Molecular beams scattering from metal surfaces: Observation of vibrational excitation in hydrogen chloride collisions with gold(111)

Chemical reactivity at metal surfaces plays a significant part in the modern technological society. Developing a deeper insight on fundamental principles underlying molecule-surface interactions is profoundly important for understanding the features of surface reactivity and development of new materials and catalysts in a predictive manner.; Energy transfer phenomena are extremely important in molecular collisions with solid surfaces. Among those, coupling of the nuclear vibrational motion to the electronic excitations in metal surfaces is still one of the least studied and understood. An innovative and highly sensitive apparatus for quantum state resolved molecule-surface energy transfer studies was built during the course of this work. The instrument includes an intense supersonic molecular beam source and a surface scattering chamber equipped with an ion detector and surface treatment and characterization capabilities. Two laser systems are employed to prepare and state-selectively ionize the molecules.; Vibrational excitation was first observed when HCl molecular beams are scattered from a Au(111) metal surface. Rotational, angular, and temporal distributions of both incident and scattered beams of HCl nu = 0 and nu = 1 were measured over a range of incidence energies and surface temperatures. The absolute vibrational excitation probability was estimated as 10 -6-10-4 over the studied range of energies, which is 2-3 orders of magnitude lower than that previously reported for NO on Ag(111).; The vibrational excitation probability was found to depend strongly on incidence kinetic energy exhibiting a threshold near Ei = ∼ 0.57 eV at low surface temperatures consistent with a direct translation to vibration (T-V) mechanical energy transfer mechanism at low surface temperatures. As the surface temperature increases, the slope of the Pnu as a function of TS exhibits a sharp increase above TS ∼ 800 K for all incidence energies. These changes in slope and threshold behavior of Pnu are consistent with a change from "mechanical" to an electronically non-adiabatic mechanism of vibrational excitation in the same scattering system. Observation of electronically adiabatic and non-adiabatic processes in the same scattering system provides a unique benchmark for theoretical development of molecular energy transfer to metal surfaces.