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Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface ``swelling'' occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10^110. 16em{0ex}K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.
Wu et al. (Mon,) studied this question.