  We use ultrafast electron crystallography to study structural changes induced
in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2,
lattice vibrations are observed to thermalize on a time scale of ~8ps. At
higher fluences approaching the damage threshold, lattice vibration amplitudes
saturate. Following a marked initial contraction, graphite is driven
nonthermally into a transient state with sp^3-like character, forming
interlayer bonds. Using ab initio density functional calculations, we trace the
governing mechanism back to electronic structure changes following the
photo-excitation.
