Band-Selective Measurements of Electron Dynamics in ${\mathrm{VO}}_2$ 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 ${\mathrm{VO}}_2$. 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.

Figure 1

Top: electronic structure of ${\mathrm{VO}}_2$ in the equilibrium insulating and metallic phase. Bottom: differential infrared absorption measurements at various wavelengths in the optical band gap region, following excitation with 60 fs pulses at 800 nm.

Figure 2

Top: static soft x-ray absorption of ${\mathrm{VO}}_2$ at the $\mathrm{V}2p$ and $\mathrm{O}1s$ absorption resonances. The high-resolution spectra were collected at beam line 6.3.2 of the ALS with 100 meV resolution. High-resolution x-ray absorption changes across $T_c$ are reported. Bottom: experimental setup for femtosecond XAS experiments.

Figure 3

(a) Femtosecond XAS measurements at the $\mathrm{V}2p$ and $\mathrm{O}1s$ resonances. Vertical error bars represent one standard deviation of the photon counting distribution. Horizontal error bars reflect the long-term drift in the delay between laser pump and x-ray probe pulses measured via cross correlation with visible femtosecond synchrotron pulses [7]. Solid lines are guides to the eye. (b) Illustration of the photo-induced rearrangement of electronic population between the $3d$-symmetry valence band and the mixed $3d_{\pi }$ orbitals. (1 left) Photoexcitation results in (2 middle) a collapse of the band gap, the creation of a nonequilibirum electron distribution, and an increase (decrease) of the $V2p$ ($O1s$) binding energies due to transient charge transfer from ${\mathrm{V}}^{4+}$ anions to ${\mathrm{O}}^{2-}$ anions. (3 right) Transient core-level shifts recover as the hot electron distribution thermalizes with the lattice, reestablishing the valency of the equilibrium metal.