Impact of laser energy and gate delay on self-absorption of emission lines in laser induced plasma spectroscopy

Abstract

Laser-induced plasma spectroscopy (LIPS) is a spectroscopy that utilizes laser induced plasma as an emission source. The most challenging part in dealing with emission lines is the self-absorption (SA) which distorts the profile and reduces emission intensity of the spectrum. Resonant lines are most prominent lines of an element in the spectrum and at the same time most prone to SA. This project focuses on the impact of experimental parameters; laser energy and gate delay on the SA coefficient of emission lines which depends on two plasma parameter namely electron temperature, Te and electron density, Ne. A sample made of Al, Mn and Zn embedded in KBr matrix was irradiated with Nd:YAG laser and the plasma signals were recorded using optical spectrometer attached to a delay unit. The atomic and ionic spectral lines of Al, Mn and Zn were observed in the spectra. The lines were verified using references and National Institute of Standards and Technology (NIST) database. Resonant lines are Al I 256.4 nm, Al I 265.6 nm, Al I 308.2 nm, Mn I 403.3 nm, Mn II 259.4 nm and Mn II 260.1 nm. The laser energy was varied from 5 to 650 mJ at a fixed gate delay of 3.75 µs, meanwhile, the gate delay was varied from 0 to 23.75 µs at a fixed laser energy of 650 mJ. The intensity of the emission lines was found increasing in response to higher laser energy. The emission lines of Al, Mn and Zn was found initially increased in intensity within first 1 µs, but then it decreased as the increasing delay time. Te was calculated using the intensity ratio method applied on Mn I 257.6 nm and Mn I 422.5 nm emission lines and Ne was determined using Stark broadening method of Ha-line 656.3 nm. The SA coefficient was calculated for both experimental parameters, by using resonant lines Al I 308.2 nm and Mn II 259.4 nm, and non-resonant lines; Al I 309.1 nm and Mn I 257.6 nm. SA coefficient has variation from 0 to 1. The maximum value of the coefficient indicates that the emission lines is free from SA. The SA coefficient was found to increase from 0.3 to 0.9 as the laser energy increased resulting from rise in Te and Ne of the plasma. Meanwhile, the increasing gate delay caused the SA coefficient to decrease from 0.9 to 0.1, where the emission lines are more prone to SA. This is due to the decreasing of Te and Ne. This work has emphasized on implementation of higher laser energy and shorter gate delay of LIPS experimental parameters as response to SA coefficient. It will save time and effort and lead to reliable plasma diagnostics, as well as pioneers in studying plasma opacity.