Phonon effects in high-energy photoemission spectra

In this paper some phonon effects on high-energy photoemission spectra for solids are discussed from a theoretical point of view. In case of photoemission from deep cores, recoil and Franck–Condon (FC) effects play some important roles: The former contributes to the peak shift following classical free atom recoil shift rule, however the latter has no influence on the shift within the harmonic phonon approximation. Both contribute to the temperature dependent peak broadening. In addition to these individual effects, the interference between recoil and FC should be considered, which contributes to the peak shift. In the harmonic approximation, the recoil shift is independent of the temperature. Beyond this approximation, we can expect that the recoil shift depends on the temperature in accordance with the observed results for some special systems. In addition to the core level photoemission, we discuss the high-energy photoemission from extended valence levels: Recoil effects and the turnover from ARPES to XPS limit are discussed.