We have used spectroscopic ellipsometry to determine the dielectric functions of thin films of γ-globulin, bovine serum albumin, and hemoglobin in the visible and near-uv photon energy range. We show that both thickness and dielectric function can be resolved for monomolecular films adsorbed on substrates with relatively low polarizability. The data which we consider to be closest to the intrinsic dielectric response of the protein films were obtained on HgTe and HgCdTe substrates. Less resolution was obtained on silicon substrates. For a density-deficient film, we were able to model the dielectric response with effective medium theories, and the void fraction could be determined.

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