Abstract The collinear long-short double-pulse laser-induced breakdown spectroscopy (long-shrot DP-LIBS) is used to detect the underwater copper sample. Fraunhofer-type signal is acquired with very short gate delay parameter, which is the first time to be reported that the collinear long-short DP-LIBS can generate Fraunhofer-type signal from underwater target. It is found that the early structure of underwater plasma is similar to the solar structure. According to the plasma image of early stage, a cold shell with low emission intensity is wrapped around a hot core with strong emission intensity. During early period, the continuum radiation from the hot core is absorbed by the copper atoms in plasma edge, causing Fraunhofer-type absorption at Cu I 324.754 nm and Cu I 327.396 nm. The experimental results show that the absorption intensity of Fraunhofer-type lines possesses a well signal stability in early stage. RSD% of signal is 1.5% with 10 ns delays and 5.3% with 30 ns delays at Cu I 324.754 nm, and RSD% is 2.7% with 10 ns delays and 3.3% with 30 ns delays at Cu I 327.396 nm. The present results sugget that the Fraunhofer-type signal generated by collinear long-short DP-LIBS is possible to be used for further underwater quantitative measurement.

中文翻译：

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使用共线长短双脉冲激光诱导击穿光谱法对铜样品进行水下测量的夫琅禾费型信号

摘要 采用共线长短双脉冲激光诱导击穿光谱（long-shrot DP-LIBS）对水下铜样品进行检测。以极短的门延迟参数获取Fraunhofer型信号，这是首次报道共线长短DP-LIBS可以从水下目标产生Fraunhofer型信号。发现水下等离子体的早期结构与太阳结构相似。根据早期的等离子体图像，发射强度低的冷壳包裹在发射强度强的热核上。早期，来自热核的连续谱辐射被等离子体边缘的铜原子吸收，在Cu I 324.754 nm和Cu I 327.396 nm处产生Fraunhofer型吸收。实验结果表明，Fraunhofer 型谱线的吸收强度在早期具有良好的信号稳定性。在 Cu I 324.754 nm 处，信号的 RSD% 为 1.5%，延迟为 10 ns，延迟为 5.3%，Cu I 为 327.396 nm，RSD% 为 2.7%，延迟为 10 ns，30 ns 延迟为 3.3%。目前的结果表明共线长短DP-LIBS产生的Fraunhofer型信号有可能用于进一步的水下定量测量。