Relaxation Phenomena in Electronic and Lattice Subsystems on Iron Surface during Its Ablation by Ultrashort Laser Pulses

Single-shot ablative spallation and fragmentation thresholds, as well as corresponding ablative crater depths, were measured on the surface of iron using optical interferometry, for different ultrashort laser pulse widths in the range tau(las) = 0.3-3.6 ps. The nonmonotonic dependence of these thresholds on tau(las) with the minimum near 1.2 ps (the characteristic electron-phonon relaxation time tau(ep)) represents transport and emission relaxation phenomena for nonthermalized and thermalized carriers, generated by sub-and picosecond laser pulses, respectively. Compared to rather slow spallative ablation, much faster-picosecond-fragmentation ablation of the iron surface through hydrodynamic expansion of its supercritical fluid ceased for tau(las) > tau(ep) as a result of evaporative cooling.