Observation of Strong Temperature Dependence and Direct-Transition Effects in Angle-Resolved X-Ray Photoelectron Spectra from the Valence Bands of Single-Crystal Tungsten

Abstract
Strong temperature dependence has been observed in angle-resolved x-ray photoelectron spectra from the tungstem valence bands, indicating the importance of phonon-assisted nondirect transitions in Brillouin-zone averaging. The spectral changes with direction at ambient temperature furthermore agree very well with the predictions of a directtransition model assuming free-electron final-state dispersion and constant matrix elements. The direct and nondirect components in the temperature-dependent spectra are also determined.