Observing Photochemical Transients by Ultrafast X-Ray Absorption Spectroscopy

Abstract

Accurate determination of the transient electronic structures, which drive photochemical reactions, is crucial in chemistry and biology. We report the detection of transient chemical changes on the picosecond time scale by x-ray-absorption near-edge structure of photoexcited aqueous [Ru(bpy)(3)](2+). Upon ultrashort laser pulse excitation a charge transfer excited state having a 300 ns lifetime is formed. We detect the change of oxidation state of the central Ru atom at its L3 and L2 edges, at a temporal resolution of 100 ps with the zero of time unambiguously determined.

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…They extended the capabilities of the previously developed fitting procedure MXAN [161] (based on a full multiple-scattering analysis [151]) to deal directly with differential XANES data. The method was successfully tested for the analysis of ps L 3 -edge XANES of [Ru(bpy) 3 ] 2+ in water solution, excited by a 400 nm ultrashort laser pulse [148,162].…”

“…sample concentration and photoexcitation features) with respect to the time-resolved XAS study. Moreover, in some cases when additional information on the ES geometry is available, the μ ES (E) contribution can be isolated, especially from the XANES portion of the XAS spectrum [148,149]. Once the ES signal is extracted, several codes can be employed to perform a combined fitting of the GS and ES structures in both the XANES [77,[150][151][152][153][154][155] and EXAFS [77,156] regions.…”

“…In most cases, a diluted solution optimized for photo-excitation is chosen, and the fluorescence detection scheme for XAS spectra acquisition is preferred to the transmission mode [145][146][147]. A generally adopted rule of thumb consists of maintaining the number of solute units and the number of photons of the pump laser pulse at the same order of magnitude (ca 10 13 -10 15 photons/pulse for commercial fs laser systems) to maximize the sample excitation [148]. (iii) The maximum intensity achievable for ultrashort X-ray pulses is not very high.…”

Tracking reaction dynamics in solution by pump–probe X-ray absorption spectroscopy and X-ray liquidography (solution scattering)

“…166,204,206 In this regard, metal L-edge spectroscopy based on the dipoleallowed 2p  3d transitions is more informative because they directly probe unoccupied valence orbitals with high resolution owing to smaller broadening of their spectral features by almost ten times compared with those of 1s-excitations. 147,[207][208][209] Specifically, TRXAS at transition-metal L-edges directly probes changes in ligand-field splitting of metal d orbitals and spin states via selective excitation of 2p 1/2 and 2p 3/2 electrons to vacancies in the 3d valence orbitals of metals with spin-orbit splitting. For the Fe(II) SCO processes, fast ligand-cage dilation and spin-state changes should be coupled to changes in Fe3d/N-2p hybridizations caused by weakening of the Fe-N bonds as well as associated changes in the ligand-field electron configurations.…”

Electron Transfer in Metal‐Organic Molecules. A Time Resolved EXAFS and Optical Spectroscopy Study

“…The number of time resolved x-ray absorption studies published so far is rather limited, yet rather far reaching 10,11 and include: a gas phase ultrafast x-ray absorption experiment 12 of gaseous SF 6 and a time resolved XAS experiment, with 14 ns resolution, on the transient molecular structure in the liquid phase. 11,13 The chemical shift of photo excited aqueous [Ru(bpy) 3 ] 2+ was also measured with~100 ps resolution 14 and later a similar tabletop x-ray system was used to study the structure of photo excited [Fe(CN) 6 ] 4-in water with 30 ps resolution. 15 The X-ray absorption near-edge structure associated with photoinduced Fe(II) spin crossover reac-tion has been studied using 70 ps, 7.1 KeV, tunable X-ray pulses derived from the Advanced Light Source.…”