Real-Time Coupled Electron-Nuclear Dynamics of Chemical Bond Formation on a Semiconductor Surface

A first-principles coupled electron-nuclear dynamics simulation based on real-time, time-dependent density functional theory and Ehrenfest dynamics quantitatively reproduces bimodal translational energy loss and angular distributions observed in experiments 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 reflected 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 impossible within such a short time (Born-Oppenheimer failure), allowing the H atom kinetic energy to be converted to interband electronic excitation of the substrate. H atom collisions at other Ge atoms also form a transient bond but exhibit no electronic excitation, resulting in a distinctly less efficient energy loss in scattered H atoms. The nuclear-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.