Abstract
The recent availability of synchrotron radiation has made EXAFS a practical tool for studying the local environment of atoms. EXAFS analysis proceeds by calculating a theoretical model using theab initio software FEFF and fitting it to experimental data. These model spectra depend on the Pair Distribution Function (PDF), which is often assumed to be a symmetric Gaussian distribution. In general, however, it is asymmetric because the potential energy function between two atoms, for any bond, is anharmonic. Results we have obtained for solid iodine (at several temperatures) and copper show that the radial asymmetry parameter, which measures the departure from evenness of the PDF, has a positive value. For iodine, this is due to pairs of atoms being bonded in a fashion similar to isolated gas molecules. In such a case, the atoms have a greater resistance to being pushed together than pulled apart due to the hard core repulsion between atoms. The effect is exaggerated considerably by London interactions with neighboring atoms.
In copper, which has a symmetric structure, the asymmetry parameter has a much smaller value because the backscatter atom feels a hard core repulsion as it moves toward the absorber as well as when it moves toward the atom symmetric to the absorber.
Both substances are compared to theoretical predictions based on the model of a perturbed harmonic oscillator. This model is consistent with our data to within experimental error.