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 X2E+ of AlO and deduce the corresponding rotational temperature. This temperature is then compared to the population of the rotational levels in the excited state B2E+ 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 B2E+ excited state provides a useful indication of kinetic temperature of the species in the plasma for delays longer than a few microseconds.

Automated summary

In this study, we investigate the thermodynamic equilibria in a laser-generated plasma by analyzing the congruence between the rotational temperatures of AlO molecules in their excited state and in their ground state. We use laser induced fluorescence spectroscopy to directly probe the population of rotational levels in the ground electronic state X2E+ of AlO and determine the corresponding rotational temperature. We find that the emission from the excited state B2E+ can provide a useful indication of the kinetic temperature of the species in the plasma for delays longer than a few microseconds.