Absorption of a nanosecond laser pulse by a picosecond laser-induced preformed aluminum plasma

The LIBS (Laser-Induced Breakdown Spectroscopy) method has already demonstrated its reliability and its robustness in many situations for the multi-elemental composition determination of samples. However, certain conditions prevent a totally satisfactory determination. For instance, the method is weakly efficient to measure with accuracy the light elements concentration in metallic matrices. Since the laser pulse used to produce the plasma contributes to its heating, using an additional pulse (double pulse configuration) provides the increase in electron temperature and density without additional ablation. A better signal-to-noise ratio and a lower limit of detection can be reached. The present paper reports the results of different experiments performed to quantify the modifications induced (1) on the electron density by the second laser pulse in a preformed aluminum plasma, and (2) on the second laser pulse itself. The related experiments have been done in the case where the plasma is produced by a picosecond laser pulse and the second laser pulse is of the nanosecond type. The electron density reaches a maximum resulting from the total ionization of the aluminum plasma volume irradiated by the second laser pulse.