Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of <= 21 mJ/cm(2), lattice vibrations are observed to thermalize on a time scale of approximate to 8 ps. 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 photoexcitation.