| We have built a rotating analyzer type ellipsometer with an ultra high vacuum sample chamber. We measured the dielectric functions of {dollar}gamma{dollar}- and {dollar}alpha{dollar}-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. {dollar}gamma{dollar}- and {dollar}alpha{dollar}-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 {dollar}times{dollar} 10{dollar}sp{lcub}-10{rcub}{dollar} 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 {dollar}alpha{dollar}-phase but the number of electrons per atom, {dollar}Nsb{lcub}eff{rcub}{dollar}/{dollar}Nsb{lcub}A{rcub}{dollar}, 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 {dollar}Nsb{lcub}eff{rcub}{dollar}/{dollar}Nsb{lcub}A{rcub}{dollar} but the increased 4f-sd valence band hybridization exactly compensates the reduced oscillator strength. Therefore the net effects of the {dollar}gammatoalpha{dollar} isostructural phase transition are an increase of optical conductivity and constancy of {dollar}Nsb{lcub}eff{rcub}{dollar}/{dollar}Nsb{lcub}A{rcub}{dollar}. |
