Resonance vibrational excitation in electron-energy-loss spectroscopy of adsorbed molecules

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Abstract
A mechanism is suggested which could lead to enhanced excitation of the low-frequency vibrational modes associated with the bond between a solid surface and a molecule adsorbed upon it or of hindered rotational modes with displacement components normal to the surface, as observed in electron-energy-loss spectroscopy. If the incident electron becomes trapped in a negative-molecular-ion shape resonance, manifesting itself in significant intramolecular overtone excitation, the molecule-surface potential-energy curve is augmented by the image potential of the negative ion. This allows displacement of the molecule from its equilibrium neutral position which in turn appears as vibrational excitation of adsorbate-substrate relative motions, upon return to the neutral curve. A simple theory for this effect is presented and numerical estimates of its magnitude are given.