Theoretical study on signal enhancement of orthogonal double pulse induced plasma

Abstract

The double pulse technology increases the energy transmission between the plasma plume and the target by coupling the laser beam, which greatly enhances the signal, reduces the detection limit of laser-induced breakdown spectroscopy (LIBS) and improves the ability of trace elemental analysis. In this paper, the emission characteristics of plasma in orthogonal double-pulse LIBS (DP-LIBS) are studied theoretically by using the hydrodynamics model, including the effects of pulse interval and spot position of the second laser pulse on the emission intensity of various species in the reheating mode. The results show that in the reheating mode, the pulse interval corresponding to the maximum intensity ratio of double pulse to single pulse occurs at 0.2–0.5 μs, while in the pre-ablation mode, it increases with the increase of pulse interval. According to the decrease of the number density of the background gas above the sample before the ablation pulse in the pre-ablation mode, the enhancement mechanism of spectral line intensity is explained from two aspects: plasma–plasma coupling effect and pressure effect. In addition, the spatial distribution and raising mechanism of plasma temperature in these modes are also explored. It shows that in the reheating mode, the plasma temperature is increased by directly absorbing laser energy, while in the pre-ablation mode, it is increased by reducing the energy loss between vapor plasma and gas plasma and weakening the shielding effect. The theoretical analysis of double pulse plasma provides a theoretical basis for the experimental research and application of DP-LIBS.