Influence of ambient gas pressure on laser-induced breakdown spectroscopy technique in the parallel double-pulse configuration

Single-pulse and double-pulse laser-induced breakdown spectroscopy experiments have been performed using two Nd:YAG lasers in the fundamental mode on a brass sample at different air pressures, ranging from 0.1 Torr to atmospheric conditions, in order to obtain information about the different ablation and plasma evolution processes in the different configurations. Neutral and ionized lines originated both by species deriving from the target and from the air environment were analysed. The temperature and electron density values were estimated in all the experimental conditions. A different behavior of the plasma emission versus the air pressure, in the case of lines deriving from the target, was observed in the single-pulse and double-pulse configurations, suggesting that the different environmental conditions in the first and the second laser ablation may be responsible in determining the plasma emission in the two cases. An interpretative model based on the cavity produced in air by the laser-induced shock wave, according to the Sedov theory of the blast wave expansion, was able to qualitatively describe the effects observed in single-pulse and double-pulse experiments. Besides, the influence of the interpulse delay time between the two laser pulses was explored in the range between 0 and 20 As. The results, according to the model proposed, provide information on the plume evolution in the single-pulse and double-pulse configurations at different air pressures. In particular, different optimum interpulse delays were found for the observation of neutral lines and ionic lines.