  We determined the optical properties of alpha- and gamma-phase Ce in the
photon energy range from 60 meV to 2.5 eV using ellipsometry and grazing
incidence reflectometry. We observe significant changes of the optical
conductivity, the dynamical scattering rate, and the effective mass between
alpha- and gamma-cerium. The alpha-phase is characterized by Fermi-liquid
frequency dependent scattering rate, and an effective mass of about 20 m_e on
an energy scale of about 0.2 eV. In gamma-Ce the charge carriers have a large
scattering rate in the far infrared, and a carrier mass characteristic of 5d
band electrons. In addition we observe a prominent absorption feature in
alpha-Ce, which is absent in gamma-Ce, indicating significant differences of
the electronic structure between the two phases.
