Hydrogen atom collisions with a semiconductor efficiently promote electrons to the conduction band

The Born-Oppenheimer approximation is the keystone of modern computational chemistry and there is wide interest in understanding under what conditions it remains valid. Hydrogen atom scattering from insulator, semi-metal and metal surfaces has helped provide such information. The approximation is adequate for insulators and for metals it fails, but not severely. Here we present hydrogen atom scattering from a semiconductor surface: Ge(111)c(2 × 8). Experiments show bimodal energy-loss distributions revealing two channels. Molecular dynamics trajectories within the Born-Oppenheimer approximation reproduce one channel quantitatively. The second channel transfers much more energy and is absent in simulations. It grows with hydrogen atom incidence energy and exhibits an energy-loss onset equal to the Ge surface bandgap. This leads us to conclude that hydrogen atom collisions at the surface of a semiconductor are capable of promoting electrons from the valence to the conduction band with high efficiency. Our current understanding fails to explain these observations. [Abstract copyright: © 2022. The Author(s).

On the dynamics of H atom scattering from metal surfaces

When a hydrogen atom collides with a surface it may either scatter or stick to the surface, depending on the energy loss the particle experiences during collision. It is established that non-adiabatic effects in form of electronic excitation dominate the energy transfer between H atom and metal surfaces. This work addresses the question whether the scattering dynamics and the associated energy transfer depend on the structural and electronic properties of the underlying surface. In order to answer this question, I performed molecular dynamics simulations with electronic friction describing H atom scattering from various metal surface facets on potential energy surfaces that are based on Effective Medium Theory. At ambient temperatures, the resulting energy loss distributions are similar for all metals, whereas drastic differences between the individual systems appear at low temperatures. Adiabatic molecular dynamics simulation of H atom scattering from a Xe(111) surface predicts a very narrow energy loss distribution which is in good agreement with experimental findings, demonstrating that electron-hole pair excitation are not important here due to the large separation of valence and conduction band in the noble gas crystal. The reaction dynamics of chemisorbed hydrogen on platinum and palladium can be described with transition state theory, where the transition state is chosen to be the gas-phase molecular hydrogen, provided that the two sensitive quantities, the partition function and the activation energy, are acquired accurately. Testing several models revealed that incorporation of quantum effects are of critical importance.2023-02-2