Dipole Active Surface Vibrational Resonances on Clean and Hydrogen Covered Ni(110).

A vibrational resonance of the surface metal atoms on the clean Ni(110) surface is observed by electron energy loss spectroscopy. The dipole coupling arises from the incomplete screening of the moving ion cores by the conduction electrons in the surface region. Surface lattice dynamical calculations are in excellent agreement with the measured spectra for the clean surface and show that there are no significant deviations in the surface force constant from the bulk value. This resonance results from a pseudo band gap in the longitudinal bulk phonon dispersion relation along the (110) direction. On the (1x2) reconstructed surface induced by hydrogen chemisorption, a dramatic decrease in intensity of the resonance is observed.