Characteristics of the secondary breakdown of DP-LIBS in bulk water with different axial focusing arrangements and laser energies

Characteristics of the secondary breakdown of dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) in bulk water were investigated with different axial focusing arrangements and laser energies of the second laser pulse. Compared to the pre-focus or in-focus arrangement, post-focus arrangement could provide both stronger emissions and better repeatability within the energy range of 20–40 mJ. Such phenomenon is directly related to the breakdown characteristics in different conditions. Especially for in-focus arrangement, i.e. the most reported case of DP-LIBS in bulk water, the breakdown all occurs at the bubble boundary rather than inside the gaseous bubble even using a quite large laser energy of 130 mJ. The simulation results further confirm the secondary breakdown could not occur directly in gas of the bubble interior under the present experimental setup.