Optical properties of a mechanically polished and air-equilibrated [111] UO2 surface by Raman and ellipsometric spectroscopy

Optical constants of a [111] UO2 crystal, aged in air for 25.5 years, were measured in the range from .75 and 5 eV using ellipsometric spectroscopy. The ellipsometric data acquired at angles of incidence of 65, 70 and 75 were modelled by a Gauss-Lorentz and a Tauc Lorentz Oscillator, leading to dielectric constants that differ substantially from Schoenes' for vacuum-annealed [111] UO2. Raman spectra taken at 632.8 nm at normal (= 0 degrees) as well as off-normal (=75 degrees) incidence show no indication of a hyper-stoichiometric surface layer, suggesting that at room temperature oxidation of UO2 to U3O7/U4O9 proceeds slower than extrapolation from higher temperature observations predict.