Abstract

The mechanisms of pulsed laser-induced ablation of strongly absorbing solutions and generation of acoustic transients are described in this paper. Aqueous solutions of K2CrO4 are irradiated by a KrF excimer laser (λ = 248 nm, FWHM = 24 ns) with moderate energy density below the plasma-formation threshold. The ablation process including the vapor cavity formation and the acoustic-wave propagation is visualized by laser flash photography. The generated pressure transients are measured using a broadband piezoelectric pressure transducer. Simultaneous optical transmission detection is also carried out in order to elucidate the vapor-phase kinetics. The mechanisms of liquid ablation and acoustic-pulse generation are investigated based on the thermoelastic behavior of the liquid medium and the evaporation dynamics. The results show that the ablation is initiated by cavitation driven by the tensile component of the produced thermoelastic stress. Three distinct stages are identified in the ablation process: the initial stage with negligible surface deformation and ejection of a small amount of vapor, the second stage with violent vapor plume and bulk liquid jet ejection that is accompanied by significant surface deformation, and the final stage of large scale hydrodynamic motion. The pressure transient is generated thermoelastically at low laser fluences, but the contribution of the vapor phase expansion and / or recoil momentum exerted by the ablation plume becomes dominant at high laser fluences.

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