Lineshape effects in photoemission from the valence states of metals

The detailed appearance of photoelectron spectra from the valence bands of metals depends on many factors, including sample quality, and the experimental energy and angular resolution. Photoemission data are a function of at least two independent variables, the electron momentum k and the energy E; the interpretation of a spectrum can depend crucially on the mode of scanning. In addition, spectra from bulk states are influenced by intrinsic effects of the surface on the wavefunctions and the photoemission matrix element, and by the uncertainty in k caused by the finite lifetime of the excited photoelectron state. Spectra from effectively 2D systems, such as quantum wells and surface states, are free from 'final state broadening' and allow for relatively straightforward interpretations of peak widths in terms of state lifetimes. Contributions to the lifetime due to defects, phonons and the electron-electron interaction can be determined. In favourable systems and with sufficient resolution of the energy and momentum, spectra reveal non-quasi-particle behaviour of the electronic states due to many-body interactions.