Simple metals

Abstract

The theory of the photoemission process is inspected taking into account those many-body effects, which manifest themselves as asymmetric line shapes and plasmons. Simple equations for the analysis of experimental spectra are deduced which allow the determination of the relative importance of the many-body plasmons (intrinsic plasmons) from the observed spectra. A commonly used formula for the background correction in XPS spectra is derived from an inspection of the photoemission process. An analysis of the core level spectra of Be, Na, Mg, and AI metal yields the magnitude of the extrinsic and intrinsic contribution to the plasmon creation rate. The valence band spectra of Be, Na, Mg, and A1 have been analysed in terms of existing band structures. It is found empirically that good agreement with existing band structures can be obtained if many-body effects of the same magnitude as observed in the core level spectra and different photoionization cross sections for the partial bands of electrons with different angular momenta are included in the analysis of the valence band spectra. 7.1 Historical Background For the understanding of the properties of metals the investigation of the simple s-p band metals like Li, Na, Mg... has always played a prominent role because of the seemingly simple model (free-electron gas) that can be used for their description. This holds also for photoemission experiments. In particular in the XPS regime the spectra of these metals show some features which do not appear to be so well defined in other cases: the zero-loss "line" (this may be a core line or a valence band) is accompanied by a series of equal distant satellite lines with decreasing intensity, due to electrons which have lost a discrete part of their energy by creation of plasmons [7.1 9]. This spectral part contains, in principle, a lot of information. On the other hand, the presence of these plasmon satellites makes the extraction of the zero-loss features a problem especially in the case of the valence band. In the simplest approximation, the valence band of a simple metal is parabolic, and therefore its density of states (DOS) shows an E 1/2 dependence. The first X-ray emission experiments in solids, some decades ago, tried to recover this parabolic shape [-7.10]. Since then many investigations of these valence bands by various methods (such as soft X-ray emission spectroscopy