Probing particle synthesis during femtosecond laser ablation: initial phase transition kinetics

The impulsive superheating of matter by an intense, ultrashort laser pulse drives material expansion into vacuum (ablation) and an associated formation of nanoparticles. The underlying dynamics of particle formation are complex and direct experimental probes of the rapid material evolution are essential. Femtosecond lasers coupled to modern synchrotrons offer an important new opportunity to probe ejecta dynamics on an atomic lengthscale. Here, the impulsive heating of a semiconductor (silicon) by an intense femtosecond laser pulse leads to material ejection and time-resolved photoemission spectroscopy probes rapid solidification kinetics occurring within the ejecta. Transient photoemission peak-shifts indicate that material is ejected predominantly as liquid droplets and that solidification occurs rapidly (<50 ps). The solidification time suggests that vacuum ejection leads to significantly enhanced undercooling compared to what has been obtained by more conventional quenching techniques; this may be of interest in attempts to 'trap' novel material states associated with extreme laser heating. Finally, a low fraction of vapor particles in the ejecta supports a view that the size-distribution of ejected particles is set by an initial fragmentation process rather than by vapor condensation.