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 loss spectroscopy (HREELS). An intense loss mode at 19 meV is assigned to perpendicular vibrations of the Na atoms. While the frequency of this mode does not shift with coverage within +/-1 meV, the intensity is strongly coverage dependent, becoming too weak to be observed above the work function minimum (similar to 0.5 ML). The absence of a frequency shift is contrary to the expectations of the Gurney model as well as more recent theoretical calculations. Work-function changes, the coverage dependence of the dynamic charge and low coverage electronic losses are all accounted for by the phenomenological Newns-Anderson model.