[Time-resolved evaluation of self-absorption in laser induced plasma from nickel sample]

Laser induced breakdown spectroscopy (LIBS) has been shown to be a promising technique for element analysis. However, self-absorption effect deeply influences the LIBS measurements. In the present paper, a Q-switched Nd:YAG laser operated at 1 064 nm was used to generate nickel plasmas in air. Four atomic lines Ni I 341.476/351.034/351.505/352.454 nm which belong to the same electronic configuration (3d9 (2D)4p-3d9 (2D)4s) of Ni were chosen for self-absorption investigation. Self-absorption of Ni I 351. 034 nm corresponding to the highest energy level 3D1 of 3d(9) (2D)4s was not observed in the plasma emission investigated. While for the other three lines, a strong self-absorption appeared at the prophase of the plasma and tended to weaken. The self-absorption at Ni I 352.454 nm was the most serious and still visible at the delay of 1100 ns, compared with the lines of Ni I 341.476/351.505 nm whose self-absorption duration is 900 and 500 ns respectively. It was also found that the self-absorption effect had power dependence and decreased with the increase in laser pulse energy. The obtained results suggest that the self-absorption effect could be alleviated by suitable atomic line selection, operating at a higher pulse energy and detecting with a longer delay. The possible reasons for the self-absorption duration difference for different lines were also discussed.