Optical properties of γ- and α-Ce by spectroscopic ellipsometry

We have built a rotating analyzer type ellipsometer with an ultra high vacuum sample chamber. We measured the dielectric functions of [gamma]- and [alpha]-Ce in the energy range from 1.5 to 5.4 eV to investigate the role of the 4f electron in the isostructural phase transition. [gamma]- and [alpha]-Ce were made in situ by evaporation of Ce on sapphire substrates at room temperature and at 25 K, respectively. All measurements were performed inside a UHV chamber at pressures lower than 1 x 10[superscript]-10 Torr.;The measured dielectric functions showed a thickness dependence. The thicker sample has the smaller optical conductivities. Using a microscopic surface roughness model we found that both overlayer thickness and void fraction increase as sample thickness increases. Repeating cooling-heating-cooling or heating-cooling-heating cycles causes the sample surface to become rougher but the relative volume fractions of both phases and the void fraction in the bulk remain unchanged.;The optical conductivity increases upon entering the [alpha]-phase but the number of electrons per atom, N[subscript]eff/N[subscript]A, contributing to the optical conductivity does not change. The valence electrons lose oscillator strength in the above energy range due to volume collapse. This reduces N[subscript]eff/N[subscript]A but the increased 4f-sd valence band hybridization exactly compensates the reduced oscillator strength. Therefore the net effects of the [gamma]→[alpha] isostructural phase transition are an increase of optical conductivity and constancy of N[subscript]eff/N[subscript]A