In the wake of collision

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Science  11 Dec 2015:
Vol. 350, Issue 6266, pp. 1321
DOI: 10.1126/science.aad8005

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When a metal atom with thermal kinetic energy hits a metal surface, it adsorbs on it with a probability of one. Moreover, it sticks to its impact site and makes not even a single jump to neighboring lattice sites (1). The mechanism behind this observation is not fully understood. On page 1346 of this issue, Bünermann et al. study a simpler system that can help to elucidate the metal/metal case. In their study, energetic hydrogen atoms impact on a gold surface. Only a fraction of the H atoms adsorb on the surface (see the figure), but the reflected atoms still lose a substantial fraction of their incident kinetic energy. The authors unequivocally identify electron-hole pair excitation as the main dissipation mechanism in these collisions (2). Apart from its obvious model character, the H/metal system is important for fields as diverse as heterogeneous catalysis, interstellar H2 production, and H storage.