Coverage-dependent frequency for Li-atom vibrations on Cu(111).

Abstract

Electron-energy-loss spectra recorded for monolayer amounts of Li adsorbed on Cu(111) show a loss peak associated with Li vibrations perpendicular to the substrate. The loss energy shifts from 38 meV at low coverage to 43 meV at 0.3 ML and remains constant for coverages between 0.3 and 0.5 ML. The loss intensity passes a maximum at a Li coverage of 0.15 ML and gradually decreases such that it is difficult to resolve a loss peak at coverages above 0.5 ML. The high loss energy indicates that the adatom resides on the surfaces rather than in substitutional sites. The frequency shift is much too large to be explained by dipole-dipole interactions. The above results are obtained with the evaporation source loaded with the natural Li isotope mixture (92.6% $^{7}\mathrm{Li}$, 7.4% $^{6}\mathrm{Li}$). Measurements with $^{6}\mathrm{Li}$ show that the increase of the vibration frequency with increasing coverage is not an isotope effect. \textcopyright{} 1996 The American Physical Society.