Delineating geometrical and electronic structure of radionuclides in environmental systems: XAS and HR-XANES studies

Synchrotron based X-ray spectroscopic techniques, owing to the brilliant source and associated X-ray beam characteristics, have been able to provide unprecedented structural information of radionuclides in environmental system. X-ray absorption spectroscopy (XAS) profiles interference pattern of an X-ray beam induced photoelectron wave with its scattered portion to produce geometrical arrangement of the probe atom in a chemical environment/structure. Unlike X-ray diffraction, longrange periodicity is not required for structural determination making XAS suitable for heterogeneous environmental samples. Comparative position of the X-ray absorption edge, in general, helps in identifying the oxidation state of the probe atom. Many a times, probe atoms are present in multiple oxidation state or with same oxidation state in different coordination geometry. Precise determination of the oxidation state and electronic structure in such cases requires sharpened spectral features and therefore high energy-resolution (HR) of the experimental setup. Johann-type spectrometer with a set of spherically bent crystals has been found to provide a sub emission line width energy resolution in hard x-ray photon-in photon-out HR-XANES measurements. Simulation of the high resolved spectral features provides important electronic structural information of the studied systems. In the present talk, the speaker will present few applications of XAS and HR- XANES along with examples of resonant inelastic scattering (RIXS) data in environmental systems. Aim of these applications has been to investigate in vitro vs. in vivo speciation of uranium and neptunium (Np) with hyperphosphorylated proteins, ternary interaction of trivalent lanthanide/actinide at oxide surfaces in presence of humic acid, reactivity of manganese dopant of magnetite surface toward neptunium, electronic structure evolution of Np carbonate complexes vis-a-vis changing oxidation state, etc. (author)