Ultraviolet photochemical reaction of [Fe(III)(C2O4)3]3− in aqueous solutions studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser

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−.

Time-resolved electron spectroscopy for chemical analysis of photodissociation: Photoelectron spectra of Fe(CO)5, Fe(CO)4, and Fe(CO)3

“…30,31,[39][40][41][42] Our study extends the demonstrated capabilities of time-resolved photoelectron spectroscopy with optical probe pulses, 18,43 and it shows how x-ray photoelectron spectroscopy complements other x-ray methods for the investigation of molecular dynamics. [44][45][46][47][48][49][50]…”

Perspective: Opportunities for ultrafast science at SwissFEL

“…More recently, Collet and co-workers extended these studies to SCO molecular crystals 137 . At SACLA, studies were performed on an Fe-complex, [Fe III (C 2 O 4 ) 3 ] 3+ in solution, 138 delivering insight into the photoinduced electronic and structural changes in the system, while the photoinduced charge carrier dynamics was investigated in the case of WO 3 nanoparticles, 139 and of TiO 2 140 . The latter studies repeated those by Santomauro et al 125 using the slicing scheme, but this time with a greatly enhanced S/N, which allowed them to detect a factor of 3 between the electron trapping time (∼100 fs) and the ensuing structural relaxation of the trap (∼300 fs).…”

Time-resolved X-ray spectroscopies of chemical systems: New perspectives

“…However, they only recorded time traces using a fixed probe energy at the Fe K-edge, which makes the conclusions not straightforward. At SACLA, studies were performed on an Fe-complex, [Fe III (C 2 O 4 ) 3 ] 3+ in solution, 85 delivering insight into the photoinduced electronic and structural changes in the system, while the photoinduced charge carrier dynamics was investigated in the case of WO 3 nanoparticles 86 . These studies are widening the scope of systems one can investigate with fs XAS at XFELs.…”