Formation of surfaceFcenters onCaF2/Si(111)

The electronic transitions responsible for the formation of F centers at the surfaces of epitaxial ${\mathrm{CaF}}_2$ films on Si(111) have been identified via photon-stimulated desorption spectroscopy. Results for films ranging from submonolayer to bulk ${\mathrm{CaF}}_2$ indicate that excitations of the Ca 3p core level result in ${\mathrm{F}}^{+}$ desorption, and hence, in F-center formation, while excitations of the F 2s do not. It is proposed that this difference is due to inward nuclear motion that occurs prior to the deexcitation of the core hole.