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 a third additional laser pulse on orthogonal double-pulse laser-induced breakdown spectroscopy (DP LIBS). Namely, a pre-ablation laser pulse and re-heating laser pulse were combined into triple-pulse LIBS (3P LIBS) to achieve plasma emission enhancement. The experiment was designed with emphasis on causing minimal sample damage by utilizing nanosecond laser pulses. It means that the energy of ablation laser pulses 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. The interpulse delays for both DP LIBS and for 3P LIBS configurations were optimized for signal enhancements. 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, it has been shown that the spectral lines with a 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 the Boltzmann plot. It was shown that the pre-ablation causes an increase of ablation crater dimensions and the re-heating increases the plasma temperature.