X-ray fluorescence spectra can give information on various chemical states, including spin states such as high-spin and low-spin. Recently, Dr. G. Venko (KFKI Research Institute for Particle and Nuclear Physics, Hungary) and his colleagues published their research on light-induced spin crossover transition. They studied the spin state of aqueous [Fe(bpy)3]2+ (bpy=2,2'-bipyridine) molecules, 60 ps after an ultrashort laser pulse excitation by time-resolved X-ray fluorescence. The laser induces a low-spin to high-spin conversion, and then the system goes back to the low-spin state on a 0.1 ns timescale. For more information, see the paper, "Picosecond Time-Resolved X-Ray Emission Spectroscopy: Ultrafast Spin-State Determination in an Iron Complex", G. Vanko et al., Angew. Chem. Int. Ed. 49, 5910 (2010).
