Estimate of the departure from excitation equilibrium of a laser-induced plasma obtained in Ar-H mixtures

International audienceLaser-induced plasmas can be used to determine the multi-elemental composition of solid, liquid or gaseoussamples. Using a pulsed laser beam focused on the sample, the resulting absorption leads to temperature increaseof several 10,000 K, therefore to a neutral phase to plasma transition. During the relaxation phase over which therecombination takes place, a continuum then lines radiation is emitted. Its analysis based on Optical EmissionSpectroscopy (OES) may lead to the composition of the plasma, therefore to that of the sample if fundamentalassumptions are fulfilled [1]. The most restrictive one is the achievement of the Local Thermodynamic Equilibrium(LTE). Indeed, since only OES is used, the excited states number density is determined. Then, the ground statesnumber density is obtained if LTE is observed and if electron temperature T_e is known, the electron density n_ebeing enough high. The characterization of LTE is therefore one of the key points of this analysis method, alsoknown as Laser-Induced Breakdown Spectroscopy (LIBS) [2].In this context, we have focused our attention on Argon-Hydrogen mixtures at atmospheric pressure. Usingnanosecond laser sources (532 nm, 6 ns, 40 mJ), laser-induced plasmas have been produced with variousHydrogen mole fractions. The spectra have been recorded and calibrated according to the gate delay over differentspectral regions. Large spectral windows have been used (around 60 nm) allowing a sufficiently high number ofArgon lines. Using relevant radiation database, the spectra have been calculated assuming LTE by integration ofthe Radiative Transfer Equation and compared to the experimental ones for each spectral window. The temporalevolution of the electron density and temperature are then derived. The comparison of these evolutions from onespectral window to the other allows to estimate the uncertainty of the (n_e, T_e) determination and therefore todiscuss the relevance of the LTE assumption