ALTERED PHOTOEMISSION SATELLITES AT CAF2-ON-SI(111 AND SRF2-ON-SI(111) INTERFACES

Bulk and interface photoemission satellite excitations, measured with x-ray photoelectron spectroscopy and x-ray photoelectron diffraction, are compared for thick, thin, and monolayer films of CaF2 and SrF2 on Si(111). Intrinsic satellites are observed for excitation of atoms in the first monolayer, both uncovered and at the buried interface, that differ from those associated with bulk atoms. For F 1s excitation, the bulk and interface satellites differ only in width and amplitude; for the cation core excitations (Ca 2p, Sr 3p, and Sr 3d), the observed e xcitation energies and intensities differ both from the equivalent bul k satellites and among the various core hole states. The results yield new information on the nature of the interface bonding, as well as on the origin of both bulk and interface satellites. Several models are considered, including differential screening, dielectric losses, cryst al-field and multiplet effects, and interface excitation. The most lik ely explanation for the new cation satellites at the CaF2/Si(111) [SrF 2/Si(111)] interface is localized excitation of the interface bond, wh ere the interface band gap is controlled by the collapse of the Ca 3d (Sr 4d) level in the presence of the Ca (Sr) core hole.