Influence of the xenon induced work function change on the xenon binding energy in photoemission of adsorbed xenon

Abstract

Photoemission of adsorbed xenon has been shown to probe the local atomic work function. The Xe binding energy has a constant value relative to the vacuum level causing the measured binding energy relative to the Fermi level to vary with the work function of the adsorption site. The alignment of the Xe core level to the vacuum level should result in peak shifts with the work function change caused by the adsorbed Xe since the substrate vacuum level shifts with this work function change. However, the Xe peak positions are invariant to the work function changes and remain aligned to the work function of the clean substrate. It is shown in this article how this invariance creates a paradox for the model of the Xe core levels aligned to the vacuum level. The paradox arises from the simultaneous invariance of the Xe and the substrate core level binding energies to the work function change along with the modified threshold for secondary electron emission. The work function change is typically described by a dipole field created by the adsorbed atom. However, this dipole field would alter the kinetic energy of the substrate photoelectrons as well as the secondary electron threshold. A compensating change in the contact potential difference could make the measured substrate levels constant but this would require the Xe level to shift which they do not. This is the basis of the paradox which cannot be resolved by ascribing the work function change to an electrostatic field such as an adsorbed dipole or screening of the image charge. It is proposed that the work function change originates from the perturbation of the conduction electron wave function by the Xe adatom as described for the effect of Xe in scanning tunneling microscopy.