Specular and off-specular high resolution electron energy loss spectroscopy of acetylene and ethylene on tungsten (100)

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Abstract
High resolution electron energy loss spectroscopy (EELS) in both specular and off-specular directions has been used to identify the vibrational modes of acetylene and ethylene on tungsten (100). The off-specular data were essential to this study since, at low coverages, some of the vibrational modes were detectable only for off-specular scattering. In addition, analysis of the relative intensities of the specular and off-specular loss peaks allows us to infer orientations of molecular dipole derivatives and bond directions for the adsorbed species. We find at 135 K that ethylene on tungsten (100) dissociates to acetylene and hydrogen for exposures less than 1 L. At saturation coverage molecular ethylene is also adsorbed. Warming of this adsorbed ethylene to 300 K causes dissociation to acetylene. Analysis of specular and off-specular loss intensities suggests a geometry with the C–H bonds lying parallel to the surface. Previous UPS data for this adsorption system can be reinterpreted giving a C–C bond length of 1.35 Å and a C–C–H bond angle of 180 °: This reinterpretation is consistent with the geometry suggested by our EELS measurements. We also present data for acetylene adsorption at room temperature and for ethylene physisorption at 82 K. Physisorbed ethylene shows vibrational losses identical to those seen in gas phase IR.