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.