Using rotating analyzer ellipsometry the authors have measured all three components of the dielectric tensor of YBa2Cu3O7 from the near-infrared to the vacuum-ultraviolet (0.7-24 eV). In the low-energy region the spectra for a-b polarization can be well modelled by a Drude free-carrier response with a frequency independent scattering rate. Below 10 eV they observe strongly anisotropic interband transitions involving the partially filled valence band and the conduction band. At higher energies the anisotropy becomes smaller. In that region they see two prominent double structures around 16 and 20 eV that they assign to transitions from lower-lying bands with O s and Ba p character to states close to the Fermi level.
 

Physica C, 192 473-80, 1992.