Influence of ambient pressure on spatial-temporal evolution of local thermodynamic equilibrium for laser-induced plasma

Laser-induced breakdown spectroscopy (LIBS) is used for quantitative detection based on the fact that the laser-induced plasma (LIP) is in local thermodynamic equilibrium (LTE). However, the radiation lifetime of the LIP is of the order of microseconds or less, resulting in the rapid change of physical properties inside the LIP, and the duration of LIP in LTE is very short. Numerous studies have shown that ambient pressure has significant influence on the LIP, but the influence of ambient pressure on the spatial-temporal evolution of LTE for LIP is still not clear, and the mechanism of the influence of ambient pressure on the LIP is obscure. In the present study, the spatially-temporally resolved spectra of the LIP were analyzed from the laser incident direction, and the influence of ambient pressure on the spatial-temporal evolution of LTE for LIP was studied. The experimental results showed that the LIP deviates from LTE when it is farther away from the plasma core and there is a longer delay time. Moreover, the results indicated that ambient pressure has a significant influence on the spatial-temporal evolution of LTE. In a higher ambient pressure environment, the ion energy level population of LIP is much closer to Boltzmann distribution and the LIP deviates from LTE faster with the increase of distance from the LIP core and slower with the increase of delay time. Finally, the moment at which the LIP core begins to depart from LTE decreases rapidly with the ambient pressure. This work is helpful for understanding the influence of ambient pressure on LIP, optimizing the experimental parameters of LIBS, and providing references for LIBS applications.