Thermal effects in angle-resolved photoemission: is it possible to separate direct and indirect transitions?

Abstract

The presence of thermal vibrations affect ARUPS measurements by reducing the intensity of direct transitions. In contrast, the intensity of indirect transitions increases with temperature, although detailed knowledge about this is scarce. It has been suggested that indirect transitions localised near direct transitions in k-space should dominate in the indirect component, which therefore will resemble the direct contribution, making separation of the two components extremely difficult. We have performed ARUPS measurements at different temperatures, on the layered compound VSe2, and on the Cu(100) surface. Using some simplifying assumptions we have achieved a reasonable decomposition of spectra into direct and indirect components. The results indicate that the indirect contributions already at room temperature are comparable in size to the direct components, and that they are characterised by strong angular dependence.