Raman intensities and interference effects for thin films adsorbed on metals

Abstract

Large oscillations in the vibrational Raman intensity have been observed for thin films adsorbed on a metal substrate as a function of film thickness. The systems studied, N2 and O2 physisorbed on Ag(111) at 14±1 K, have previously been shown to be unenhanced, i.e., the observations are not caused by the surface enhanced Raman (SERS) effect. The observed oscillations are due to multiple beam interference caused by reflections at the film boundaries of the incident laser light, and, to a lesser extent, the Raman scattered light. We extend the two-dimensional theoretical development of Moscovits and co-workers for Raman scattering in thin films to include more general three-dimensional experimental geometries. We derive expressions for the total intensity as a function of film thickness, incident laser polarization and angle, and scattered light polarization and angle. The resultant expressions are applicable to Raman scattering from any dielectric thin film adsorbed on a substrate with known optical parameters. Although complicated, the general expressions have been numerically evaluated for our experimental conditions using a commercial mathematical package on a personal computer. The calculated Raman intensities are in excellent agreement with the experimental results. The observed period of the intensity oscillation relates the film thickness to the exposure allowing the sticking probabilities to be determined. The sticking probabilities of N2 on N2 and O2 on O2 at 14±1 K are 0.84±0.2 and 0.82±0.2, respectively.