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/cm², lattice vibrations are observed to thermalize on a time scale of ≈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 sp³-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photoexcitation.