COVERAGE-DEPENDENT FREQUENCY FOR LI-ATOM VIBRATIONS ON CU(111)

Electron-energy-loss spectra recorded for monolayer amounts of Li adso rbed on Cu(111) show a loss peak associated with hi vibrations perpend icular to the substrate. The loss energy shifts from as meV at low cov erage to 43 meV at 0.3 hit and remains constant for coverages between 0.3 and 0.5 ML. The loss intensity passes a maximum at a Li coverage o f 0.15 hit and gradually decreases such that it is difficult to resolv e a loss peak at coverages above 0.5 ML. The high loss energy indicate s that the adatom resides on the surfaces rather than in substitutiona l sites. The frequency shift is much too large to be explained by dipo le-dipole interactions. The above results are obtained with the evapor ation source loaded with the natural Li isotope mixture (92.6% Li-7, 7 .4% Li-6). Measurements with Li-6 show that the increase of the vibrat ion frequency with increasing coverage is net an isotope effect.