A TEST OF SURFACE SELECTION-RULES FOR SURFACE-ENHANCED RAMAN-SCATTERING - THE ORIENTATION OF ADSORBED BENZENE AND MONOSUBSTITUTED BENZENES ON GOLD

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 v1 ring mode is also consistent with the essential absence of the v2 (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 e1g relative to the e2g 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. © 1990 American Chemical Society.