Direct probing of spin state dynamics coupled with electronic and structural modifications by picosecond time-resolved XAFS.

The first direct observation of the transient spin-state in a disordered magnetic system with time-resolved XAFS is reported. By observing the evolution of the Fe(II) 1s-3d transition, the spin crossover transition from the (1)A(1) low spin state to (5)T(2) high spin state has been directly observed on a picosecond time scale. Moreover, observation of the transient spin state with time-resolved XAFS allows for the investigation of the variations in the electronic state and molecular structure. This unique experimental technique probes the excited states involved in the ultrafast photoinduced reactions in disordered magnetic systems.