Negative ion resonance states of SF6 physisorbed on the graphite surface investigated by high-resolution electron energy loss spectroscopy

The study of the adsorption and electron driven reactions of etching molecules at surfaces is relevant both to the field of microfabrication and to the characterisation of the basic physics of molecule-surface dynamics. We have investigated the negative ion resonance states of sulphur hexafluoride (SF6) physisorbed upon graphite (HOPG) at ∼85 K using high-resolution electron energy loss spectroscopy (HREELS) for both monolayer and multilayer coverages. The resonant energy profile of the ν3 vibrational mode of SF6 exhibits two peaks in the monolayer regime, at 1.75±0.5 eV and at 5.5±0.5 eV. The lower-lying resonant state is attributed to capture into the 6a1g molecular orbital, and higher-energy resonance is assigned to the 6t1u state. In the multilayer regime, there is a positive shift of ∼1 eV in the energy profile of these two peaks, which is attributed to the reduced influence of the image potential. No evidence for phase transitions has been found.