Correlation between early-stage expansion and spectral emission of a nanosecond laser-induced plasma from organic material

Laser-Induced Breakdown Spectroscopy (LIBS) has been used since 40 years on typical samples such as metals, alloys, rocks. Detection of organic hazards or analysis of biological compounds under atmospheric pressure with LIBS represents a new challenge. For this purpose, we need better understandings of the physico-chemical properties of the plasma in atmosphere and their influences on the LIBS signal. As a model sample of organic materials, Nylon 6-6 has been studied under nanosecond ablation at different wavelengths (1064 nm and 266 nm) and energies (from 1 to 5 mJ) in order to observe the influence of these parameters. Shadowgraph technique is used to image the plasma at its early stage of expansion (0 to 40 ns). Time-resolved LIBS signal is recorded for longer times (50 ns to 5 μs). In the infrared regime, the expansion of the plume is faster along the laser axis, perpendicular to the sample surface. On the contrary, for UV ablation, the expansion of the plume is quite isotropic. We can also observe different regimes of expansion due to Laser-Supported Detonation Waves (LSDW) above 3 mJ in the UV regime. In particular, these observations provide us ideas to understand the kinetics of the CN emission in the LIBS signal. In the IR regime, a formation of CN due to carbon present in the sample and nitrogen in the air via the reaction 2C + N₂ → 2CN can be observed. In the UV regime, the direct ablation of CN bonds is clearly seen but other effects like screening and recombination due to LSDW have also been observed.