The self-absorption temporal evolution of spectral lines emitted from laser-induced plasmas

The interference of self-absorption (SA) in measurement of spectral lines in laser-induced plasmas (LIPs) needs to be addressed urgently, since the SA affects the spectral line shape, i.e., the line intensity decreases and line broadening changes. Here, the time resolved SA coefficients of six lines (Ca I 422.67 nm, Ca II 396.84 nm, Mg I 285.21 nm, Mg II 280.27 nm, Al I 394.40 nm, Al II 781.23 nm) emitted from laser induced soil plasmas are investigated based on the investigation of the temporal evolution of electron temperature (T_e) and electron density (N_e). The SA coefficient of line Ca I 422.67 nm decreases sharply in the delay time of 0.2-0.6 μs, and tends to be maintained in the 0.6-2.0 μs temporal window. While other five lines (Ca II 396.84 nm, Mg I 285.21 nm, Mg II 280.27 nm, Al I 394.40 nm, Al II 781.23 nm) SA coefficients decrease sharply in the 0.2-1.2 μs delay time due to the decreasing Stark broadening of those lines with the decreasing T_e and N_e correspondingly. While, in the 1.2-1.6 μs temporal window, the changes in SA coefficients of these five lines become steady because the plasmas property parameters (T_e and N_e) are stable after the delay time of about 1.0 μs. This indicates that the best time window for the accurate analysis of element concentration using LIPs is from 1.2 μs to 1.6 μs.