为了研究样品温度对激光诱导击穿Cu等离子体特征参数的影响, 以黄铜为研究对象, 在优化的实验条件下采用波长为532 nm的Nd∶YAG纳秒脉冲激光诱导激发不同温度下的块状黄铜, 测量了Cu等离子体的特征谱线强度和信噪比; 同时在局部热平衡条件下利用Boltzmann斜线法和Stark展宽法分析计算了不同的样品温度条件下等离子体电子温度和电子密度。 实验结果表明, 在激光功率为60 mW时, 随着样品温度的升高, Cu的特征谱线强度和信噪比逐渐增加, 样品温度为130 ℃时达到最大值, 然后趋于饱和。 计算表明, 黄铜样品中Cu元素Cu Ⅰ 329.05 nm, Cu Ⅰ 427.51 nm, Cu Ⅰ 458.71 nm, Cu Ⅰ 510.55 nm, Cu Ⅰ 515.32 nm, Cu Ⅰ 521.82 nm, Cu Ⅰ 529.25 nm, Cu Ⅰ 578.21 nm八条谱线在130℃的相对强度相较于室温(18 ℃)下分别提高了11.55倍、 4.53倍、 4.72倍, 3.31倍、 4.47倍、 4.60倍、 4.25倍、 4.55倍, 光谱信噪比分别增大了1.35倍, 2.29倍、 1.76倍、 2.50倍、 2.45倍、 2.28倍、 2.50倍, 2.53倍。 分析认为, 升高样品温度会增大样品的烧蚀质量, 相对于温度较低状态增加了等离子体中样品粒子浓度, 进而提高等离子体发射光谱强度。 所以, 适当升高样品温度能够提高谱线强度和信噪比, 从而增强LIBS技术检测分析光谱微弱信号的测量精度, 改善痕量元素的检测灵敏度。 同时研究了改变样品温度时等离子体电子温度和电子密度的变化趋势。 计算表明, 当样品温度从室温上升到130 ℃的过程中, 等离子体的电子温度由4 723 K上升到7 121 K时基本不再变化。 这种变化规律与发射谱线强度和信噪比变化趋势一致。 分析认为, 这主要是由于在升高样品温度的初始阶段, 激光烧蚀量增大, 等离子体内能增大, 从而导致等离子体电子温度升高。 当激光烧蚀样品的量达到一定值后不再变化, 激光能量被激发溅射出来的样品蒸发物以及尘粒的吸收、 散射和反射, 导致激光能量密度降低, 电子温度趋于饱和, 达到某种动态平衡。 选用一条Cu原子谱线(324.75 nm)的Stark展宽系数计算激光等离子体的电子密度, 同时研究改变样品温度时等离子电子密度的变化趋势, 计算表明在样品温度为130 ℃时, Cu Ⅰ 324.75 nm对应的等离子电子密度相较于室温(18 ℃)条件下增大了1.74×10¹⁷ cm^-3。 该变化趋势与电子温度的变化趋势一致。 适当升高样品温度使得电子密度增大, 从而提高电子和原子的碰撞几率, 激发更多的原子, 这是增强光谱谱线强度的原因之一。 由此可见, 升高样品温度是一种便捷的提高LIBS检测灵敏度的有效手段。

To investigate the influence of sample temperature on the characteristic parameters of laser-induced Cu plasma, brass was used as the study object. A Nd: YAG nanosecond pulsed laser with 532 nm wavelength was adopted under optimized experimental conditions to excite and to induce the breakdown of massive brass under different sample temperatures, and the characteristic spectral line intensity and signal-to-noise ratio of the Cu plasma were measured. The Boltzmann diagonal line and Stark broadening methods were used to analyze and calculate the electron temperature and electron density of the plasma under different sample temperatures. When the laser power was 60 mw, the characteristic spectral line intensity and signal-to-noise ratio of Cu gradually increased as the sample temperature increased and tended to be saturated after reaching maximum values under 130 ℃. The relative intensities of eight spectral lines—Cu Ⅰ 329.05, Cu Ⅰ 427.51, Cu Ⅰ 458.71, Cu Ⅰ 510.55, Cu Ⅰ 515.32, Cu Ⅰ 521.82, Cu Ⅰ 529.25, and Cu Ⅰ 578.21 nm—of Cu in the brass sample increased 11.55, 4.53, 4.72, 3.31, 4.47, 4.60, 4.25, and 4.55 times under 130 ℃ compared with those under room temperature (18 ℃), and the spectral signal-to-noise ratios increased 1.35, 2.29, 1.76, 2.50, 2.45, 2.28, 2.50 and 2.53 times respectively. Elevating the sample temperature would increase the ablation mass of the sample and plasma particle concentration compared with those under relatively low temperature and consequently would enhance the plasma emission spectral intensity. Therefore, appropriately elevating the sample temperature could increase the spectral line intensity and signal-to-noise ratio to enhance the measurement accuracy of LIBS technology in detecting and analyzing weak spectral signals and to improve its detection sensitivity for trace elements. The variation tendency of the electron temperature and electron density with the change of sample temperature was investigated. In the calculation, the electron temperature of the plasma was basically unchanged when it was increased from 4 723 to 7 121 K when the sample temperature increased from room temperature to 130 ℃. This change law was consistent with the variation tendency of the emission line intensity and signal-to-noise ratio. This condition was mainly due to the increase in the laser ablation quantity and internal energy of the plasma at the initial phase of the sample temperature rise, which increased the electron temperature of the plasma. No additional change in the sample quantity under laser ablation was observed after reaching a certain value, and the laser energy was absorbed, scattered, and reflected by excited and sputtered sample evaporants and dust particles; such process reduced the laser energy density. As a result, the electron temperature tended to be saturated, and several dynamic balances were reached. The Stark broadening coefficient of the 324.75 nm Cu atomic spectral line was selected in this study to calculate the electron density of the plasma. The variation tendency of the plasma electron density with the change of sample temperature was evaluated. The plasma electron density that corresponded to Cu Ⅰ 324.75 nm when the sample temperature was 130 ℃ increased by 1.74×10¹⁷ cm^-3 compared with that under room temperature (18 ℃). This variation tendency was consistent with that of electron temperature. Appropriately elevating the sample temperature increased the electron density and the probability for electron and atom collision, which excited many atoms. This process was one of the reasons for the enhancement of spectral line density. Thus, elevating the sample temperature is a convenient and effective means to improve LIBS detection sensitivity.