Core-Exciton Decay in Photoemission and the Nonmetal - Transition.

Ultra thin films or overlayers of materials, normally metallic in the bulk case, can exhibit nonmetallic characters. Typically, these systems undergo a nonmetal-to-metal transition with changing film density, crystalline structure, or thickness. The purpose of this thesis is to identify this electronic phase transition and to investigate the corresponding fundamental mechanisms by studying the detailed electronic structure. In particular, I attempted to look at the evolution of electronic structure in films undergoing this transition. The core -exciton decay in the resonant photoemission was probed, from both theoretical and experimental points of view, to correlate with the change of film metallicity. Resonant photoemission, combining with normal photoemission, was found to be a sensitive and successful method to identify the overlayer nonmetal-metal transition, both from static and dynamic pictures. In most of this work, we concentrate on the studies of the evolution of electronic structure of ultra thin films of divalent metals, on different crystalline surfaces. The formation of new Hg electronic states arising from the electron orbital hybridization between adjacent adatoms, the formation of quantum well states in the overlayers, and the evolution of mercury shape resonance due to 5d to epsilonf excitation, all provide indications of when mercury overlayers undergo a nonmetal to metal transition. This transition has been found to be associated the changes in adatom coordination number. On both Cu(100) and W(110), the interactions between the Hg adatoms and the substrates are very weak and the surface bonding is more like covalent bonding at low coverages. The Hg overlayers on these two surfaces resembles free-standing layers, and the metallicity of the overlayers is largely determined by the nearest neighbor interactions of Hg adatoms. Comparing Hg overlayers on Ni(111) where there exists a nonmetal to metal transition caused by the structure phase transition, and free clusters in which a metallization process exhibits with changing cluster size, we conclude that the average coordination numbers of 6 (or larger) or a hexagonal adlayer for mercury is required for the onset of metallicity. On the other hand, the Hg overlayers on Si(111)--a semiconductor surface, exhibit always metallic even in low coverages. This completely different behavior has been found to be caused by the strong surface bonding between the Hg adatoms and silicon substrate. A nonmetal to metal transition in alkali earth metal monolayers on transition metal surfaces has also been observed. The intensity change of the resonances related to Ba 5pto5d/4f and 5s to6p excitations suggests that there exists a nonmetal-metal transition in the Ba overlayers on Ni(111) though the overlayer structure is not clearly identified. On the other hand, for the magnesium overlayers on Mo(112), the dramatic changes of the density of states, the dispersion of the bands near E_{F}, and screening, have been observed across the nonmetal to metal transition in the overlayers. The changes of the resonance photon energy and the intensity of Mg to epsilond excitation with different coverages indicate a obvious correlation between the electronic structure (particularly final state screening effects) and the overlayer structure. The commensurate to incommensurate transition beyond 0.5 monolayer of coverage corresponds to the overlayer nonmetal-metal transition, which is due to the hybridization of Mg s and p bands and represents a transition from a localized (bondlike) to a delocalized (bandlike) phase for divalent atoms.