Optical properties of YHx (0≤x≤2.1) thin films

Abstract Thin single-crystalline films of YH x have been prepared by molecular beam epitaxy in a hydrogen atmosphere, thus avoiding the usual Pd capping layer. Near-normal incidence reflectivity and ellipticity measurements have been performed in the photon energy range from 1.2 meV to 5 eV and 1 to 10 eV, respectively. With increasing H concentration but x ≤ 2 , the absorption decreases below 2 eV and stronger structures develop above 2 eV. For x > 2 , an additional peak is found in the infrared showing a pronounced x and temperature (4 K ≤ T ≤ 295  K) dependence. Its behavior is discussed in terms of an order–disorder transition of hydrogen atoms.