Volume and surface photoemission from tungsten. II. Experiment

Energy-distribution spectra of photoelectrons emitted normal to three single-crystal faces of tungsten have been measured for photon energies between 7.7 and 21.2 eV. The results are interpreted in terms of one-dimensional electronic properties along the symmetry lines in $k$ space that correspond to the emitting crystal faces. The emitted spectra may be considered to consist of three types of contributions. One part is due to electrons that, after excitation by direct interband transitions in the bulk, have left the crystal unscattered. This contribution may be described by the energy density of a one-dimensional joint density of states along a symmetry line in the Brillouin zone. Another part, evident for photon energies above 12 eV, is due to electrons that have suffered inelastic scattering processes after optical excitation. This part carries information on the density of conduction states along the symmetry line under observation. Finally, a third contribution to the emitted spectrum is assigned to surface emission. This part reflects the surface density of states and bears evidence of a narrowing of the $d$ bands near the surface. A theoretical surface density of states, derived from a simple model that scales the width of the $d$ bands by the square root of an effective number of neighbor atoms, is found to describe the energy distribution of the electrons emitted by the surface effect satisfactorily.