Theory for laser-induced ultrafast phase transitions in carbon

The response of carbon to femtosecond laser pulses of arbitrary form, diffe rent durations, and different intensities is studied theoretically. We perf orm molecular dynamics simulations based on a microscopic electronic Hamilt onian. We include in our model the theoretical description of the pulse for m, the electron thermalization, and diffusion effects explicitly. We apply our method to diamond and C-60 crystals. For the diamond case, 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. Thi s ultrafast collective motion of the atoms occurs within a time scale short er than 100 fs. The laser-induced melting of a C-60 crystal under pressure is also analyzed. In this case, an ultrafast melting of the system occurs. We discuss the mechanisms underlying these nonequilibrium phase transitions .