Far infrared optical excitation in metallic uranium sulfide

Previous near normal incidence reflectivity measurements on US single crystals from 12 to 0.03 eV have been extended down to 0.0018 eV (15 cm^?1). A broad plateau with a reflectivity of 90±2% is observed between 40 and 400 cm^?1 with a further increase of the reflectivity below 40 cm^?1. A Kramers-Kronig transformation of the data shows the existence of a resonance at 315 cm^?1. From a comparison with recent neutron data and other arguments we deduce that this resonance is due to the excitation of a transverse optical phonon coupled to an f→d or d→f interband transition.