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(1976).(53) D. Allara, private communication, measured the infrared spectrum with p polarized light at low angles of a film of 10 layers of cadmium arachidate on silver, made in our laboratory. (54) K.

A family of combination rules, from which van der Waals coefficients for interactions between unlike molecules may be estimated in terms of those for the corresponding like interactions and the static polariabilities, is tested empirically and related to an approximate parameterization of the frequency-dependent polarizability function. The treatment includes two- and three-body dipole force constants and the higher-multipole terms of the second-order two-body dispersion energy. The rules give agreement within a few percent (about the same accuracy as the input data) when compared with accurate unlike coefficients for systems involving H, He, Ne, Ar, Kr, Xe, Li, Na, K, Rb, Cs, Hg, H2, N2, CH4, He(21S), He(23S), He+, Li+, Na+, K+, Rb+, Cs+. Knowledge of n like coefficients and polarizabilities permits calculation of n(n − 1) / 2 unlike two-body and n(n − 1)(n + 4) / 6 unlike three-body force constants.

Vibrational spectra of tetrathiafulvalene (TTF) adsorbed on silver and gold surfaces have been observed via surface-enhanced Raman scattering (SERS). When adsorbed onto silver and gold island films at room temperature and pressure, TTF exists primarily as the radical cation TTF+. This appears to be the first SERS observation of an adsorbed cation in a nonelectrochemical environment. The oxidation of TTF to TTF+ is probably promoted by surface impurities which raise the work function of the metal surface. Some support for this idea comes from kinetic measurements of TTF adsorption onto colloidal gold surfaces; at short times TTF exists predominantly as the partially oxidized species TTF0.3+, which converts slowly in time to TTF+, consistent with an increased work function due to slow deposition of impurities onto the gold surface. Since the oxidation state of many charge-transfer compounds can be quantitatively determined by Raman spectroscopy, SERS spectra of TTF and related molecules provide a means to explore the nature of charge transfer between metal surfaces and adsorbates. We discuss some potential applications, including the relationship between charge transfer and the SERS mechanism itself.

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