Band-Selective Measurements of Electron Dynamics inVO2Using 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 ${\mathrm{VO}}_2$. Symmetry- and element-specific x-ray absorption from $\mathrm{V}2p$ and $\mathrm{O}1s$ core levels (near 500 eV) separately measures the filling dynamics of differently hybridized $\mathrm{V}3d\mathrm{\text{−}}\mathrm{O}2p$ electronic bands near the Fermi level.