Light-modulated XAFS spectroscopy for photo-induced structural changes

The light-modulation method was applied to XAFS spectroscopy as a useful tool for the electronic and structural studies of light-excited states. Visible light for electronic excitation was modulated by an optical chopper and concomitant change in the x-ray absorbance was extracted by a lock-in amplifier. It is essential to reduce noises arising from several sources, especially those generated by the x-ray monochromator. Light-induced changes were measured successfully by the present approach with high S/N ratios for the Fe and Co K-edge XANES and EXAFS spectra of spin transition compounds. It is also possible to construct the pure EXAFS k³χ(k) for the light-induced state from the light-modulated EXAFS difference spectrum. Detailed experiments including the signal amplitude and phase shift information revealed that the two components with different rate constants can be discriminated by changing the modulation frequency. The light-modulated XAFS method is found to be applicable to the systems with the excited-state ratio less than several percents. It has the following advantages; (1) simple experimental setup, which requires only a visible-light source, an optical chopper, a lock-in amplifier, and a conventional XAFS beamline, and (2) free from the ground-state components because the light-induced absorbance difference is directly detected.