PHOTOEMISSION AND PHOTON SCREENING IN LAYERED CRYSTALS

Previously it was shown that above the plasma frequency the photoemiss ion intensities crucially depend on the screening of the photon field by virtual plasmon excitation at the surface of the solid. This, usual ly being neglected in the most precise calculations within the one-ste p model, led to a considerable improvement of the agreement between th eory and experiment. Here, we regard as an additional effect the bulk microscopic screening by taking into account the possibility of local fields. Because the discrepancies between the theory without screening corrections and experiment extend to frequencies below the plasma fre quency, we consider these frequencies here. The outlined procedure is not limited to this scope. We report on calculations of the full diele ctric function and the one-step photoemission spectra for TiSe2, a lay ered material where those effects are expected to become transparent.