Coverage-dependent shifts ofsandpresonances 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 Θ 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 ${\Theta }_{\mathrm{K}\mathrm{\approxeq }0.06}$ monolayers (ML). Extrapolation of our results to the zero-coverage limit yields a position of the 4p level at 2.7±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±0.2 eV above $E_F$. Therefore, a single K adatom is mainly ionic. At Θ≥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.