Dynamic effects of a pre-ablation spark in the orthogonal dual-pulse laser induced breakdown spectroscopy

We have observed dynamic effects of a pre-ablation spark on the signal intensity in the orthogonal dual-pulse laser-induced breakdown spectroscopy. We applied pre-ablation and ablation laser pulses with significantly reduced energy for an aluminum metal in open air. Under this experimental condition, the well-known signal enhancement through the increase in ablated mass was negligible. The Al I and II emissions were investigated by both top-view and spatially-resolved side-view collection modes. In this low laser power regime, dynamic effects of a pre-ablation spark on the signal intensity were clearly revealed. The principal factor of signal enhancement is the increase in temperature. Without the mass removal enhancement, effective rarefaction leads to decrease in the Al I emission intensity and simultaneous increase in the Al II emission intensity. This is attributed to the role of Saha equilibrium. Selective prolongation of emission lifetime only for the enclosed part of the analyte plasma in the rarefied region and other fluid-dynamic effects of a pre-ablation spark have been visualized by wavelength-selected time-space correlation maps of plasma emissions.