Escape probability of photoelectrons from silver sulphide

Silver sulphide films, grown in situ under UHV conditions by exposing the clean silver to a sulphur beam, have been studied by X-ray induced photoelectron spectroscopy. The films composition and chemical state were determined by quantitative analysis, binding energy and Auger parameter analysis. The escape probability as a function of depth of origin for S 2s and S 2p photoelectrons leaving a Ag2S surface at three emission angles α=0°, 39° and 60° with respect to the photon flux propagation direction has been measured and compared to the predictions of the Monte Carlo calculations. The theoretical model applied accounted for both, elastic as well as inelastic electron scattering in a near-surface region of the sample. The escape probability as a function of depth of origin for the S 2s photoelectrons at α=0° exhibited a non-monotonic behaviour with a maximum beneath the surface at a depth of 0.6–0.8 nm. This complex behaviour cannot be explained within models neglecting elastic scattering of photoelectrons. In contrast, the escape probability of the S 2s photoelectrons measured for α=39°, 60° and the escape probability of the S 2p photoelectrons measured for α=0°, 39° showed a monotonic dependence decreasing with the depth that can be approximated by a simple exponential function. The mean escape depth values are determined from the measured and calculated escape probabilities of S 2s and S 2p photoelectrons.