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

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.