Valence-electronic structure of potassium adsorbed on Cu(001) deduced from work-function change and electron-energy-loss spectroscopy

The work-function variation for a potassium-covered Cu(001) surface has been analyzed in the framework of the Muscat-Newns theory. The K 4s resonance, with a halfwidth of 1.0–1.5 eV, was found to be positioned at 1.5±0.1 eV above the Fermi level at zero coverage. With increasing coverage, the valence resonances were shifted downward in energy due to depolarization effects. At a coverage of 0.18 (about 50% of the full monolayer coverage, 0.37), the 4s resonance reached 0.1 eV above the Fermi level. A moderate increase of the work function above a coverage of 0.18 was attributed to the metallization of a monolayer. Electron-energy-loss spectroscopy exhibited two loss peaks, one of which was assigned to a charge-transfer excitation from the Cu 3d band to the K 4s resonance and the other was assigned to the K 4s-4$p_z$ transition. The variation in energy losses and loss intensities of these peaks was in close accordance with the behavior of the valence resonances deduced from the work-function change. At coverages higher than 0.18, the 4s-4$p_z$ transition was shifted from 1.7 eV (0.1 eV larger than the 4s-4p energy separation in a free K atom) to 2.3 eV. This was interpreted as indicative of the collective nature of the transition in condensed phases. A previously reported two-dimensional condensation of K atoms was found to follow closely the ionic-to-neutral change and to coincide with the metallization of a monolayer. The influence of alkali-metal atoms on coadsorbed molecules is also discussed on the basis of the electrostatic potential around a K atom calculated for a low-coverage monolayer.