Experimental determination of the temperature range of AlO molecular emission in laser-induced aluminum plasma in air

Measurements with laser-induced breakdown spectroscopy (LIBS) usually take place in the atmospheric air. For quantitative analysis of metallic elements, oxidation may represent an important issue which can significantly modify the stoichiometry of the plasma. Molecule formation in plasma should be therefore studied and taken into account in the LIBS practice. In this work, we experimentally investigated the temporal evolution and transformation of the plasma induced on an aluminum target by a nanosecond infrared (1064nm) laser in the atmospheric air, in terms of its temperatures over a large interval of time from hundreds of nanoseconds to tens of microseconds. Such evolution was then correlated to the temporal evolution of the emission intensity from AlO molecules in the ablation plume. In particular, for a given ablation laser pulse energy, the appearance of the molecular emission while the plume cools down allows determining a minimal delay, τmin, which corresponds to a maximal value of the temperature, Tmax, below which the molecular emission begins to be clearly observed and to grow as a function of the delay. Such delay or such temperature indicates the longest delay or the lowest temperature for laser-induced plasma to be suitable for a correct analysis of metallic elements without significant influence of the alternation of the stoichiometry by oxidation. In our experiment, the values of τmin and Tmax have been determined for a range of ablation laser pulse energies from 5mJ to 50mJ. These values lie respectively in the range of 3 to 15μs for τmin, and 4500K to 6600K in terms of the molecule temperature for Tmax. Beyond the practical interest for LIBS, our results provide also insights to the kinetics of the AlO molecule formation in laser-induced plasma. •Determination of the temperatures in laser-induced plasma up to tens of microseconds•Determination of the molecule temperature by fitting the emission spectrum•The delay and the temperature for the emission from AlO molecules in plasma