Introduction

Abstract

There have been two dominant models used in describing the chemical and electronic behavior of a surface. One is the atomistic model, or the “surface molecule” model; the other is the band model, often called for emphasis the “rigid band” model. The atomistic model has been preferred in general in discussions of the chemical processes at a solid surface; the rigid band model is often preferred in discussions of electron exchange between the solid (especially a semiconductor) and a surface group, where, for example, the conductivity of the solid is changed. The atomistic model describes the solid surface in terms of surface sites, atoms, or groups of atoms at the surface, essentially ignoring the band structure of the solid. The rigid band model describes the surface in terms of surface states, localized electronic energy levels available at the surface, to a great extent ignoring the microscopic details of atom/atom interaction between the surface species and its neighboring substrate atom.