On the use of laser-induced fluorescence to probe the thermodynamic equilibrium in laser-generated plasmas

In this work, we investigate thermodynamic equilibria in a laser-generated plasma from ro-vibrational population distributions in the AlO molecule. We address the congruence between the rotational temperatures of diatomic molecules in their excited state and in their ground state, the latter being assumed to correspond to the kinetic temperature of the species in the plasma. The model system consists of AlO molecules produced by ablation of an alumina target in ambient air using a nanosecond laser pulse. Using laser induced fluorescence spectroscopy, we can directly probe the population of the rotational levels in the ground electronic state X2Σ+ of AlO and deduce the corresponding rotational temperature. This temperature is then compared to the population of the rotational levels in the excited state B2Σ+ deduced from the thermal B − X rovibronic emission. In such plasma, AlO molecules in the excited state are believed to be formed by chemical reaction and might be strongly out of equilibrium, but we find that emission from the B2Σ+ excited state provides a useful indication of kinetic temperature of the species in the plasma for delays longer than a few microseconds.