Abstract
Precise binding energies for the outer valence bands of the alkali (A) and Halogen (H) ions in the Li, Na, K, Rb and Cs halides have been determined using 40.81 eV ultraviolet photoelectron spectroscopy (UPS). Following corrections for polarization effects in the region of photoexcitation according to the model of Mott and Littleton, the data is compared with the Born model for strongly ionic crystals. The direct determination of the band gap in these materials allows ionicity values to be assigned using the thermochemical model due to Pauling.