  The complex interplay between several active degrees of freedom (charge,
lattice, orbtial and spin order) is thought to determine the electronic
properties of many oxides, but the respective role of the various contributions
is often extremely difficult to determine. Vanadium dioxide (VO$_2$) is a
particularly notorious example. Here we report on combined ultrafast electron
diffraction (UED) and infrared transmissivity experiments in which we directly
watch and separate the lattice and charge density reorganizations that are
associated with the optically-induced semiconductor-metal transition (SMT) in
VO$_2$. These studies have uncovered a previously unreported photoinduced
transition to a metastable state with the periodic lattice distortion (PLD)
characteristic of the insulator intact, but differing by a 1D rearrangement of
charge density along the octahedrally coordinated vanadium dimer chains and a
transition to metal-like mid IR optical properties. The results demonstrate
that UED is capable of following details of both lattice and electronic
structural dynamics on the ultrafast timescale.
