Fraunhofer-type signal for underwater measurement of copper sample using collinear long-short double-pulse laser-induced breakdown spectroscopy

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.