Surface-ion core-level shifts in alkali halides

Abstract

Surface-ion core-level shifts from evaporated films of K, Rb, and Cs chlorides, bromides and iodides have been identified in high resolution photoemission spectra. The measured shifts of the alkali-metal surface ions in the six NaCl-structure compounds average 0.45 ± 0.03 eV to higher binding energy. Subtracting from these shifts the change in Madelung potential at the surface gives the corresponding change in the surface polarization energies, averaging 0.17 ± 0.02 eV. Shifts of the halide surface ions are found to be significantly smaller. All of these results are in qualitative agreement with theoretical expectations of surface Madelung potential and polarization energy changes, both of which are positive for the cation but of opposite sign for the anion. The data from the Cs compounds have a much larger scatter due to the polymorphism upon thin film growth.