Investigation of local thermodynamic equilibrium in laser-induced plasmas: Measurements of rotational and excitation temperatures at long time scales

We studied the rotational temperature of diatomic molecules in the context of laser induced plasma from a solid target. In particular, its temporal evolution is investigated at long time scales (≥ 30 μs). The measured values are compared to ionic and atomic excitation temperatures and the issue of local thermodynamic equilibrium is discussed. The investigation was carried out using an aluminium oxide (Al2O3) target doped with titanium (Ti) and iron (Fe). The ionic and the atomic excitation temperatures are deduced from the Ti II lines and the Fe I lines respectively. For the molecular temperature, a temporally resolved study of the aluminium monoxide (AlO) blue-green spectrum was carried out. We show that underthese experimental conditions, a complete thermodynamic equilibrium is not reached for up to 50 μs after the laser pulse. The plasma is identified as cold plasma, with two different temperatures: the electron kinetic temperature and the heavy species kinetic temperature.