Phase factor in extended x-ray-absorption fine structure

An investigation is made to ascertain the physical processes that affect the phase-shift factor in the extended x-ray-absorption fine structure. The tetrahedrally coordinated series of CuBr, ZnSe, GaAs, and Ge was measured and analyzed to ascertain the effect of ionic bonding. Comparison was made between the theoretical phase shifts of Teo and Lee and the measurements using the zero of energy E0 as a variable. The most reliable criterion found for determining E0 was to set the intercept of the phase difference between theory and experiment to zero at k=0. The results of this analysis verified that the phase shift is not strictly transferable, but much of the lack of transferability can be accounted for by varying E0 in accordance with the above criterion. Different values of E0 are required for each shell of neighboring atoms. The largest cause of the lack of transferability is the breakdown of the small-atom approximation, which can be accounted for by a shift in E0 of -13 and -8 eV for the first and second shells of the samples, respectively. The ionicity of bonding can be accounted for by a shift in E0 of about 5 eV per electron-charge transfer. Evidence is presented for charge transfer occurring after x-ray excitation of the Cu and Zn atoms in these materials.