Optical properties of sub-monolayer molecular films

Abstract

Equations for the change in both the specular reflectance and the ellipsometric parameters for a flat surface following formation of a sub-monolayer molecular film are derived by taking explicit account of the particulate nature of the adsorbed film, while treating the substrate as a continuum. A dielectric constant tensor, ε, can be defined for the adsorbed film which, upon choosing an effective film thickness, d_e, so that ε is isotropic whenever the mean molecular polarizability of the adsorbed species is isotropic, takes a form essentially identical to that obtained using the Lorentz–Lorenz equation. In general, both ε and d_e vary with coverage in the range zero to one monolayer. For films consisting of island patches, however, ε is constant and d_e is proportional to coverage; while for films consisting of more or less randomly distributed molecules, d_e is approximately equal to the molecular diameter at all coverages up to one monolayer and ε varies with coverage.