Vibrational spectra of ammonia chemisorbed on platinum (111): II. The electron scattering mechanism

We have measured the angle and energy dependence of the inelastic cross sections for NH₃ chemisorbed on Pt(111). The relative contributions of dipole and nondipole processes in the on-specular intensities were determined. The angular dependence (off-specular scattering) was found to be more useful than the impact energy dependence for discrimination between the two scattering mechanisms. For both molecular states of NH₃/Pt(111), strong impact scattering contributions were found in the NH stretching mode intensities. One particular mode, the degenerate stretch ν_d(NH)] was selectively excited by impact scattering. The energy dependence showed an enhancement of all of the inelastic intensities at low impact energies (1 eV). This enhancement was found to be a result of a surface reflectivity increase, coupled with an increase in the inelastic cross section. The latter increase is not predicted by dipole theory. Determination of the molecular orientation was not possible, due to the complex contributions of dipole and impact scattering.