THE ELECTRONIC-STRUCTURE OF CSI ADLAYERS ON W(110)

The electronic structure of thin CsI adlayers deposited on W(110) at r oom temperature is studied by ion impact electron spectroscopy (IIES) with He+ and He2+ ions, metastable impact electron spectroscopy (MIES) with He atoms, ultraviolet photoelectron spectroscopy (UPS), work fu nction measurements, and to some extent with electron energy loss spec troscopy (EELS). The transition from the electronic structure of singl e adsorbed molecules to that of the bulk structure could be studied by following the emergence of bulk properties, in particular the bandgap , as a function of the CsI exposure. The experimental results are cons istent with the following conclusions: The first step in the layer gro wth is molecular adsorption of CsI perpendicular to the surface with t he iodine end of the molecule pointing toward the substrate accompanie d by decrease of the surface work function. The next step, starting wi th the occurrence of a pronounced work function minimum, is the format ion of two-dimensional islands formed by CsI molecules. The results ar e consistent with lateral growth of the islands until the substrate is largely covered by CsI. The electronic structure of the adlayer canno t be distinguished anymore from that of CsI bulk material by means of electron spectroscopic techniques at exposures which are more than thr ee times that of the occurrence of the work function minimum. The prod uction of vacancies in the I- (5p) and Cs+ (5p) orbitals under the inf luence of the ionizing radiation is studied as a function of the CsI e xposure. Their relevance for the desorption processes induced by elect ronic transitions (DIET) is discussed.