Angular-resolved ultraviolet photoemission spectroscopy and its application to the layer compounds TaSe2and TaS2

Abstract

Photoelectron energy spectra have been measured on the layer compounds $1T$-Ta${\mathrm{Se}}_2$, $1T$-Ta${\mathrm{S}}_2$, and $2H$-Ta${\mathrm{Se}}_2$ as a function of both polar and azimuthal angle of emission. Light was incident normally upon the sample and the photon energy in most of the experiments was 10.2 eV. The measurements were performed in a specially constructed ultrahigh vacuum chamber using a movable retarding potential-energy analyzer. The variations with polar angle of the positions of peaks in the spectra have been used to plot experimental energy bands which are found to compare favorably with the first-principles calculations of Mattheiss. The azimuthal dependence of the photoemission, and in particular the Ta $d$-emission intensity, was found to display strong anisotropies. X-ray orientation of the $1T$ samples revealed that if one assumes that the Ta $d$ photoelectrons originate on Ta atoms, then they propagate preferentially along directions which avoid neighboring chalcogen atoms. The relationship with atomic orbitals and band theory is discussed. In the hexagonal crystal $2H$-Ta${\mathrm{Se}}_2$, residual threefold rotational symmetry is observed in the Ta $d$ emission and is attributed to the shortness of the photoelectron escape depth.