Surface EXAFS

Abstract

In the last decade there has been an increased wealth of information on surface properties of solids, both for atomically pure (or clean) surfaces of metals and semiconductors and for surfaces with adsorbed atoms and molecules. The latter systems have many fundamental properties, such as 2-dimensional phase transitions and electronic instabilities. In order to describe these dynamical properties of surfaces, it is necessary to know accurately the atomic structural coordinates of the first one or two layers at the surface. Since 1977–78 studies^1,2 of surfaces using synchrotron radiation have provided new structural information with a precision of ±0.03 Å, which is better than the accuracy achieved using most other surface-structure probes. In this chapter, we shall describe the essential physics of two experimental methods which are explicitly designed to determine surface structures by exploiting the properties of elemental specificity and sensitivity to local structure. The two methods—surface extended X-ray absorption fine structure (sExAFS) and angle-resolved photoemission extended fine structure (ARPEFS) are closely related, as shown below. In a heuristic sense, it is useful to think of sExAFS and ARPEFS, respectively, as the integral and differential manifestations of the same phenomenon, by analogy with the broader relationship between absorption and angle-resolved photoemission. This phenomenon is the spectral extended fine structure due to the elastic scattering of photoelectrons originating from core level photoexcitation of surface atoms.