Electron energy loss vibrational spectra of cyclopropane on Cu(111): Negative ion formation at 6 eV

Abstract

High-resolution electron energy loss (HREELS) spectra of cyclopropane adsorbed on Cu(111) were recorded over a range of primary electron energies. Vibrational excitation functions mapped out over the 2-18 eV primary energy range reveal a shape resonance centered at approximately 6 eV. The internally normalized excitation functions permit a separation of the observed losses into two families of modes, one associated with ring vibrations and one with CH vibrations. Although all of the loss peaks draw intensity from the shape resonance, there is a preferential enhancement of the CH bands and, in particular, the CH stretching loss. The shape resonance is assigned to 4e′ in D3h, symmetry on the basis of the excitation spectra and by reference to gas phase studies. By applying the selection rules appropriate for resonance scattering, the point group symmetry for adsorbed cyclopropane is determined to be either C3v or Cs(σv). Additional arguments are given in favor of a C3v, geometry.