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 (>75degrees) 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 60degrees are small because of the method by which the results were normalized. (C) 2002 Elsevier Science B.V. All rights reserved.