The complex interplay among several active degrees of freedom (charge, lattice, orbital, and spin) is thought to determine the electronic properties of many oxides. We report on combined ultrafast electron diffraction and infrared transmissivity experiments in which we directly monitored and separated the lattice and charge density reorganizations that are associated with the optically induced semiconductor-metal transition in vanadium dioxide (VO2). By photoexciting the monoclinic semiconducting phase, we were able to induce a transition to a metastable state that retained the periodic lattice distortion characteristic of the semiconductor but also acquired metal-like mid-infrared optical properties. Our results demonstrate that ultrafast electron diffraction is capable of following details of both lattice and electronic structural dynamics on the ultrafast time scale.

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Radio-frequency compressed ultrafast electron diffraction has been used to probe the coherent and incoherent coupling of impulsive electronic excitation at 1.55 eV (800 nm) to optical and acoustic phonon modes directly from the perspective of the lattice degrees of freedom. A biexponential suppression of diffracted intensity due to relaxation of the electronic system into incoherent phonons is observed, with the 250 fs fast contribution dominated by coupling to the E_{2g2} optical phonon mode at the Γ point (Γ-E_{2g2}) and A_{1}^{'} optical phonon mode at the K point (K-A_{1}^{'}). Both modes have Kohn anomalies at these points in the Brillouin zone. The result is a unique nonequilibrium state with the electron subsystem in thermal equilibrium with only a very small subset of the lattice degrees of freedom within 500 fs following photoexcitation. This state relaxes through further electron-phonon and phonon-phonon pathways on the 6.5 ps time scale. In addition, electronic excitation leads to both in-plane and out-of-plane coherent lattice responses in graphite whose character we are able to fully determine based on spot positions and intensity modulations in the femtosecond electron diffraction data. The in-plane motion is specifically a Γ point shearing mode of the graphene planes and the out-of-plane motion an acoustic breathing mode response of the film.

The temporal evolution of the charge density distribution in femtosecond laser produced electron pulses was studied using electron-laser pulse cross correlation techniques and compared to analytical predictions and simulations. The influence of propagation time and weak magnetic focusing were both investigated. Our results show that ultrashort electron pulses develop a relatively uniform internal charge density as they propagate, which is in good agreement with analytical predictions, and that weakly focusing an ultrashort electron pulse results in an increased internal charge density towards the leading edge of the pulse.

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