Scattering of molecules from metal surfaces: Nitric oxide/gold(111).

A detailed study of rates and pathways of energy transfer in gas-surface interactions is crucial to elucidating the dynamics of chemical reactions on surfaces. Most theories of chemical processes at surfaces are based on the Born-Oppenheimer or adiabatic approximation, in which interacting nuclei move on a single potential energy surface usually corresponding to the ground electronic state of the system. However, the validity of this approximation is suspect in the case of metal surfaces because metals have a continuum of electronic states. In a set of state-selected molecular-beam experiments aimed at probing this matter, Alec Wodtke and coworkers scattered highly vibrationally excited NO molecules at low incidence energies successively from Au(111) and LiF(001) surfaces. It was observed that while most NO molecules underwent striking, multiquantum vibrational relaxation on scattering from Au(111), only few NO molecules lost a small fraction of their vibrational energy on scattering from LiF(001). Such qualitatively different behavior between the metal and insulating surfaces led the authors to suggest that the substantial vibrational energy transfer in NO/Au(111) resulted from a coupling of molecular vibration to electron-hole pair excitations in the metal via nonadiabatic electron transfer. This thesis theoretically investigates the underlying causes of vibrational inelasticity in NO/Au(111) and attempts to clarify the competition between adiabatic and nonadiabatic mechanisms of energy transfer at metal surfaces. The study involves the construction of a two-state model diabatic Hamiltonian of NO/Au(111) using density functional theory. While adiabatic dynamics on the calculated ground-state potential energy surface of NO/Au(111) demonstrated good, qualitative agreement with results of scattering experiments on NO(nu = 0)/Au(111), the simulations produced negligible vibrational relaxation of NO(nu = 15), the vibrational state used in experiments by the Wodtke group. In stark contrast, nonadiabatic dynamics, using the independent-electron surface hopping method in conjunction with a multi-state extension of the two-state model Hamiltonian, satisfactorily reproduced the observed multiquantum vibrational relaxation of NO(nu = 15) on Au(111). These results emphasize the importance of going beyond the Born-Oppenheimer approximation in theories of chemical processes at metal surfaces.