In situ characterization of infra red femtosecond laser ablation in geological samples. Part B: the laser induced particles

Abstract

The analytical study of infra red femtosecond laser induced particles has been performed using Transmission Electron Microscopy (TEM) and Low Pressure Impaction (ELPI). Various natural and synthetic matrices have been tested: monazite (phosphate), zircon (silicate), NIST610 (glass), spinel (oxide), quartz (silicate structure), silicon (semiconductor) and Nordic gold (metallic alloy). Three types of particles are systematically observed: very rare large round spherical particles (d_p ≈ 1 μm) whose composition is close to the initial sample, spherical particles of smaller size (d_p ≤ 250 nm) and agglomerates of d_p ≈ 10 nm particles. Chemical compositions of the latter two are complementary with respect to the ablated sample. Isolated occurrence of hydrodynamic sputtering may explain the creation of rare large droplets. Other particles are probably generated from the irradiated matter in the supercritical state during the cooling process and plasma expansion. A recent model provides a strong basis to describe vapour to particle conversion and further condensation/coalescence processes for simple systems (single component). Additional assumptions must be included to apply the model to our observations of complex multi-elemental systems. A qualitative interpretation may be proposed on the basis of fractionated condensation/coalescence and further agglomeration of particles, depending on plasma pressure and the ablated elements properties (mainly density and melting period) as well as the thermal evolution of the plume. This interpretation is discussed and validated for each sample type. Previous results concerning ablation mechanisms using the same system are included in our model. The generation of particles from a vapour phase confirms that vaporization is the main ablation mechanism in the femtosecond regime. Moreover, the possible presence of molecular sized clusters in the initial plasma, which can accelerate the nucleation process, strongly suggests that fragmentation is the secondary ablation mechanism. Finally, the present study is an experimental validation for recent femtosecond laser ablation simulation, and it brings new insights for interpreting particles generation processes for complex systems. Correlations between laser ablation ICP-MS measurements must now be made with the present results.