Spatiotemporal and spectroscopic investigations of the secondary plasma generated during double-pulse laser-induced breakdown in bulk water

Abstract

Double-pulse laser-induced breakdown spectroscopy (DP-LIBS) has been demonstrated to enable efficient elemental analysis of bulk water. Its applications are based on the fundamental understanding of the characteristics of the secondary plasma. However, due to the poor stability of the secondary breakdown, the inherent formation and evolution processes of the secondary plasma within the first laser formed bubble (LFB) remain unknown. By post focusing the second laser beam at the rear boundary of the first LFB (post-focus arrangement), the generated secondary plasma can provide a stable system for understanding its spatiotemporal behaviors. Based on this post-focus arrangement, the secondary plasma was characterized by using temporally resolved spectroscopy, monochromatic fast imaging and shadowgraph techniques. It was shown that the emission of underwater DP-LIBS presented transient increments within 1 μs. Such a distinctive temporal behavior was closely related to the back and forth movement of the secondary plasma within the first LFB, due to its spatial confinement effect. Species-specific imaging studies reveal that the interactions between the secondary plasma and the surrounding water vapor can give rise to the dissociation of water molecules, as well as the excitation of hydrogen and oxygen atoms. Meanwhile, the molecular emissions (CaOH) appeared only after 2 μs, but lasted for 32 μs, suggesting a slower plasma relaxation within the first LFB. Further shadowgraphic investigations highlight the quasi-static, low-pressure environment of the first LFB, which can reduce energy losses and enhance optical radiation from the secondary plasma.