激光波长对水中金属元素激光诱导击穿光谱探测的影响

针对激光诱导击穿光谱(LIBS)在海洋应用中的问题,对1 064和532nm两个激发波长下水中LIBS光谱特性进行探测分析,以比较其烧蚀效果。通过激光诱导等离子体的时间分辨光谱,分析水下等离子体电子密度随时间的演化规律,1 064nm激光诱导等离子体寿命约为1 200ns,而532nm激光激发情况下等离子体寿命仅约为600ns。基于光在水中的传输特性和LIBS的实验结果,建立了获得最佳LIBS探测效果所需的入水前激光脉冲能量Eiopt(r)与探测距离r的关系,并应用到水下原位探测的模拟分析。结果表明,当探测距离不大于5cm时,所需的入水前1 064nm激光单脉冲能量小于100mJ,该激发波长可用于LIBS的水下探测;当探测距离增至10cm时,所需的入水前532nm激光单脉冲能量只需30mJ左右。因此,当原位探测距离增加时,则需考虑选择532nm激光作为烧蚀光源。

Effects of laser wavelength on detection of metal elements in water solution by laser induced breakdown spectroscopy

In the present paper, aiming at the problem of laser induced breakdown spectroscopy (LIBS) applyication in ocean detection, the effects of laser wavelength on the detection of Ca in water solution were investigated. The evolvement characteriza tion of electron density was studied by analyzing the time resolved spectra of the plasma. The experimental results show that the lifetime of plasma is about 1 200 and 600 ns respectively induced by 1 064 and 532 nm laser. Based on the optical transmission characteristic and the LIBS experimental results, the dependence of needed laser energy before incidence into water E(iopt)(r) for optimal detection effect on the detection distance in water was found, and the dependence was simulated by applying to the in situ detection in water solution. The simulated results suggest that the needed laser energy of 1 064 nm laser before incidence into water is about 100 mJ when the detection distance is no larger than 5 cm. When the detection distance increases to 10 cm, the needed laser energy of 532 nm before incidence into water is only about 30 mJ. So it should be considered to choose 532 nm laser as the ablation source with the increase in the in situ detection distance.