THEORETICAL-ANALYSIS OF THE ELECTRONIC-STRUCTURE OF THE STABLE AND METASTABLE C(2X2) PHASES OF NA ON AL(001) - COMPARISON WITH ANGLE-RESOLVED ULTRAVIOLET PHOTOEMISSION SPECTRA

Using Kohn-Sham wave functions and their energy levels obtained by den sity-functional-theory total-energy calculations, the electronic struc ture of the two c(2X2) phases of Na on Al(001) are analyzed, namely, t he metastable hollow-site structure formed when adsorption takes place at low temperature, and the stable substitutional structure appearing when the substrate is heated thereafter above 180 K or when adsorptio n takes place at room temperature from the beginning. The experimental ly obtained two-dimensional band structures of the surface states or r esonances are well reproduced by the calculations. With the help of ch arge-density maps, it is found that, in both phases, two pronounced ba nds appear as the result of a characteristic coupling between the vale nce-state band of a free c(2X2)-Na monolayer and the surface-state/res onance band of the Al surfaces; that is, the clean (001) surface for t he metastable phase and the unstable, reconstructed ''vacancy'' struct ure for the stable phase. The higher-lying band, being Na derived, rem ains metallic for the metastable phase, whereas it lies completely abo ve the Fermi level for the stable phase, leading to the formation of a surface-state/resonance band structure resembling the bulk band struc ture of an ionic crystal.