Ablation of metals with picosecond laser pulses: Evidence of long-lived non-equilibrium surface states

Abstract

Experiments on laser ablation of metals in air, in vacuum, and in similar irradiation conditions, revealed that the ablation thresholds in air are up to three times lower than those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient non-equilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the rate of energy transfer from the bulk to the surface layer to build the high-energy tail, which determines the lifetime of this non-equilibrium state, exceeds other characteristic timescales such as the surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this non-equilibrium surface state allowing thermal evaporation to proceed before the surface cools. It was experimentally observed that the difference between the ablation depth in vacuum and that in air disappears at the laser fluencies 2–3 times in excess of the vacuum threshold value. The material removal at this level of the deposited energy density attains the features of the non-equilibrium ablation similar for both cases. We find, therefore, that the threshold in vacuum corresponds to non-equilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.