A density functional (DFT-GGA) study on the modification of the Si(0 0 1) surface optical response upon adsorption of 9,10-phenanthrenequitione and oxidation is presented. In the first case it is found that intramolecular pi-pi* transitions as well as adsorption-modified Si bulk states contribute to the optical signal. The molecular contributions differ strongly from the respective signals of gas-phase molecules, indicating the need for a cautious interpretation of experimental data. The calculations for oxidized Si structures show that local Si lattice deformations accompanying the oxidation of Si bulk bonds directly at the silicon-silicon oxide interface give rise to pronounced optical anisotropies that explain the experimental findings very well. In contrast, calculations for translationally invariant oxide structures fail to reproduce the experiment. This indicates the oxidation to occur layer-by-layer and strong disorder of the silicon oxide layers immediately above the interface. (c) 2005 Elsevier B.V. All rights reserved.