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

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Abstract
High resolution electron energy lossmeasurements 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 (∼s p 2.5), 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.