Line shapes and relaxation effects in core-level photoelectron spectra of chemisorbed species

Publisher Website
Google Scholar
Abstract
Asymmetric broadening and relaxation shifts observed in the photoelectron spectra from adsorbate core levels are explained in terms of intra-atomic and extra-atomic relaxation effects. A simple model is set up in which the creation of a hole in an adsorbate core level in photoemission is followed by intra-atomic relaxation which pulls the adsorbate valence resonant states downward towards larger binding energies. It is shown that the response of the system to this perturbation involves singular shake-up and charge-transfer processes which give rise to shifts and drastic redistribution of the spectral weight in the core state. In addition we find that the latter effect is subject to the "zero-work sum rule" which states that the mean energy of the core level is unaffected by relaxation processes. As a consequence of this sum rule a maximum at the unrelated energy may appear in the core spectrum under the conditions which are briefly discussed. In systems in which the natural width of the adsorbate core level is negligible compared to that of the valence level, the core spectrum displays singular broadening at the relaxed (elastic) threshold and tailing off towards larger binding energies. In the case when the decay of the core hole must be taken into account this singularity is smeared out by lifetime broadening but the spectrum retains an overall asymmetric shape. The strength of these effects is found to depend strongly on the initial occupation of the adatom valence states and the magnitude of the intra-adatom relaxation shift.