Multiple Scattering and Bond Angle Determination

Abstract

While EXAFS spectroscopy can provide structural information about the local environment of the absorbing atoms in terms of radial distribution functions (distances), no direct method of determining angular information is available; except, perhaps, for elaborate measurements on single crystals utilizing polarized X-rays. Furthermore, the very same advantageous characteristics of EXAFS (short-range, single-scattering) are also its serious limitations: distance determinations out to only ca 4Å. The situation, however, changes dramatically when atoms (including the X-ray absorbing atom and its neighbors) are arranged in a linear or nearly collinear fashion. In such cases, EXAFS contributions from neighboring atoms as far as 8Å can be observed. For these systems, both the amplitude and the phase of the EXAFS of a more distant neighbor are significantly affected by the intervening atom(s). In particular, the amplitude is greatly enhanced and is therefore commonly called “focusing” effect. The short-range single-scattering theory of EXAFS fails in these situations and one must take into account multiple scattering processes involving the intervening atoms.