Studies Of The Solid-Liquid Interface Via Laser Excited Surface Plasmon Spectroscopy

Abstract

The chemistry of the solid-liquid interface is of intense practical and theoretical interest, since a vast majority of heterogeneous reaction, adsorption, and epitaxy processes occur there. It is also among the most difficult of systems to study. One must have a probe which is extraordinarily sensitive, in order to observe the first few atomic layers, and very selective, in order to separate the effects of the surface associated species from those of the solvent or neat liquid in close proximity. To fulfill these requirements and to retain the advantages of optical spectroscopic techniques requires that the electric fields employed for excitation of interfacial molecules be spatially localized. When the solid component of the interface is one of the non-lossy metals, the above constraints can be satisfied by exciting a surface plasma resonance of a thin metallic film. Both theoretical and experimental work has shown that enhancement of Raman signals at Ag-liquid interfaces can be as high as 4 x 104. In addition the excitation of surface plasma resonances at metallic interfaces in the Kretschmann configuration has been shown to be extremely sensitive to adsorption of components from the ambient.4-7 In general the position of the plasmon resonance along the wavevector axis shifts and broadens upon adsorption or overcoating. Thus the exci-tation of surface plasma resonances at metal-liquid interfaces offers the possibility of simultaneously studying both adsorption-desorption kinetics, via observation of the reflectivity, and interfacial geometries, via enhanced signal vibrational spectroscopy of the adsorbates themselves.