Light-induced spin crossover probed by ultrafast optical and X-ray spectroscopies

The photoinduced energy and structural relaxation of aq. iron(II)-tris-bipyridine ([FeII(bpy)3]2+), upon excitation into its singlet metal-to-ligand charge transfer (1MLCT) band, and the population of its short-lived (?0.6 ns) high-spin excited state have been characterized by combined ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence up-conversion reveals a very short lived 1MLCT emission band, which decays in ?20 fs by intersystem crossing to a 3MLCT, whose very weak emission is also obsd. Transient absorption studies show that the latter decays in