Molybdenum X-ray absorption edges from 200 to 20,000eV: The benefits of soft X-ray spectroscopy for chemical speciation

Abstract

We have surveyed the chemical utility of the near-edge structure of molybdenum x-ray absorption edges from the hard x-ray K-edge at 20,000 eV down to the soft x-ray M 4,5 -edges at ~230 eV. We compared, for each edge, the spectra of two tetrahedral anions, MoO 4 and MoS 4 2-. We used three criteria for assessing near-edge structure of each edge: (i) the ratio of the observed chemical shift between MoO 4 2-and MoS 4 2-and the linewidth, (ii) the chemical information from analysis of the near-edge structure and (iii) the ease of measurement using fluorescence detection. Not surprisingly, the K-edge was by far the easiest to measure, but it contained the least information. The L 2,3 -edges, although harder to measure, had benefits with regard to selection rules and chemical speciation in that they had both a greater chemical shift as well as detailed lineshapes which could be theoretically analyzed in terms of Mo ligand field, symmetry, and covalency. The soft x-ray M 2,3 -edges were perhaps the least useful, in that they were difficult to measure using fluorescence detection and had very similar information content to the corresponding L 2,3 -edges.Interestingly, the soft x-ray, low energy (~230 eV) M 4,5 -edges had greatest potential chemical sensitivity and using our high resolution superconducting tunnel junction (STJ) fluorescence detector they appear to be straightforward to measure. The spectra were amenable to analysis using both the TT-multiplet approach and FEFF. The results using FEFF indicate that the sharp near-edge peaks arise from 3d → 5p transitions, while the broad edge structure has predominately 3d → 4f character. A proper understanding of the dependence of these soft x-ray spectra on ligand field and site geometry is necessary before a complete assessment of the utility of the Mo M 4,5 -* Corresponding author: sjgeorge@lbl.gov, +1 510 486 6094, FAX +1 510 486 5664. Supporting Information(1) Crystallographic data of Na 2 MoS 4 in CIF format.

Kinetics of Molybdenum Adsorption and Desorption in Soils

“…1) is attributed to dipolar 1s → 4d electronic transitions (Farges et al, 2006). Previous studies found that the enhanced pre‐edge feature is indicative of Mo=O bonds in the Mo tetrahedral configuration (George et al, 2009; Wichard et al, 2009). In this study, a similar pre‐edge feature can be found in the 20 mM Na 2 MoO 4 solution sample, indicating that the Mo coordination environment is predominantly tetrahedral under such reaction conditions (Fig.…”

Accurate X-ray Absorption Spectra near L- and M-Edges from Relativistic Four-Component Damped Response Time-Dependent Density Functional Theory

“… Calculated 4c-DR-TDDFT (VDZ/aVDZ) XAS spectra near molybdenum M 4,5 -edges of MoS 4 2– using different xc functionals and their comparison with experimental results. 120 To ease comparison, the spectra were shifted so that the M 5 -line maxima are centered on the experimental value. (a) Theory and (b) experiment (reprinted from ref ( 120 ) with permission from Elsevier).…”

Characterization of foliar manganese (Mn) in Mn (hyper)accumulators using X‐ray absorption spectroscopy

“…In addition, the degree of Mn complexation varies enormously with pH, highlighting the need to base hypotheses on a range of information. Soft X‐ray (L‐edge) absorption spectroscopy techniques, which are potentially more sensitive to the chemical environment around the absorbing ion, may also serve to better resolve the ligands binding foliar Mn (George et al. , 2009).…”