Mechanisms for low-energy-electron vibrational excitation of adsorbates: H on W(100)

Vibrational-excitation spectra for the saturated coverage of H on W(100) at ∼ 300 K have been measured by inelastic low-energy (<30 eV) electron scattering. In the specularly reflected beam direction, a single vibrational peak is observed and is attributed to H vibrating normal to the surface ($z$ displacement). However, vibrational modes corresponding to H displacement parallel to the surface ($x$ and $y$ directions) are also observed if the electrons are collected away from the specular direction. These two different polarization modes are easily distinguished from the dependence of their intensities on the scattering angle. Scattering from the substrate is the dominant process in determining the elastic and vibrationally inelastic intensities, as is reflected in the impact-energy dependence of the cross sections. An understanding of the mechanisms for vibrational excitation of molecules adsorbed on surfaces can be obtained from the dependence of the measured cross sections on the scattering angle and impact energy and a comparison of calculations from dipole and localized potentials.