Femtosecond laser-induced breakdown spectroscopy of sea water

• Lines of Na I, K I, Ca I–II, Mg I–II, H I, N I, O I and N2 were recorded.
• Recombination determines characteristic decay time of line intensity.
• Three-body recombination time was used to estimate electron density.
• Excitation rate constants allow to determine relation of detection limits.

The composition of the line and band spectra of the plasma induced by a femtosecond laser pulse on the surface of sea water is determined. The temporal behaviors of the intensity of the continuum and the Ca II, Mg II and Na I lines are investigated. It is shown that the time dependence of the intensity of the Na I line is described by a monoexponential function. The characteristic decay times of the line intensities of Mg II and Na I were used to estimate the three-body recombination times. Using these values, we estimate the electron number density and the feasibility of Local Thermodynamic Equilibrium (LTE) criterion. A method involving excitation rate constants is proposed for the comparison of detection limits. For a plasma generated on a liquid surface, the following relation among detection limits will be obtained: LOD(Na) < LOD(K) < LOD(Ca) < LOD(Al) < LOD(Mg) < LOD(Zn).