Angular distribution of EELS intensities: CO on Pt(111)

We present the angular distribution of inelastic intensities corresponding to the excitation of the vibrational modes of CO chemisorbed on Pt(111). The experimental values are compared with those predicted by dipole scattering. All the vibrational modes, including the C-metal, stretch, show experimental values larger than theoretical ones. The difference is attributed to vibrational excitation by impact scattering. The impact scattering cross section is found to be energy dependent and increases for low primary energies (2 eV). The differential cross section is, however, smaller than that observed for resonant scattering in free CO.