采用长脉宽激光(500 μs)作为激发光源, 研究了长脉宽激光在低能量密度条件下诱导击穿土壤等离子体的光谱特性。 实验发现, 在长脉宽激光的激发下等离子体光谱特性与纳秒及超短脉宽激光相比存在较大的差异, 在402~409 nm波段和420~436 nm波段, 土壤等离子体光谱均没有出现强烈的连续背景辐射阶段, 所激发产生的等离子体发光寿命可持续220~270 μs。 特征谱线PbⅠ405.78 nm和CrⅠ425.43 nm分别在210和190 μs时出现, 且谱线强度均随时间延长而逐渐增加, 分别在延时320和350 μs时达到最大值。 研究结果表明, 采用长脉宽激光作为激发光源, 增加了激光与物质相互作用的时间, 有助于激发形成发光时间较长的“准稳态等离子体”; 应用内标法建立了Pb和Cr元素校准曲线, 8次重复实验得到分析线与内标线净信号强度比值的相对标准偏差为2.21~6.35%, 长脉宽激光激发产生的等离子体发光稳定; 计算得到土壤中Pb和Cr元素的检测限分别为34.7和40.0 mg·kg-1, 达到国家土壤环境质量标准规定的一级含量要求; 长脉宽激光激发产生等离子体的电子温度为6 612 K, 电子密度为3.7×1017 cm-3, 达到了局部热平衡状态。

Laser induced breakdown spectroscopy with long-pulse laser(500 μs) was used to generate plasma of soil sample in air. The spectroscopy emission characteristic of soil plasma was investigated under the low power-density conditions. Intense continuum background could not be detected (402~409 and 420~436 nm) and the long-pulse laser induced plasma had a longer overall life time (about 220~270 μs), which was different from the dynamic characteristics using nanosecond laser and ultra-short pulse laser. Besides, the spectral lines of Pb Ⅰ405.78 nm and Cr Ⅰ425.43 nm appeared at about 210 and 190 μs. Intensity of Pb Ⅰ405.78 nm and Cr Ⅰ425.43 nm increased as time passed by, reaching to its maximum at 320 and 350 μs, respectively. The study results showed that increased interaction time between laser and sample contributed to the formation of “quasi-stable state plasma”. The relative standard deviation was 2.21%~6.35% concluded by 8 times repeated experiments, which showed a better stability of soil plasma by using a long-pulse laser. The detection limits of Pb and Cr were 34.7 and 40.0 mg·kg-1, respectively, which was below the trace element thresholds for Class 1 soil used in the environmental quality standard in China. Parameters characterizing a laser-induced plasma were obtained with the temperature of 6612 K and electron density of 3.7×1017 cm-3 in the condition of long-pulse laser. Experimental results showed that it was in local thermodynamic equilibrium.