A surface enhanced resonance Raman study of cobalt phthalocyanine on rough Ag films: Theory and experiment

Abstract

We present a combined experimental/theoretical study of the coverage and frequency dependence of surface (enhanced) resonance Raman scattering [S(E)RRS] of cobalt phthalocyanine (CoPc) on CaF2 roughened silver films. The experimental spectra indicate a rather strong coverage dependence at very low coverage for excitation at or close to the molecular resonance frequency, with a peak at 0.07 monolayer (ML) followed by a rapid decrease above that. This coverage dependence differs strongly with observations on smooth films, where a much weaker dependence is observed. At very low coverage on the rough Ag films S(E)RRS enhancements comparable to SERS are observed. To model this coverage dependence, we consider the electromagnetic interactions between the adsorbate and substrate, with the substrate modeled as a metal spheroid and the adsorbate as a layer with variable coverage. Two models for this layer are considered, an effective medium model in which the layer is taken to have a coverage dependent dielectric constant, and a coupled dipole model in which both the molecules and metal are taken to be polarizable dipoles. The dependence of field enhancement and S(E)RRS intensity is studied for these two models as a function of frequency, coverage, and Stokes shift. It is found that although there are differences between the two models, the coverage dependence is similar in both, with peak intensities at about 0.1 ML for reasonably prolate spheroids. These models also demonstrate that the drop in intensity above 0.1 ML arises from damping of the plasmon resonance by the adsorbed layer. Interadsorbate interactions are found to play a role in determining the coverage dependence of the S(E)RRS intensity that is secondary to this damping. The large enhancements seen below 0.1 ML suggest that excited state quenching by the surface is not important for this nonfluorescent molecule.