Isotopic analysis and plasma diagnostics for lithium detection using combined laser ablation–tuneable diode laser absorption spectroscopy and laser-induced breakdown spectroscopy

Abstract

Laser-Induced Breakdown Spectroscopy (LIBS) enables rapid stand-off measurements of potentially hazardous material which is particularly favourable for applications in the nuclear industry. Isotopic characterisation with LIBS remains challenging because of the narrow separation of isotopic emission lines, which demands large, high resolution spectrometers. To address this, laser ablation (LA)-Tuneable Diode Laser Absorption Spectroscopy (TDLAS) has been combined with LIBS to analyse laser-produced plasma of lithium. Isotope ratio calculations were performed by fitting multiple peaks to the Li absorption spectra, achieving a relative error of 13% at later delay times after line broadening had reduced. Double-pulsed laser ablation resulted in narrower absorption peaks but more noise in the absorption spectra. LIBS and LA-TDLAS spectra were used simultaneously to calculate plasma temperature and electron density for both single- and double-pulsed laser ablation. The Doppler broadening of absorption peaks from LA-TDLAS was used to calculate the plasma temperature after long time delays of 200 μs and the temperature followed an exponential decay which was extrapolated back to early delay times in order to predict the temperature throughout the plasma lifecycle. The electron density was calculated using the Stark broadening of emission peaks from the LIBS spectra, which were temperature-corrected by accounting for the Doppler broadening. Both single- and double-pulsed-laser ablation plasmas exhibited maximum electron densities of around 1016 to 1017 cm−3, although the decay rate was reduced by around a factor of 2 using DP-laser ablation. We have shown that the combination of emission and absorption spectroscopy with LIBS and LA-TDLAS is useful for isotopic analysis and calculating laser ablation plasma properties. We have demonstrated that double pulsed laser ablation has the potential to enable more isotopic pairs to be analysed due to the narrower absorption linewidth.