Simultaneous XAFS measurements of multiple samples.

A four-channel ionization chamber has been designed, constructed and tested. This ionization chamber allows X-ray absorption spectra to be collected in transmission from up to four samples simultaneously. This results in spectra that are free of systematic uncertainty in relative energy alignment introduced by scan-to-scan stability of the monochromator or of numerical uncertainty associated with a post-processing alignment algorithm, allowing, in a single shot, an absolute measure of edge shift between four samples of different valence. As four samples can be measured in parallel, the time expended over the course of an experiment to cycle the measurement environment between its rest state and the measurement condition is substantially reduced. The ionization chamber is simple in design and could be implemented at virtually any XAFS beamline with a horizontal fan of radiation such as that provided by a bend magnet or wiggler.

Effect of hydrogen adsorption on the x-ray absorption spectra of small Pt clusters.

Hydrogen adsorption on Pt(6)H(n) clusters leads to striking changes in the Pt L(2,3) x-ray absorption spectra. These effects are interpreted using a self-consistent real space Green's function approach. Calculations show that they are due largely to changes in the atomic background contribution to x-ray absorption (i.e., atomic x-ray absorption fine structure) and to reduced Pt-Pt scattering at the edge, while Pt-H multiple scattering is relatively weak. The origin of both effects is traced to the change in the local Pt potential due to Pt-H bonding.

Pt L-edge XANES as a probe of Pt clusters.

The sensitivity of Pt L-edge XANES to local geometric and electronic structure in various Pt(n) clusters is investigated using the ab initio self-consistent FEFF8 code. Calculations based on FEFF8 are found to be in good agreement with experiment. For pure Pt clusters the XANES can distinguish between 2- and 3-dimensional clusters. Self-consistency is important in determining the variation of XANES with cluster size. The effect of a support is also studied. In Pt-Cl(x) clusters the presence of a Cl-Pt bond leads to a "hybridization peak," i.e., a peak in the Cl d-density of states (d-DOS) mixed with Pt d-states. For Pt-H clusters hydrogen addition is well correlated with the growth of a broad shoulder on the white line. This change is attributed largely to AXAFS, i.e., to a corresponding change in the atomic background absorption.