VIBRATIONAL AND ELECTRONIC EXCITATIONS OF NA MO(100)/

The vibrational and electronic excitations of Na atoms chemisorbed on Mo(100) have been investigated with high-resolution electron energy lo ss spectroscopy (HREELS). An intense loss mode at 19 meV is assigned t o perpendicular vibrations of the Na atoms. While the frequency of thi s mode does not shift with coverage within +/-1 meV, the intensity is strongly coverage dependent, becoming too weak to be observed above th e work function minimum (similar to 0.5 ML). The absence of a frequenc y shift is contrary to the expectations of the Gurney model as well as more recent theoretical calculations. Work-function changes, the cove rage dependence of the dynamic charge and low coverage electronic loss es are all accounted for by the phenomenological Newns-Anderson model.