Quantum study of laser-induced initial activation of graphite-to-diamond conversion

Recently (Science 2009, 325, 181), femtosecond-resolved electron energy loss spectroscopy (FEELS) was used to map the structural changes of graphite upon laser irradiation, revealing the change from sp(2) to sp(3), i.e., diamond-like, hybridization. With a laser excitation energy of 2.39 eV and a fluence of 1.5 mJ/cm(2), the most pronounced changes were observed in approximately 180 fs, a time similar to the temporal resolution of the technique. The presence of the laser field turns the electronic wavefunction into a wavepacket whose quantum dynamics governs the onset of the structural rearrangement. Density functional theory calculations with a quantum propagator that include the laser field show that the charge density of graphite expands between the layers in an ultrafast process of the order of approximately 10 fs. Calculations as a function of the field/fluence further show different values of the maximum bond order reached at the stationary state. The experimentally used value is at the crossover between two regimes. It is tempting to associate the second regime with the electron organization necessary to achieve ablation or melting. The application of the model demonstrates its potential for examining the dynamical nature of the charge density and chemical bonding as it forms.