Surface core-level shifts in CaF2-on-Si(111) films : experiment and theory

Using a combination of in situ x‐ray photoelectron spectroscopy and x‐ray photoelectron diffraction, we have resolved surface core‐level shifts for Ca and F atoms in thin (three–eight layers) CaF2‐on‐Si(111) films. These values are 0.7±0.1 eV towards higher binding energy for Ca 2p electrons and 0.6±0.1 eV towards lower kinetic energy for F KVV electrons. These shifts are in good agreement with a simple extra‐atomic electrostatic model in which the shifts are due to the change in both the Madelung potential and the polarization response (relaxation) at the surface. We find no evidence for altered ionicity at the CaF2(111) surface.