Theory for Laser Induced Ultrafast Phase Transitions in Carbon

The response of carbon to femtosecond laser pulses of arbitrary form,

different durations and different intensities is studied

theoretically. We perform molecular dynamics simulations based on an

electronic Hamiltonian. We include in our model the theoretical

description of the pulse form, the electron thermalization and

diffusion effects explicitly. We apply our method to diamond and

graphite. In the case of diamond, we show that a femtosecond laser

pulse induces a nonequilibrium transition to graphite, which takes

place for a wide range of pulse durations and intensities. This

ultrafast collective motion of the atoms occurs within a time-scale

shorter than 100 fs. The microscopic mechanisms for laser ablation of

graphite are also analyzed. Two distinct ablation thresholds can be

observed. We compare our results with experiment.