  We follow in real-time and under controlled conditions the oxidation of the
organic semiconductor rubrene grown on SiO_2 using spectroscopic ellipsometry.
We derive the complex dielectric function epsilon_1 + i epsilon_2 for pristine
and oxidized rubrene showing that the oxidation is accompanied by a significant
change of the optical properties, namely the absorption. We observe that
photo-oxidation of rubrene is orders of magnitude faster than oxidation without
illumination. By following different absorption bands (around 2.5eV and 4.0eV
for pristine rubrene and around 4.9eV for oxidized rubrene) we infer that the
observed photo-oxidation of these films involves non-Fickian diffusion
mechanisms.
