Interference effects in surface enhanced Raman scattering by thin adsorbed layers

The intensity of the surface enhanced Raman scattering signal from an organic molecule (usually pyrazine) adsorbed on a rough, coldly deposited Ag surface was monitored as a function of dose. The interference maxima observed in the SERS signal as a function of film thickness are not of constant intensity. Model calculations were performed assuming that the organic film adsorbed on the Ag surface is a stratified three phase medium and the modulation in SERS intensity to be the result of interference effects both in the incident and Raman scattered light. Simulations of experimental results using this model give semiquantitative agreement between the experimental and calculated SERS intensity modulation obtained with pyrazine films as a function of increasing film thickness. Experiments and model calculations were also performed for benzene, iodobenzene, and ethylene films and a method for determining the (nonbulk) values of the densities of these films is described. The implications of this interference effect on the photodecomposition of pyrazine at and near rough Ag surfaces is discussed. We reinterpret the increase in photochemical reaction rate observed with increasing pyrazine doses as being due to interference effects in the intensity incident at the Ag surface and not due to a tuning of the surface plasmon as suggested in a previous study [Wolkow and Moskovits, J. Chem. Phys. 87, 5858 (1987)].