Energy disposal during desorption of D2 from the surface and subsurface region of Ni(111).
The recombination of surface and subsurface D atoms on Ni(111) has been studied using resonance-enhanced multiphoton ionisation (REMPI) to measure the internal state and translational energy distributions of the desorbing product. By detecting D2 formed during temperature-programmed desorption we were able to examine the reaction between subsurface and surface D atoms, and the recombination of two D atoms chemisorbed on the surface. Translational energy distributions for D2 formed by recombination of surface D are very sensitive to coverage. Desorption from a low coverage surface produced a translational energy release of 2.6 kT, but a thermal rotational distribution, reflecting an entrance channel barrier to dissociative chemisorption on the clean Ni(111) surface. Sticking probabilities predicted from detailed balance are consistent with molecular beam adsorption measurements. Desorption from D coverages above 0.5 ML resulted in a sub-thermal energy release, desorption being mediated by a molecular precursor state with D2 dissociation occurring via a non-activated, trapping-dissociation channel. In contrast, the reaction of subsurface D produces translationally hot D2, with a mean energy approaching 8 kTs at 180 K. This is consistent with the energetics for direct recombination of a chemisorbed D atom with a metastable subsurface D atom, which overcomes an activation barrier to resurface of between 0.35 and 0.47 eV depending on D concentration. The energy release decreases at higher temperature, probably as a result of a reduction in the energy of resurfacing D as the subsurface D concentration drops. This low energy component is attributed to accommodation of resurfacing D which is unable to react directly, followed by slow thermal desorption via the high coverage, surface D recombination channel. No internal rotational or vibration excitation was found in D2 formed by reaction of subsurface D.
Surface Science Research Centre, University of Liverpool, Liverpool, UK L69 3BX.
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Wright
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Skelly
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Hodgson