PREDICTION OF EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTRA FROM MOLECULAR INTERACTION MODELS - NA-MGO(100) INTERFACE((H2O)(N))

A theoretical approach is presented that can be used to model extended x-ray-absorption fine-structure (EXAFS) spectra for complex systems a t finite temperatures. This method has the ability to compute, directl y, the effects of thermal motion on EXAFS spectra and to deconvolve th e EXAFS signal into contributions from individual scattering paths. Cl assical molecular-dynamics simulations of the interface are used to ge nerate configurations corresponding to a given temperature and are use d to compute the Debye-Waller factors for all (single and multiple) sc attering paths. Interface geometries and first- (r(eff)), second- (sig ma(2)), and third-order cumulants are computed directly from the confi gurations and input into the multiple-scattering x-ray-absorption fine -structure code, FEFF6, which calculates the EXAFS spectra. This metho d is applied to predict EXAFS spectra of sodium ions adsorbed at the M gO (100) interface at three types of surface sites-flat terrace, step edge, and step corner (kink). The calculations indicate that the exper iments should only be able to detect signals from sodium ions adsorbed onto defect sites-step edges and corners. in situ spectra are compute d from models of Na+ at the aqueous-MgO interfaces using sodium ions i mbedded in finite water clusters. The strong overlap of features attri butable to the water oxygens and the nearest-neighbor surface oxygens may complicate the structural analysis from the experimental spectra; however, the scattering from the magnesium atoms and focusing multiple -scattering paths at longer distances are clear signatures of the inte rface.