VIBRATIONAL-MODES OF HYDROGEN ADSORBED ON PT(111) - ADSORPTION SITE AND EXCITATION MECHANISM

The vibration spectrum of hydrogen adsorbed at {greater-than or approximate}90 K on Pt(111) has been studied by electron energy loss spectroscopy. It is shown that impact scattering gives an important contribution to the vibrational cross section. This has the consequence of removing the limitations of the surface selection rule". So all the vibrational modes perpendicular and parallel to the surface are observed. At high coverage two vibrational bands at 550 and 1230 cm-1 are assigned to the symmetric (a1) and asymmetric (E) stretching modes of atomic hydrogen adsorbed in a three-fold hollow site. At low coverage some slight modifications are observed. The correlation of these results in particular with flash desorption spectroscopy is discussed. The measurements of the loss intensities in and out of the specular reflection allows us to determine the differential cross sections for dipole and impact scattering. Some correlation between the cross section and structure appearing in the specular elastic intensity versus voltage curve for the hydrogencovered surface is observed. © 1979."