LAYER-BY-LAYER RESOLVED CORE-LEVEL SHIFTS IN CAF2 AND SRF2 ON SI(111)- THEORY AND EXPERIMENT

Using x-ray-photoelectron spectroscopy and Auger-electron spectroscopy , we have resolved surface, bulk, and interface Ca and F core-level em ission in thin films (3-8 triple layers) of CaF2 and SrF2 on Si(lll). We confirmed these assignments using x-ray-photoelectron diffraction ( XPD) and surface modification. XPD was also used to identify the growt h modes of the films as being either laminar or layer plus islands; in the latter case we have resolved buried and uncovered interface F and Ca/Sr emission. We compare the observed energy differences between su rface, bulk, and interface emission to theoretical estimates of the ex tra-atomic contributions to emission energies. We find excellent agree ment considering only the Madelung (electrostatic) potentials for the initial-state contribution and polarization response for the final-sta te contribution, including the effect of tetragonal strain. Small disc repancies for emission from metal atoms bonded to the Si substrate are interpreted in terms of chemical shifts.