In situ characterization of infrared femtosecond laser ablation in geological samples. Part A: the laser induced damage
Résumé
Infrared femtosecond laser induced damage has been studied in order to determine, with analytical protocols, the processes involved in laser ablation in this regime. Transmission Electron Microscopy (TEM) coupled with Focused Ion Beam (FIB) milled cross-sections of natural ablated monazite were used. Craters were formed using N = 1 and 3 shots, E0 = 0.1 and 0.8 mJ per pulse and τ = 60 fs. Observations revealed that laser settings induce little changes in the nature and size of damaged structures. The crater bottom forms a ∼0.5 μm layer composed of melted and recrystallized monazite grains, and spherical ∼10 nm voids. The underlying sample shows lattice distortions, progressively attenuated with depth, typical of mechanical shocks (thermoelastic relaxation and plasma recoil pressure). No chemical difference appears between these two domains, excluding preferential vaporization and thus laser induced chemical fractionation. Correlations with existing molecular dynamics (MD) simulations indicate that the deep distorted lattice probably undergoes spallation whereas the upper layer rather goes through homogeneous nucleation. Nevertheless, these processes are not pushed forward enough to induce matter removal in the present conditions. In consequence, photomechanical fragmentation and vaporization, requiring higher energy density states, would rather be the main ablation mechanisms. This hypothesis was supported by an additional study focused on the laser produced aerosols. Further links to LA-ICP-MS measurements can then be developed.