Optical properties of thallium films studied with synchrotron radiation

Abstract

Near-normal-incidence reflectance (2–11 eV) and multiangle reflectance measurements (11–30 eV) of thallium films evaporated on low-temperature (77 K) Pyrex substrates have been performed in ultrahigh vacuum (≥5×${10}^{\mathrm{-}10}$ torr). Optical and dielectric functions have been obtained by Kramers-Kronig analysis and multiangle techniques. The results are discussed in terms of direct interband transitions, both at critical points and between parallel bands, by considering the relativistic augmented-plane-wave (APW) and linear muffin-tin orbital calculations of the band structure of thallium. The optical data, i.e., ${\omega }^2$ ${\epsilon }_2$ spectra, are well described as far as 7 eV by the APW calculation with the value α=(2/3 for the exchange coefficient, if one takes for the peak of the calculated density of states of s-like symmetry the s peak observed in x-ray photoemission experiments. The optical conductivity σ then exhibits sharp edges at 12.4 and 14.7 eV due to the onset of transitions from core spin-orbit-split 5d states to the Fermi level. Both the optical-absorption edges and the spin-orbit–splitting energies are found to be in good agreement with x-ray photoemission data.