Theory of electron-hole pair excitations in unimolecular processes at metal surfaces. I. X-ray edge effects

Abstract

A theory of the dynamics of molecular processes at solid surfaces must necessarily deal with those aspects of the solid which provide dissipative or irreversible reaction channels, thus giving the particular process a direction in time. While the heat-bath aspects of the solid are often considered from the phonon point of view, there is increasing speculation that the substrate electron-hole pair excitations may be a significant rate-determining factor, at least for metals. This belief is supported by various conclusions which have emerged from theoretical studies of time-dependent perturbations acting on extended Fermi systems, as physically realized in core-level spectroscopies of solids. We present here one phase of a study of surface reaction dynamics, focusing on the irreversible coupling of nuclear motion of an incident beam of atoms or molecules with the substrate electrons. For the cases in which the incident particle undergoes a substrate-induced diabatic transition in its internal electronic state, a sudden localized perturbation on the electrons is turned on. In analogy with the x-ray edge problem, an infrared divergent spectrum of electron-hole pairs is created which could give rise to irreversibility. Specific examples are considered and the ramifications on such observable quantities as sticking coefficients are detailed.