High-resolution X-ray spectroscopy of rare events: a different look at local structure and chemistry

Abstract

The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal Kbeta fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG- are presented. Weaker features of the Kbeta spectrum resulting from valence-level and 'interatomic' ligand to metal transitions contain detailed information on the ligand- atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard (approximately 10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.

Figure 1

Mn K fluorescence spectrum of Mn^IIO. Inset: schematic transition scheme.

Figure 2

K fluorescence features for Mn^IIO (solid lines), Mn^IVO₂ (dashed lines) and KMn^VIIO₄ (circles) and corresponding shifts of Kα₁ (squares), Kβ_1,3 (diamond) and Kβ_2,5 (triangles).

Figure 3

First moment of the Mn Kβ_1,3 peak (left) and the XANES inflection-point energy (right) of photosystem II as a function of flash number. The S₁ state corresponds to zero flashes.

Figure 4

Kβ_1,3 spectrum of crystallites from SG-1 spores after 11.5 h in 10 mM Mn^IICl₂ solution (circles). Top: best fit using only Mn^II and Mn^IV. Bottom: best fit allowing for Mn^II, Mn^III and Mn^IV.

Figure 5

Kβ″ region of two sets of models, where F (solid lines) is interchanged with Cl (dashed lines). Top insets: local Mn structure for each set.

Figure 6

X-ray Raman spectra of (top to bottom): paraffin, coronene and asphaltene. Inset: measured energy resolution of the elastic peak, resulting from convolution of the incident-beam resolution and the analyzer resolution.