Surface vibrational spectroscopy with angle-dependent electron energy loss spectroscopy: Acetylene chemisorption on Pd(111)

High resolution electron energy loss measurements and LEED observations for normal and deuterated acetylene chemisorbed on Pd(111) are reported. Analysis of specular and off-specular energy loss data permits classification of the vibrational modes in terms of long-range dipole and short-range impact scattering mechansims. Impact scattering intensities are found to be relatively strong for some modes and comparable in strength to intensities of dipole-excited modes. The vibrational data indicates that chemisorbed acetylene is strongly rehybridized (∼sp2.5), in approximate agreement with earlier photoemission work. Comparsion is made to vibrational assignments for C2H2 chemisorbed on Ni(111) and Pt(111), as well as relevant model compounds. A surface bonding geometry is proposed, which involves acetylene coordination at threefold sites on the Pd surface in the (√3×√3)R30° structure.