IMPACT OF THE ASYMMETRIC PAIR DISTRIBUTION FUNCTION IN THE ANALYSIS OF XAFS

Asymmetric distributions were once considered to pose a serious limita tion to EXAFS spectroscopy, but with present analysis methods it is no w possible to go beyond the simple extraction of static structural par ameters and to determine the effective pair potential and contribution s to relative motion of atomic pairs. This paper reviews methods of in cluding asymmetry in EXAFS analysis such as cumulants, splice method, fitting to model functions and using Monte Carlo or molecular dynamic simulations. Determining the pair potential is emphasized. The quantum treatment of anharmonic oscillators is summarized and the possibility of specifying double-well pair potentials is indicated. Applications ranging from the strong covalent bonding in semiconductors to the weak er ionic and metallic bonding in bulk and surface systems are given. T hermal expansion coefficients of bonds, premelting and liquids are als o discussed.