EXCITATION OF PHONONS AND FORWARD FOCUSING IN X-RAY PHOTOEMISSION FROM THE VALENCE-BAND

A model is presented to explain the transition of the angular distribu tion of valence-band photoelectrons from the low-energy regime, charac terized by emission only along the Mahan cones, to the high-energy reg ime, characterized by emission patterns very similar to those of core- level photoelectron diffraction. The two main ingredients of the model are the excitation/absorption of phonons during the photoemission, an d the multiple scattering of the electron in the final state. Using si mple assumptions about the valence-band initial states of Al(001) and a Debye model to describe the lattice vibrations, we calculate angular distributions of photoelectrons that are in excellent agreement with recent experimental results.