SOME VIBRATIONAL PROPERTIES OF CLEAN AND WATER-VAPOR EXPOSED CU(111) NA,K/

When Na (K) is adsorbed on Cu(111) and the coverage is below half of a full monolayer electron energy loss spectra show a peak at 21 meV (13 meV) ascribed to the excitation of Na (K) vibrations perpendicular to the surface. From the intensity of the loss peak at low alkali metal coverage a dynamic charge of approximately 0.5e (0.6e) is obtained ass uming dipolar scattering. The value agrees well with theoretical estim ates obtained from the dependence of the induced dipole moment on the distance from the substrate surface and provides strong support for th e description of the adsorbate as partly ionic at low monolayer covera ges. If the Na covered Cu(111) surface is exposed to water vapour ther e is a rapid uptake only if there is more than 10% of a full monolayer of Na on the surface. The existence of a Na precoverage threshold is revealed by the exposure induced vibrational losses and work function changes. Above threshold there is, upon H2O exposure, an increase of t he work function and strong changes in the low loss energy region of t he electron energy loss spectra with peaks appearing at 18 meV and 36 meV loss energy.