Photon Factory Activity Report 2022 #40 (2023)

BL-13A/2021G047, 2021PF-S002, 2018S2-003

Operando Time-Resolved Soft X-ray Absorption Spectroscopy for Photoexcitation
Processes of Metal Complexes in Solutions

Fumitoshi Kumaki,1 Masanari Nagasaka,2,3,* Ryo Fukaya,1,4 Yasuaki Okano,2 Shohei Yamashita,1,4Shunsuke Nozawa,1,4 Shin-ichi Adachi,1,4 Jun-ichi Adachi1,4,*1
Department of Materials Structure Science, School of High Energy Accelerator Science,SOKENDAI, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
2Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan3
Department of Functional Molecular Science, School of Physical Sciences, SOKENDAI, Myodaiji,
Okazaki 444-8585, Japan
4
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research
Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
Operando time-resolved soft X-ray absorption
spectroscopy (TR-SXAS) has the advantage to investigate
photochemical reactions of metal complexes in solutions
since soft X-ray region below 2 keV include K-edges of C,
N, and O and L-edges of 3d transition metals. The TRSXAS experiment at the Fe L-edge has been applied to thespin-state conversion of the iron polypyridyl complex inaqueous solution after the photoexcitation [1]. On the otherhand, there are few reports on the TR-SXAS measurementsof the ligands in metal complexes using the N K-edge
compared to that of the central metal using the Fe L-edge.
Recently, we have developed the liquid cell for soft Xray absorption spectroscopy (XAS) in transmission modeby controlling the liquid thickness precisely from 20 to2000 nm [2]. In this study, we have developed the TRSXAS system including our transmission-type liquid cellby synchronizing the soft X-ray probe pulses from thesynchrotron radiation with the laser pump pulses and havemeasured the photoexcitation process of ironphenanthroline ([Fe(phen)3]2+) aqueous solutions from theligand side by using N K-edge XAS [3].
2 Experiment
The TR-SXAS system was developed at the soft X-ray
beamline BL-13A. The experiments were performed
during the hybrid mode of the PF ring. The laser pumppulses (515 nm, 290 fs) from the second harmonic
generation of a Yb:KGW crystal laser (Pharos, LightConversion) were focused on the liquid cell by using acenter-holed aluminum mirror. Since soft X-rays passthrough the hole of the mirror, the laser pulses were almost
coaxially introduced to the liquid cell with soft X-ray
pulses. A trigger clock delay system and a frequency
synchronization module were used for the synchronizationof the laser pump pulses with soft X-ray probe pulses. In
the liquid cell, the liquid layer was sandwiched between
the gold-coated SiC and the bare SiC membranes. XAS
spectra were measured by detecting soft X-ray intensities
with an avalanche photodiode detector. An aluminum filter
was installed in the front of the detector for removing thelaser beam.

3 Results and Discussion
Figure 1(a) shows the N K-edge XAS spectra of[Fe(phen)3]2+ aqueous solutions in the ground (low spin)
and photoexcited (high spin) states with the 60 ps delay
time of soft X-ray probe pulses from the laser pump pulses.
The C=N π* peak in the high spin state is shifted to thelower photon energy compared to that in the low spin state.
Figure 1(b) shows the difference spectrum of the high spinstate from the low spin state obtained from the XAS spectra
shown in Fig. 1(a). The difference spectrum clearly shows
the increase of the peak at the lower energy side and thedecrease of that at the higher energy side.

Fig. 1: (a) N K-edge XAS spectra of [Fe(phen)3]2+
aqueous solutions in the ground state (laser off) and thephotoexcitation state (laser on). (b) The difference
spectrum of the photoexcitation state relative to theground state obtained from the XAS spectra shown in (a).

Photon Factory Activity Report 2022 #40 (2023)

Figure 2 shows the temporal evolution of the peakintensity difference at the energy position of α (398.97 eV),
where the delay time of the soft X-ray probe pulses wasscanned relative to the laser pump pulses. The time
constant from the N K-edge XAS is 550 ± 12 ps, which is
close to that obtained by the Fe K-edge XAS (690 ps) [4].
The slight deviations in the time constants might be causedby the differences in the absorption edges or the different
thermal environments.
Since liquid cells are in an atmospheric condition andcan control the reaction condition such as liquid
temperature, electrode potential, etc., the present TRSXAS system would be applied to various chemical andbiological phenomena in solutions, such as photocatalytic
reactions,photoelectrochemical
reactions,andphotochemical reactions of antenna pigment-protein
complexes in photosystems.

Fig. 2: The temporal evolution of the peak intensity
difference at the energy position of α (398.97 eV) shownin Fig. 1(b).
References[1] N. Huse et al., J. Am. Chem. Soc. 132, 6809 (2010).
[2] M. Nagasaka and N. Kosugi, Chem. Lett. 50, 956(2021).
[3] F. Kumaki et al., J. Chem. Phys. 158, 104201 (2023).
[4] S. Nozawa et al., J. Am. Chem. Soc. 132, 61 (2010).
* nagasaka@ims.ac.jp* jun-ichi.adachi@kek.jp