Local thermodynamic equilibrium in a laser-induced plasma evidenced by blackbody radiation

Abstract

We show that the plasma produced by laser ablation of solid materials in specific conditions has an emission spectrum that is characterized by the saturation of the most intense spectral lines at the blackbody radiance. The blackbody temperature equals the excitation temperature of atoms and ions, proving directly and unambiguously a plasma in local thermodynamic equilibrium. The present investigations take benefit from the very rich and intense emission spectrum generated by ablation of a nickel-chromium-molybdenum alloy. This alternative and direct proof of the plasma equilibrium state reopens the perspectives of quantitative material analyses via calibration-free laser-induced breakdown spectroscopy. Moreover, the unique properties of this laser-produced plasma promote its use as radiation standard for intensity calibration of spectroscopic instruments.

Optical diagnostics of gold plasmas produced by infrared laser ablation

“…However, for any electron density, it is possible to find highly excited levels where the states are close enough for condition (5) to hold (partial LTE). The LTE conditions are often found in LPPs of relatively high electron density and relatively low temperature, when collisional processes are far more important than radiative ones and radiative processes do not affect population distributions [45][46][47].…”

Two-step procedure for trace element analysis in food via calibration-free laser-induced breakdown spectroscopy

“…The temperoral evolution was approximated by functions f (t) = At B represented by the continuous lines. This approximation was shown to describe closely the variations n e (t) and T (t) in plasmas produced by laser ablation of different materials under indentical irradiation conditions [32,40,24]. (0,0) Figure 9: Molecular bands and atomic transitions emitted from major and most abundant minor elements.…”

“…Based on the assumption of local thermodynamic equilibrium (LTE), the main difficulty of these techniques comes from the requirement of model validity [20,21]. Indeed, LTE is only established in plasmas of sufficiently large electron density [22], and continuum emission associated to free charges in the plasma limits the signal-to-noise ratio of the explored atomic or molecular emission [23,24]. Consequently, calibration-free laser-induced breakdown spectroscopy (CF-LIBS) was mostly applied to measure elements with sufficiently high concentration values.…”

Modelling of Plasma Temperatures and Densities in Laser Ablation Plumes of Different Metals

“…The effective background pressure is 22-35 Pa at room temperature, depending on the target material. The model assumes local-thermal equilibrium in the expanding plasma plume, which is a reasonable assumption for short timescales and expansion into a background gas [31,32].…”