Exact atomic absorption background for Rb K-edge EXAFS

The atomic absorption background can directly be determined only for very few elements. In one instance the absorption of Rb vapor above K-edge is measured: the spectrum approximates to a very high degree the atomic background for EXAFS of Rb+ ion in aqueous solution of RbNO3. It includes several sharp multielectron excitation features -resonances and absorption edges -which are poorly described by the usual polynomial or spline ansatz. Using it as a starting background approximation for the ion EXAFS, the harmonic components are reliably reconstructed: removing them from the original ion absorption spectrum, the exact Rbi atomic background is recovered. It differs from the vapor absorption spectrum only in the [1s3d] region where an additional resonance appears as a replica of the ''white line'' at the Rb+ K edge. The recovered shape of the exact atomic background can be extended by parametrisation to the neighbor elements.