X-ray absorption spectroscopy is one of the most powerful probes of molecular structures. So far, applications have been limited to the steady state and/or quite slowly changing systems. Recently, Professor M. Chergui (Ecole Polytechnique Federale de Lausanne, (EPFL), Switzerland) and his colleagues reported a very impressive ultrafast X-ray absorption experiment. There is a large class of Fe(II)-based molecular complexes that show two electronic states closely spaced in energy: a low-spin (LS) singlet and a high-spin (HS) quintet state. They therefore exhibit spin crossover (SCO) behavior, wherein conversion from a LS ground state to a HS excited state (or the reverse) can be induced by small changes in temperature and pressure or by light absorption. The studies were done for an aqueous solution of [FeII(bpy)3]2+, which serves as a model system for the family of Fe(II)-based SCO complexes. A 100-mm-thick free-flowing liquid jet of an aqueous solution of 50 mM [FeII(bpy)3]2+ was excited by an intense 400-nm laser pulse (115-fs pulse width, repetition rate 1 kHz), and a tunable femtosecond hard X-ray pulse from the slicing source was used to probe the system in transmission mode at 2 kHz. The X-ray flux was about 10 photons/pulse at 7 keV. The time resolution was under 250 fs. By recording the intensity of a characteristic near edge absorption spectral feature as a function of laser pump/X-ray probe time delay, the very early stages of photo excitation in Fe(II)-based complexes were clarified. For more information, see the paper, "Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex", Ch. Bressler et al., Science, 323, 489 (2009).
