Triple-pulse LIBS: Laser-induced breakdown spectroscopy signal enhancement by combination of pre-ablation and re-heating laser pulses
The goal of this work is to examine the effect of third additional laser pulse on the orthogonal double-pulse laser-induced breakdown spectroscopy (DP LIBS) experiment. Namely, the pre-ablation laser pulse and re-heating laser pulse were combined into the triple-pulse LIBS (3P LIBS) to achieve the plasma emission enhancement. The experiment was designed with the emphasis to a minimal sample damage. It means that energy of ablation laser pulse was set to the minimal value for which the intensities of selected spectral lines were above the limits of detection. This energy as well as the energies of the pre-ablation and the re-heating laser pulses were kept constant for all experimental arrangements. All possible configurations were studied - single pulse, pre-pulse DP LIBS, re-heating DP LIBS and triple-pulse LIBS. The interpulse delays for both DP LIBS and for 3P LIBS configurations were optimized separately in order to obtain the highest possible enhancement for each of them. The 3P LIBS results showed up to 5-fold improvement of the signal to background ratio compared with both DP LIBS arrangements and up to 228-fold improvement in comparison to conventional SP LIBS. In addition, the relationship of signal enhancement and the upper-level energy of selected spectral lines was studied. It has been shown that the spectral lines with higher value of excitation energy prove higher enhancement in both DP LIBS and 3P LIBS configurations. To explain this effect, the dimensions of ablation craters were measured for each configuration using a 3D optical microscope and the temperature was determined from the slope of Boltzmann plot. It was shown that the pre-ablation causes an increase of ablation crater dimensions and the re-heating increases the plasma temperature.