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 attributed to materials ablation threshold and hence to the population density, while the other part of plasma heating and hence to the temperature.

我们研究了水下激光诱导击穿光谱的发射光谱强度的脉冲间变化。发射光谱强度及其色散作为激光脉冲在浸入水中的金属目标表面的注量的函数进行测量。作为分散指数的变异系数在某个注量处显示最小值。根据基于玻尔兹曼分布的理论谱线强度的误差传播分析，低通量处的色散归因于布居密度和原子激发温度的变化。人口密度与温度呈负相关，这一致地解释为脉冲能量不稳定的分为两部分，即，