Surface effects on angular distributions in X-ray-photoelectron spectroscopy

The spatially varying inelastic mean free paths of photoelectrons emitted from a solid surface have been calculated from an extended Drude dielectric function, that considers the characteristic oscillator strength, damping constant, and critical-point energy for each subband of valence electrons. The results reveal that the additional inelastic scattering probability due to surface effects can be regarded as a surface excitation probability on the vacuum side and can be taken into account by use of a surface excitation parameter. The X-ray-photoelectron spectroscopy formalism in which elastic-scattering effects have been accounted for has been extended to include surface effects. The results show that surface effects lead to a reduction of photoelectron intensities at small emission angles and a sharp decrease at large angles (>75°) since surface excitations are most probable for glancing electrons. However, the difference between the results obtained with and without surface effects for emission angles less than 60° are small because of the method by which the results were normalized.