Abstract
Time-resolved X-ray absorption spectroscopy was performed for aqueous ammonium iron(III) oxalate trihydrate solutions using an X-ray free electron laser and a synchronized ultraviolet laser. The spectral and time resolutions of the experiment were 1.3 eV and 200 fs, respectively. A femtosecond 268 nm pulse was employed to excite [Fe(III)(C2O4)3]3− in solution from the high-spin ground electronic state to ligand-to-metal charge transfer state(s), and the subsequent dynamics were studied by observing the time-evolution of the X-ray absorption spectrum near the Fe K-edge. Upon 268 nm photoexcitation, the Fe K-edge underwent a red-shift by more than 4 eV within 140 fs; however, the magnitude of the redshift subsequently diminished within 3 ps. The Fe K-edge of the photoproduct remained lower in energy than that of [Fe(III)(C2O4)3]3−. The observed red-shift of the Fe K-edge and the spectral feature of the product indicate that Fe(III) is upon excitation immediately photoreduced to Fe(II), followed by ligand dissociation from Fe(II). Based on a comparison of the X-ray absorption spectra with density functional theory calculations, we propose that the dissociation proceeds in two steps, forming first [(CO2•)Fe(II)(C2O4)2]3− and subsequently [Fe(II)(C2O4)2]2−.
Highlights
Action spectroscopy requires the narrow spectral width provided by monochromatized X-rays, e.g., that of ca. 1 eV, which is considerably smaller than that of the direct X-ray beam employed in the dual beam multiplex detection method
We investigate the ultrafast dynamics of a photochemical reaction of [Fe(III)(C2O4)3]3À upon 268 nm photoexcitation from the viewpoint of near-edge X-ray absorption fine structure (NEXAFS) at the Fe K-edge
We extracted three spectral components, as shown in Figure 5(b), in which the black line indicates the X-ray absorption spectrum of the parent and other colors indicate the spectra of transient species
Summary
We reported TRXAS of aqueous solutions carried out with a dual beam multiplex detection method using SACLA. In this method, a transmission grating splits a hard X-ray pulse (FWHM 1⁄4 30 eV) into two intense beams, one of which is transmitted through a sample solution and used as a signal beam.. Action spectroscopy, in which the X-ray photon energy is scanned while monitoring the emission intensity of the X-rays, photoelectrons, or photoions, has been proven to achieve higher sensitivity than X-ray direct absorption spectroscopy It is not immediately clear whether action spectroscopy is superior to direct absorption spectroscopy using the dual beam multiplex detection method because the intensity fluctuations of the X-ray pulses are enhanced by monochromatization of the radiation from the SASE (self-amplification of spontaneous radiation) source.
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