Transmittance and optical constants of Ho films in the 3-1340 eV spectral range

The optical constants n and k of holmium (Ho) films were obtained in the 3-1340-eV range from transmittance measurements performed at room temperature. Thin films of Ho with various thicknesses were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Ho films were deposited onto thin C-film substrates supported on high transmittance grids. Transmittance measurements were used to obtain the extinction coefficient k of Ho films. The refractive index n of Ho was calculated with Kramers-Kronig analysis; in order to do this, k data were extrapolated both on the high and on the low energy parts of the spectrum by using experimental and calculated k values available in the literature. Ho, similar to other lanthanides, has a low-absorption band below the O-2,O-3 edge onset; the lowest absorption was measured at similar to 22 eV. Therefore, Ho is a promising material for filters and multilayer coatings in the energy range below the O-2,O-3 edge in which most materials have a large absorption. Good consistency of the data resulted from the application of f and inertial sum rules.