Real-Time Coupled Electron-Nuclear Dynamics of Chemical Bond Formation: Hydrogen Scattering from a Semiconductor Surface

TL;DR

This study uses first-principles simulations to reveal how hydrogen atoms transfer energy electronically during scattering from a semiconductor surface, showing site-specific bond formation and ultrafast electron transfer events.

Contribution

It introduces a novel first-principles approach to simulate coupled electron-nuclear dynamics, elucidating site-selective nonadiabatic energy transfer mechanisms at a semiconductor surface.

Findings

Hydrogen scattering involves site-specific transient Ge-H bond formation.

Ultrafast electron transfer occurs during bond formation, causing electronic excitations.

Energy transfer mechanisms differ from those at metal surfaces.

Abstract

A first-principles coupled electron-nuclear dynamics simulation based on real-time, time-dependent density functional theory and Ehrenfest dynamics quantitatively repro-duces bimodal translational energy loss and angular distributions observed in experiment for hydrogen atom scattering from Ge(111)-c(2*8). The theory elucidates a site-selective mechanism of electronically nonadiabatic energy transfer associated with the formation of different Ge-H bonds. When a hydrogen atom approaches a Ge rest-atom, it is strongly accelerated toward the potential minimum forming a transient Ge-H bond and then re-flected by the repulsive wall. This transient bond formation triggers an ultrafast electron transfer event from the rest-atom to an adjacent Ge-adatom, involving several crossings between valence and conduction bands of the substrate. Electronic equilibration is impos-sible within such a short time (Born-Oppenheimer failure) allowing the H-atom kinetic energy to be converted to inter-band electronic excitation of the substrate. H-atom colli-sions at other Ge atoms also form a transient bond but exhibit no electronic excitation, resulting in distinctly less efficient energy loss in scattered H-atoms. The nucle-ar-to-electronic energy transfer observed in this system reflects the electronic dynamics of covalent bond formation at a semiconductor surface, a mechanism that is quite distinct from previously identified nonadiabatic energy transfer mechanisms at metal surfaces mediated by electronic friction or transient negative ions.