Magic angle HAXPES

Abstract

The use of higher energy X-ray sources for photoelectron spectroscopy is receiving considerable attention due to the increased availability of laboratory-based instrumentation and an improved insight into the structures and interfacial properties of technological materials. In traditional X-ray photoelectron spectroscopy the design of the instrument often compensates for anisotropy in photoelectron emission through consideration of the angles between the X-ray source and the electron analyser. X-ray polarisation and non-dipole effects in photoemission are usually assumed to be negligible. However, for high energy XPS (HAXPES) both may be significant. Polarisation at synchrotron sources is an important consideration and non-dipole effects are generally more significant at higher photon energies. In this article we demonstrate that, for certain polarisations, ‘magic angle’ geometries exist that minimise the effects of both dipole and non-dipole contributions in photoemission. However, it is not possible to find such geometries for unpolarised X-rays; achieving a ‘magic angle’ geometry in HAXPES requires the X-rays to have a degree of linear polarisation of 1/3 or greater.