Coverage-dependent shifts of<i>s</i>and<i>p</i>resonances of alkali metals chemisorbed on Al(111)

Abstract

The electronic structure of alkali-metal adatoms on Al(111) has been studied experimentally by inverse photoemission and ultraviolet photoemission spectroscopy. The unoccupied np resonances of Cs, K, and Na shift towards the Fermi level ${E}_{F}$ with increasing alkali coverage \ensuremath{\Theta} from zero to monolayer coverage. Simultaneously, the ns resonances observed by inverse photoemission shift downwards and cross ${E}_{F}$. For K/Al(111) the 4s resonance emerges in the photoemission spectrum below the Fermi edge at ${\ensuremath{\Theta}}_{\mathrm{K}\mathrm{\ensuremath{\approxeq}}0.06}$ monolayers (ML). Extrapolation of our results to the zero-coverage limit yields a position of the 4p level at 2.7\ifmmode\pm\else\textpm\fi{}0.1 eV above ${E}_{F}$ for a single K atom on Al(111) and the center of gravity of the 4s resonance (full width at half maximum equals 0.6 eV) at 0.7\ifmmode\pm\else\textpm\fi{}0.2 eV above ${E}_{F}$. Therefore, a single K adatom is mainly ionic. At \ensuremath{\Theta}\ensuremath{\ge}0.10 ML the alkali-metal interaction induces the formation of two-dimensional bands and the overlayer becomes metallic. Our experimental results clearly corroborate the Gurney-type theoretical models of alkali-metal chemisorption.