Separation of weak EXAFS signal from atomic background in Br and Rb samples

In EXAFS analysis of Rb-doped silicate glasses and of cetyl- trimethylammonium bromide (CTAB) adsorbed on carbon black, the spline approximation of the atomic background is inadequate. Instead, a residuum obtained from absorption spectra of Rb and Br ions in aqueous solution after removal of the structural signal is used. Another iteration cycle whereby the residuum from one sample in the atom-ion pair is used in analysis of the other one, gives satisfactory convergence. In this way, definite FEFF models of Rb glass and CTAB are constructed, and a better approximation for atomic absorption obtained as residual

Effects of the molecular potential on coexcitations of valence electrons in the KK-shell photoeffect of 3p3p and 4p4p elements

Photoabsorption spectra of gaseous hydrides of $3p(PH_{3},H_{2}S, HCl)$ and $4p$ elements $(GeH_{4},AsH_{3},H_{2}Se,HBr)$ are measured in the energy region within 50 eV above the $K$ edge, to study coexcitations of valenceelectrons by photoeffect in the $K$ shell. The analysis of the valence coexcitations is extended to Ar, Kr, and $SiH_4$. Relative probabilities and energies of states in the individual coexcitation channels are recovered bymodeling the spectral features with a minimal ansatz based on the features in the contiguous noble gas. Theextracted parameters are compared to the results of theoretical calculations for molecules (ORCAcode) andfree atoms (Hartree-Fock code). The experimental results confirm that the valence coexcitations in the $3p$ and $4p$ hydride molecules can be satisfactorily described by a two-step process, with the shake of the outer electronfollowing the excitation of the core electron. The total probability—relative to the $K$-edge jump—of the shake-upprocesses shows a steady decrease from 19% in Si to 14% in Cl, and from 15% in Ge to 12% in Br. Theexperimental values for Ar (12%) and Kr (10%) are in accord with the trend. The dominant contribution isthe transition to quasiatomic orbitals, in contrast with the deeper coexcitation channels in hydride moleculeswhere transition to molecular orbitals prevails

Atomic background in 4p4p elements

In the 4p elements from G-a to Kr the atomic background is determined by removing the ,structural component from Ample EXAFS spectra of chalcogenid¢ glasses, solutions and vapors. The baclq~und comprises sharp features due to ls3d and ls3p multielecVon photoexcitations, such as found in the atomic absorption spectrum of gaseous krypton. A spline background, generated by the FEFF "autobk" routine accounts properly for the long-wavelength components, but misses the sharp details of the amplitude of-l% of the K edge jump. This is sufficient to spoil the ~ parameters in EXAFS analysis in some cases

Multielectron photoexcitations in x-ray-absorption spectra of 4p4p elements

By removing the structural x-ray-absorption fine-structure signal from K-edge absorption spectra of 4pelements from Ga to Br in chalcogenide glasses and other samples, the pure atomic absorption is recovered. Its main components are shake-up and shake-off channels of the [1s3p] and [1s3d] group. The [1s3p]4p shake-up channel is observed to close off along the series as the 4p shell is progressively filled