selection ru les, characterizing these related energy-loss spectroscopic methods in fluence the vibrational information content. The ''dynamic polarizabil ity'' surface-selection rules followed by SERS enable most adsorbate n ormal modes to readily be detected and identified, aided by H/D isotop ic substitution, even for such flat oriented adsorbates. The relative Raman band intensities combined with adsorption-induced frequency shif ts also provide a reliable guide to chemisorbate structure and surface bonding. These SERS characteristics compare favorably with correspond ing EELS data: while both techniques provide detailed vibrational info rmation for ''flat'' oriented aromatic molecules, the former method al lows at least as complete a spectroscopic analysis to be undertaken, y et is uniquely applicable to transition metal-nonvacuum interfaces. To luene chemisorption on palladium yields rich SER spectra that also sig nal a ''flat'' aromatic orientation via metal ring pi interactions, ag ain similar to the bonding deduced by EELS at related metal-UHV interf aces. Benzonitrile, however, yields markedly different SER spectral fe atures compared with toluene that are indicative of chemisorption prim arily via the nitrile substituent rather than the aromatic ring. This binding geometry contrasts the flat adsorbate orientation deduced by E ELS on copper and gold. The difference is probably due to the influenc e of solvent (and possibly also interfacial charge) in the electrochem ical environment. While the SERS band intensities are attenuated progr essively for increasing transition-metal thickness, this ''overlayer-f ilm'' strategy is viable even for markedly thicker (similar to 5-10 nm ) layers. The likely broad-based utility of the approach for in-situ s urface vibrational characterization of such catalytically significant interfaces is considered in light of these findings.