STRONG MODE-SELECTIVE RESONANT VIBRATIONAL-EXCITATION IN CHEMISORBED MOLECULES - AN ELECTRON-ENERGY-LOSS SPECTROSCOPY STUDY OF HCOO GAAS(100)

High resolution electron energy loss spectroscopy (HREELS) has been us ed to study the vibrational properties of the surface formate species (HCOO) chemisorbed on GaAs(100). Measurements carried out over a wide range of incident electron energies indicate that at least two of the vibrational modes of the adsorbate, namely the symmetric and asymmetri c OCO stretches, are excited preferentially, even in specular scatteri ng geometry, by a resonance scattering process involving formation of a temporary negative ion. The resonance, centred at similar to 11 eV, is characterised by a large enhancement in the intensity of not only t he fundamental vibrations, but also the appearance of a number of over tone and combination bands. The width and asymmetric profile of the re sonance for the symmetric OCO stretch suggests there are two resonant states involved. The two final states, of a(1) and b(1) symmetry, corr espond respectively to in-phase and out-of-phase combinations of the f (z)3 terms, based on the C-O bonds. The observation of both of these v ibrational modes by resonance scattering suggests a breakdown of the m ode-selective vibrational excitation selection rule.