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 ~< 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. 
Ramani K. Raman, Yoshie Murooka, Chong-Yu Ruan, Teng Yang, Savas   Berber, David Tom\'anek