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.
@article{arxiv.0807.1377,
title = {Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography},
author = {Ramani K. Raman and Yoshie Murooka and Chong-Yu Ruan and Teng Yang and Savas Berber and David Tománek},
journal= {arXiv preprint arXiv:0807.1377},
year = {2009}
}