Momentum-dependent energy losses in core level photoemission spectra of poorly conducting metals.

Angular resolved photoemission spectra of 5d core levels from Bi111 exhibit a component which is split off in energy by approximately 200 meV. The energy as well as the spectral width and relative intensity of the split off structure are strongly modulated by the final state electron momentum k(perpendicular) and k||, resembling the periodicity of the surface and bulk Brillouin zone. The data can be explained by a k-dependent energy loss function Im[-1/epsilon(k)(omega)] composed of indirect and direct interband transitions from various parts of the bulk Brillouin zone.