The light-modulation method was applied to XAFS spectroscopy as a useful tool for electronic and structural studies of light-excited states. The visible light for electronic excitation was modulated by an optical chopper and the concomitant change in the X-ray absorbance was extracted by a lock-in amplifier. It is essential to reduce any noise arising from several sources, especially those generated by the X-ray monochromator. Fe and Co K-edge XANES and EXAFS spectra were measured using the present approach for spin transition compounds. The light-modulated XAFS method is found to be applicable to systems with an excited-state ratio of less than several percent. Detailed discussion is also given concerning quantitative measurements, including the signal amplitude and phase shift information.

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A reaction between N2 and trans-Ru(II)Cl2L (L=2,5,9,12-tetramethyl-2.5,9,12-tetraazatridecane) in aqueous solutions has been investigated by XAFS spectroscopy. Ru K-edge EXAFS clearly revealed that there exists an N2-bridged binuclear complex and that the Ru-Ru distance is 4.96 A with the coordination number of 0.5. The Ru K-edge energies of the N2 complexes are closer to that of Ru(III)Cl2L+ rather than that of Ru(II)Cl2L, indicating back donation from the Ru dpi to the N2 pi* orbitals.

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