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Electron-hole pair excitation determines the mechanism of hydrogen atom adsorption

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Science  11 Dec 2015:
Vol. 350, Issue 6266, pp. 1346-1349
DOI: 10.1126/science.aad4972

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Sticking hydrogen atoms to surfaces

The simplest case of adsorption at a surface—that of a hydrogen atom—is actually quite complicated. This is because it is not clear how this light atom can transfer enough momentum to the heavy surface that it can slow down and stick. Bünermann et al. prepared highly energetically controlled hydrogen atoms (see the Perspective by Brune). On a gold surface, inelastic collisions occurred during adsorption, but not when an insulating layer of xenon atoms was used.

Science, this issue p. 1346; see also p. 1321

Abstract

How much translational energy atoms and molecules lose in collisions at surfaces determines whether they adsorb or scatter. The fact that hydrogen (H) atoms stick to metal surfaces poses a basic question. Momentum and energy conservation demands that the light H atom cannot efficiently transfer its energy to the heavier atoms of the solid in a binary collision. How then do H atoms efficiently stick to metal surfaces? We show through experiments that H-atom collisions at an insulating surface (an adsorbed xenon layer on a gold single-crystal surface) are indeed nearly elastic, following the predictions of energy and momentum conservation. In contrast, H-atom collisions with the bare gold surface exhibit a large loss of translational energy that can be reproduced by an atomic-level simulation describing electron-hole pair excitation.

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