INTERPRETATION OF SPECTROSCOPIC ELLIPSOMETRY DATA ON PROTEIN LAYERS ON GOLD INCLUDING SUBSTRATE-LAYER INTERACTIONS

Interface effects are often neglected when interpreting ellipsometric data on thin organic films on solid substrates. We find that on gold s ubstrates such effects give rise to spurious absorption seen as a nonz ero imaginary part of the optical response function of adsorbed protei n layers. With spectroscopic ellipsometry it is revealed that these ef fects are more pronounced in the visible part of the spectrum than in the ultraviolet part. We conclude that the standard three-phase model (ambient-layer-substrate) in general is insufficient for an accurate a nalysis of the layer properties. Large errors in the deduced refractiv e indices, film thicknesses, and surface concentrations may arise. A f our-phase model for interpretation of spectroscopic ellipsometric data on protein layers on gold is proposed as an improvement. In this mode l the interaction between the adsorbed layer and the gold substrate is included and modeled in effective medium approximations. The physical mechanism behind the interaction is suggested to be depletion of elec trons from a small zone in the gold substrate, close to its surface. B y using this new model, a considerable improvement is obtained and the spectra obtained are free from the unphysical absorption in the visib le part of the photon energy spectrum. The time development of the int eraction between the protein layer and the gold substrate leads to a s uggestion of a two-state adsorption model for protein adsorption. The studies in this report are limited to layers of ferritin and albumin, but we think that the results are applicable to other protein layers a s well.