Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

NW2A/2003G294

Time-Resolved XAFS Measurements for Laser-Induced Excited Stateof Tris(2,2’-bipyridine)ruthenium(II) Complex
Yasuhiro INADA*, Akane SUZUKI, Masaharu NOMURA
KEK-PF, Tsukuba 305-0801, Japan
Introduction
The electronic states of ruthenium(II) complexes withdiimine ligands are widely investigated in relation to their
good performance as the light sensitizer for the reductive
catalysis system and the tris(2,2’-bipyridine)ruthenium(II)2+complex ([Ru(bpy)3] ) has been a recent subject of thetime-resolved X-ray absorption spectroscopy [1]. In thisstudy, the time-resolved XAFS measurements in the orderof ns have been carried out using a single-bunch property
of PF-AR to investigate not only the electronic state but
also the local structure around the Ru center.
Experiments
The time-resolved XAFS spectra were measured at
NW2A by using the X-ray pulse selector, by which the Xray pulse frequency was reduced to 946 Hz. The excited
2+state of [Ru(bpy)3] was generated by a pulse YAG laser(2 ns pulse width, 532 nm). Because all X-ray pulses aresynchronized with the YAG laser pulses, the X-ray
detector does not need to have the time resolution. Thetime delay of an X-ray pulse after the YAG laser pulse
was varied from 5 ns to 100 µs. A conventional XAFS
scan was performed at a constant time delay and repeatedby changing it. The incident and transmitted X-ray
intensities were measured using the ionization chambers.
Results and Discussion
As seen in Figure 1, in which the normalized XANES
spectra are shown at 5, 200, and 500 ns after theexcitation, the absorption edge is slightly shifted to lower
energy. The time-course plot of X-ray absorbance at
22.130 keV given in Figure 1 indicates that the energy
shift is completed within 5 ns and that the shifted edge isrecovered to the original energy in the order of a few 100
ns. This time scale is almost consistent with the life time2+of ca. 300 ns for the triplet MLCT state of [Ru(bpy)3] . Itshould be however noted that the shift direction of theabsorption edge may not be correct because the spectra
are numerically normalized so as the absorbance jump tobecome 1.
The Fourier transform (FT) magnitudes of the EXAFS
data are depicted in Figure 2. The local structure aroundthe Ru center at 500 ns after the laser irradiation is almost
comparable to that of the ground state. For the triplet
MLCT state, it is found that the peak position observed at
ca. 1.6 Å (phase-shift uncorrected) assigned to the Ru–N
interaction is slightly shifted to the lower distance
together with the decrease in intensity.
The changes observed in Figures 1 and 2 are very small
and thus a great attention should be paid for the structure

analyses. In this study, the size of excitation laser was
about 5 mmφ at the sample position while that of X-ray
2
beam was ca. 0.5 × 0.3 mm . It is very important to focus
the laser light to increase the content of the excited state,leading to an increased reliability of the structure analysis.
Acknowledgment
This study was carried out under the collaboration withProf. S. Koshihara (Tokyo Institute of Technology) andhis ERATO project members.
References[1] M. Saes, C. Bressler, R. Abela, D. Grolimund, S. L.
Johnson, P. A. Heimann, M. Chergui, Phys. Rev. Lett. 90,047403 (2003).
* yasuhiro.inada@kek.jp

Figure 1: Normalized XANES spectra of laser-excited
2+[Ru(bpy)3] at 5 (red), 200 (sky blue), and 500 (blue)
ns after the 532 nm YAG laser pulse. The change in Xray absorbance at 22.130 keV is given in the inset.

Figure 2: Fourier transform magnitudes of the time2+resolved EXAFS data for [Ru(bpy)3] . The meaningsof the line indication are the same with Figure 1.

Users' Report45