Introduction

A complete characterization of a solid surface requires knowledge of not only what atoms are present but where they are. Just as in the bulk, it is not that the atomic coordinates as such are of much direct interest. Rather, our concerns generally will center on electronic and magnetic properties and it is the geometrical arrangement of the surface atoms that largely determines the near-surface charge and spin density. Put another way, the nature of the surface chemical bond depends in detail on surface bond lengths and bond angles. The corresponding bulk structural issues normally are resolved by x-ray diffraction. Unfortunately, the extremely large penetration depth and mean free path of x-rays severely limits their routine use for surface crystallography. Consequently, much effort has been devoted to the invention and application of alternative experimental approaches to surface-specific structural analysis. Although a number of common techniques will be discussed below, it is a sobering fact that no single surface structural tool has emerged that can be used as easily and reliably as x-rays are used for the bulk.

Appeal to theory does not offer much relief. In principle, a solid adopts the crystal structure that minimizes its total energy. We know how to write down an exact expression for this energy; it is a parametric function of the exact position of all the ions in the material.