Analysis of pulse-to-pulse fluctuation in underwater Laser-Induced Breakdown Spectroscopy on the basis of error propagation calculation

We study the pulse-to-pulse variation of the emission spectral intensity of underwater laser-induced breakdown spectroscopy. Emission spectral intensity and its dispersion were measured as a function of the fluence of the laser pulse at a metal target surface immersed in water. The coefficient of variation, which is an index of the dispersion, showed a minimum at a certain fluence. The dispersion at the low fluences was attributed to the variations of the population density and the atomic excitation temperature, according to the error propagation analysis of the theoretical spectral line intensity based on the Boltzmann distribution. The population density and the temperature showed a negative correlation, which is consistently explained by the unstable division of pulse energy into two parts, i.e., the early part of a pulse is attributed to materials ablation and hence to the population density, while the other part to plasma heating and hence to the temperature. [Display omitted] •Pulse-to-pulse fluctuation of underwater laser-induced breakdown spectroscopy is studied.•Effect of laser fluence at the target surface upon the spectral intensity is studied.•The coefficient of variation of the spectral intensity shows a minimum at a certain fluence of the laser pulse.•Serious self-absorption effect on the fluctuation of spectral intensity at higher fluences.•The pulse-to-pulse fluctuation was consistently explained by the fluctuation of the ablation threshold of the target.