[Study on the Spatial Distribution of Laser Induced Plasma Emission Underwater with Different Laser Energies]

Laser-induced breakdown spectroscopy (LIBS), as a promising in-situ underwater detection technology, has received extensive attention in the field of ocean exploration. Improving the remote sensing ability of LIBS is crucial in bringing this technology into practical applications, hence higher laser energies, i.e. over threshold, are required. To characterize the plasma induced with super threshold energies and have a better understanding of resulted moving breakdown process, some extensive investigations into KCl water solution have been carried out with spectra-image jointed analysis. The spatial span and the brightest spot position of plasma radiation, with different laser energies from 1 to 20 mJ, were determined from the recorded plasma images. It was found that the plasma stretched from 0.49 to1.83 mm, as the laser pulse energy increased from 1 to 20 mJ, with a center position shift of 0.79 mm towards the incident laser beam. The obvious power dependence has also been observed from the obtained spatial resolved atomic spectra. Although the axial distributions at different energies were similar, both the position and the intensity of potassium atomic emission maximum varied. The optimal laser energy was determined to be 5 mJ with the emission intensity maximum higher than that at any other investigated energies. The obtained results suggested that the power dependence of atomic emission should be taken into account on increasing laser energy to meet the needs of stand-off LIBS applications. The FWHM and signal to background ratio of K Ⅰ 769.90 nm under different laser energies have also been investigated.