Angle-resolved photoemission study of the valence bands of W(011) in the photon energy range 1100-1250 eV: Observation of strong direct transitions and phonon effects

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Abstract
Angle-resolved photoemission spectra from valence bands of a clean W(011) single crystal, obtained using synchrotron radiation in the range 1100-1250 eV and emission normal to the surface, were observed to show strong dependences on both photon energy and temperature. The spectral variations with excitation energy at ambient temperature agree very well with the predictions of a simple bulk direct-transition model assuming free-electron final-state dispersion and constant matrix elements. The strong temperature dependence observed indicates the importance of phonon-assisted nondirect transitions in Brillouin-zone averaging.