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 τ_las = 0.3–3.6 ps. The nonmonotonic dependence of these thresholds on τlas with the minimum near 1.2 ps (the characteristic electron-phonon relaxation time τ _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 τ_las > τ _ep as a result of evaporative cooling.