Model Studies of Electron Transfer and Reaction Dynamics at Surfaces

Doktorsavhandling, 1996

Charge-transfer is of key importance in many physical, chemical and biological processes, including surface reactions. This thesis consists of several theories for and model studies of surface reactions that are initiated by electron transfer between the surface to a molecule. Large non-adiabatic effects can result from a fast dissociation process initiated by an "early" electron transfer (surface harpooning). The probability for a chemical reaction between a molecule and a metal surface is tied to such an electron transfer. For a chlorine molecule impinging on a potassium metal surface, the harpooning electron results in a dissociative sticking with unit probability (Paper III). The reaction heat is released in non-adiabatic detectable quantum jumps in the form of electron (exo-electron) and light emission (surface chemiluminescence). Signals in this way sent out of the material can be used to monitor the dissociation dynamics. For instance, the Cl2-velocity dependence of such yields and spectra can be interpreted in terms of a simple model (Paper III). The excess energy can also give rise to the emission of ions and neutral atoms (Paper IV) or in a transient mobility of the dissociation fragments. Such a mobility of oxygen atoms on an aluminum surface, formed in the course of dissociative sticking of oxygen molecules is studied in Paper II, where only a limited transient mobility is found. Non-adiabatic transitions can give rise to simultaneous desorption and dissociation of adsorbed molecules. In Paper V the branching ratio for these processes is calculated by wave-packet propagation in realistic potentials for an oxygen molecule on the Pt(111) surface. Such dissociation fragments might act as hot precursors, reacting with predosed CO and then forming CO2. A proper description of the dynamics requires good accounts of the forces acting on the particles involved, i.e. good potential-energy surfaces (PES). The feasibility of a first-principles calculation of such molecule-surface PES is done for a hydrogen atom on a Cu surface in Paper I. With this PES the absorption and transient mobilities of hydrogen atoms impinging on the Cu(111) surface are studied (Paper I). The transient mobility can result in an enhanced reactivity for hydrogen atoms reacting in the Eley-Rideal scenario with predosed deuterium atoms, forming HD.