Study of material ablation and plasma radiation in double-pulse laser induced breakdown spectroscopy at different delay times: Modeling and numerical simulation

A one-dimensional numerical model is presented on a copper sample to investigate double-pulse laser induced breakdown spectroscopy (DP-LIBS). The effect of the inter-pulse delay time on the material ablation, plasma homogeneity, and signal enhancement is examined. The dynamics of laser ablation, plume expansion, plasma formation, and plasma radiation of the ionized and neutral atoms in the presence of helium background gas at a pressure of 1 atm are studied. A heat conduction equation is solved in the sample and is coupled to the fluid dynamic equations through the Knudsen layer relations. Saha-Eggert equations are utilized to investigate the plasma formation. The influence of plasma shielding, due to the photoionization and inverse bremsstrahlung processes, is considered. Continuous radiation, bremsstrahlung and recombination radiations, and spectral emissions of the plasma are examined. The optimum inter-pulse delay time for maximizing the neutral and ionized spectral emissions is determined. The results reveal that the ablation rate in DP-LIBS is significantly higher than that of single pulse laser induced breakdown spectroscopy (SP-LIBS) and reaches its maximum at an optimum inter-pulse delay time due to the decrease in the recondensation of the ablated plume. Furthermore, it is demonstrated that in DP-LIBS, the ablation profile is smoother and its continuous radiation decreases much earlier than that of SP-LIBS. Although the double-pulse mode improves the signal to background ratio, it leads to more inhomogeneity in the plasma.