Unoccupied band structure of NbSe2 by very low-energy electron diffraction: Experiment and theory

A combined experimental and theoretical study of very low-energy electron diffraction at the (0001) surface of 2H-NbSe₂ is presented. Electron transmission spectra have been measured for energies up to 50 eV above the Fermi level with k^|| varying along the ΓK line of the Brillouin zone. Ab initio calculations of the spectra have been performed with the extended linear augmented plane-wave k.p method. The experimental spectra are interpreted in terms of three-dimensional (3D) k^||-resolved one-electron band structure. Special attention is paid to the quasiparticle lifetimes; by comparing the broadening of the spectral structures in the experimental and the calculated spectra, the energy dependence of the optical potential -iV_i is determined. A sharp increase of V_i at 20 eV is detected, which is associated with a plasmon peak in the Im[-1/ɛ] function. Furthermore, the electron energy-loss spectrum and the reflectivity of NbSe₂ are calculated ab initio and compared with the optical experiments. The obtained information on the dispersions and lifetimes of the unoccupied states is important for photoemission studies of the 3D band structure of the valence band.