Prediction of extended x-ray-absorption fine-structure spectra from molecular interaction models: Na⁺(H₂O)_n-MgO (100) interface

A theoretical approach is presented that can be used to model extended x-ray-absorption fine-structure (EXAFS) spectra for complex systems at finite temperatures. This method has the ability to compute, directly, the effects of thermal motion on EXAFS spectra and to deconvolve the EXAFS signal into contributions from individual scattering paths. Classical molecular-dynamics simulations of the interface are used to generate configurations corresponding to a given temperature and are used to compute the Debye-Waller factors for all (single and multiple) scattering paths. Interface geometries and first- (r_eff), second- (σ²), and third-order cumulants are computed directly from the configurations and input into the multiple-scattering x-ray-absorption fine-structure code, FEFF6, which calculates the EXAFS spectra. This method is applied to predict EXAFS spectra of sodium ions adsorbed at the MgO (100) interface at three types of surface sites—flat terrace, step edge, and step corner (kink). The calculations indicate that the experiments should only be able to detect signals from sodium ions adsorbed onto defect sites—step edges and corners. In situ spectra are computed from models of Na⁺ at the aqueous-MgO interfaces using sodium ions imbedded in finite water clusters. The strong overlap of features attributable 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 interface.