Band-selective measurements of electron dynamics in VO2 using femtosecond near-edge x-ray absorption

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from "laser-sliced" electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.     

Ultrafast X-ray science at the advanced light source

On May 19, 2004, 250 guests from all over the world joined the DESY research center to celebrate 40 years of research with synchrotron radiation at DESY in Hamburg. “The first measurements with the light beam from the DESY ring accelerator started in 1964. DESY was one of the seed laboratories in which the worldwide success story of research with synchrotron radiation began,” Albrecht Wagner, chairman of the DESY Board of Directors, explained in his welcoming address. “Today, more than 1,900 scientists from 31 countries come to DESY every year to carry out experiments with synchrotron radiation.”

Forty years ago, synchrotron radiation at DESY started from scratch. At the beginning of the 1960s, the radiation generated by the electrons in the bending magnets of their new 6 GeV electron synchrotron was regarded by DESY particle physicists as an unwanted, disruptive effect.     

Picosecond X-ray absorption spectroscopy of a photoinduced iron(II) spin crossover reaction in solution

In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron-nitrogen (Fe-N) bond is lengthened by 0.21+/-0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change in the metal-ligand interactions as reflected by the altered density of states of the unoccupied metal orbitals. Our results constitute the first direct measurements of the dynamic atomic and electronic structural rearrangements occurring during a photoinduced FeII spin crossover reaction in solution via picosecond X-ray absorption spectroscopy.