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 molecu les at surfaces is relevant both to the field of microfabrication and to th e characterisation of the basic physics of molecule-surface dynamics. We ha ve investigated the negative ion resonance states of sulphur hexafluoride ( SF6) physisorbed upon graphite (HOPG) at similar to 85 K using high-resolut ion electron energy loss spectroscopy (HREELS) for both monolayer and multi layer coverages. The resonant energy profile of the v(3) vibrational mode o f 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 6a(1g) molecular orbital, and higher-energy resonance is assigned to t he 6t(1u) state. In the multilayer regime, there is a positive shift of sim ilar to1 eV in the energy profile of these two peaks, which is attributed t o the reduced influence of the image potential. No evidence for phase trans itions has been found. (C) 2000 Elsevier Science B.V. All rights reserved.