TSUKUBA, JAPAN - Scientists at KEK, led by Shunsuke Nozawa, along with other scientists from at Tokyo Institute of Technology, Institute for Molecular Science, and Japan Science and Technology Agency have directly probed a spin state dynamics coupled with electronic and structural modifications by picosecond time-resolved XAFS.

Controlling the dynamic magnetic properties has attached a lot of research interest in resent years. However, in disordered magnetic systems, it is particularly hard to study the spin dynamics directly with conventional optical methods. In this study, we report the first direct observation of the transient spin-state in the disordered system by time-resolved XAFS using a pulsed hard x-ray beam. By detecting the evolution of the inner-atomic transition of the divalent iron ion, the transient spin-crossover transition from ¹A₁ low spin state to ⁵T₂ high spin state has been directly observed on the picosecond timescale. Furthermore, the time-resolved investigation in the x-ray absorption near-edge structure and the extended x-ray absorption fine structure provided dynamic information on the altered electronic states of the iron as well as information on the structural change in the Fe-N bond elongation of 0.017 nm resulting from the formation of the high-spin excited state. This unique experimental technique probes the excited states and provides a new level of understanding of ultrafast reactions in spin crossover transitions as well as in magnetic photocatalysts, nanomagnets, biological magnetic systems, and photo-induced phase transition.

This work was published online in the Journal of the American Chemical Society online publication on December 18, 2009: Shunsuke Nozawa, Tokushi Sato, Matthieu Chollet, Kouhei Ichiyanagi, Ayana Tomita, Hiroshi Fujii, Shin-ichi Adachi and Shin-ya Koshihara, "Direct Probing of Spin State Dynamics Coupled with Electronic and Structural Modifications by Picosecond Time-Resolved XAFS ".
      http://pubs.acs.org/doi/full/10.1021/ja907460b

Figure 1  :  Scientists at KEK, led by Shunsuke Nozawa, along with other scientists from at Tokyo Institute of Technology, Institute for Molecular Science, and Japan Science and Technology Agency have directly probed a spin state dynamics coupled with electronic and structural modifications by picosecond time-resolved XAFS.

Figure 2  :  Fe K-XAFS of [FeII(phen)3]+2 in the aqueous solution (solid line) and the transient difference 50 ps after the laser excitation (square). (Courtesy: High Energy Accelerator Research Organization).

Figure 3  :  Photoinduced ultrafast spin crossover reaction accompanied by the structural change.

Figure 4  :  Dynamics of the molecular structure attributed to the photoinduced spin crossover transition.