Enhanced Raman spectroscopy of CO adsorbed on vapor-deposited silver

Abstract

Raman spectra of CO condensed on vapor-deposited Ag at 11 K show two signals in the CO stretching region, one originating from a monolayer next to the metal, the other from the bulk CO film above it. In addition, a low-frequency band associated with the former was detected, as were overtones of both the low- and high-frequency bands of the surface CO. Raman scattering by the former was found to be some 106 times more intense than by the latter on a per molecule basis. The state of polarization of the tow signals was investigated as a function of angle of incidence, as was the excitation profile of the signal from the CO next to the metal, using the bulk CO signal as a reference. The behavior of the latter could only be understood when one took into account coherence effects arising from the superposition of rays directly incident on or scattered by a molecule with those reflected from the metal before impinging on, or after scattering from, the molecule. These effects could give rise to apparent polarizing or depolarizing effects and warrant attention in matrix isolation Raman spectroscopy as well as in surface Raman spectroscopy. Taking all observations into account, the enhanced Raman is suggested to be a form of resonance Raman by CO-covered, metal surface irregularities which act as ’’adsorbed,’’ giant cluster compounds, the metal core of which is large enough to sustain conduction electrons, thereby allowing the conduction electron resonance to become an electronic state of the pseudoadsorbed cluster. It is this electronic state which is in resonance with the incident illumination. The relationship between this electronic resonance and surface plasma resonance in an isolated metal sphere is pointed out. The expected enhancement of molecules adsorbed on several metals other than silver is estimated in the context of the model proposed. Group IB metals are shown to be premier among the enhancers, although Cu and Au should be most efficatious in the red.