Dynamics of Hydrogen Atoms Scattering from Surfaces

Hydrogen atoms interactions with single crystal surfaces are the simplest processes in surface science, which are of both practical and fundamental interest. Chemical reactions of hydrogen atoms with single crystal surfaces lead to adsorption. Despite the fact that it has been studied for decades, the dynamics of hydrogen atom adsorption is still not fully understood. Adsorption involves the impinging hydrogen atom losing its initial translational energy and dissipating the energy of the chemical bond formed with the surface. How hydrogen atoms dissipate their initial translational energy to adsorb on a surface is still an open question. In principle, a hydrogen atom may lose its initial translational energy to the vibrations of the surface atoms or to electron-hole pair excitations. To answer this question in detail, a state-of-the-art UHV machine was built to study atom-surface scattering. During my doctoral study, I conducted experiments on hydrogen atom scattering from single crystal surfaces, including Au(111) and Pt(111), Xe covered Au(111) and epitaxial graphene on Pt(111). High resolution scattering angle resolved translational energy loss distributions have been obtained. The goal of this research is to achieve a detailed understanding of the mechanisms of H atom interaction with different kinds of single crystal surfaces, and especially of the translational energy transfer between the atom and the surfaces, which is fundamentally important to the adsorption processes. To compare hydrogen atom translational energy loss to metals and insulators, I studied scatterings of hydrogen atoms from Au(111) and Xe layer on Au(111). Hydrogen atoms scattering from insulating Xe layer exhibit a small energy loss and a narrow translational energy distribution and can be understood using a binary collision model. In contrast, Hydrogen atoms scattering from Au(111) show a large energy loss and a broad translational energy distribution, indicating that a broad continuum of accepter states in the solid contribute to the translational energy loss. A MD simulation self-consistently including the non-adiabatic electronic excitations agrees with the experiments of hydrogen atom scattering on metal. In contrast, calculations neglecting the electronic excitations cannot capture the essence of the measurements, indicating hydrogen atoms scattering from metal is strongly non-adiabatic. Exchanging H atoms with D atoms only leads to minor change in the translational energy loss distribution. This is explained by a cancellation effect, where the phonon excitation is enhanced for D but the electron-hole pair excitation is reduced. To study the dynamics of chemically activated adsorption of hydrogen atoms, I did a series of experiments on hydrogen atoms scattering from epitaxial graphene, which has a barrier to C-H bond formation of several hundreds of meV. The scattered hydrogen atoms exhibit a bimodal distribution for translational energy and scattering angle. The fast component in the distribution originates from atoms scattered back without crossing the adsorption barrier and is near-elastic and near-specular. The slow component originates from atoms scattered back after crossing the adsorption barrier and exhibits large and rapid energy loss (energy loss power around 1019 W/mole atom). By monitoring the ratio between the slow and the fast component, we determine that the adsorption threshold is in the range of 0.41 eV to 0.48 eV for the H atom, and 0.43 eV to 0.47 eV for the D atom. Combined with DFT based AIMD calculations, we conclude that the fast component is H/D atom quasi-elastic scattering on a corrugated surface. The large and rapid energy loss of the slow component is caused by the formation of a transient reaction complex. Due to the π resonance structure of graphene, H atom colliding with one C atom will cause simultaneous displacements of the neighboring C atoms, leading to the formation of a transient reaction complex. Large portion of the energy loss is due to the inelastic scattering mechanism. Surface IVR provides insights of the phonon excitations in graphene