Study of Self-Absorption of Emission Magnesium Lines in Laser-Induced Plasmas on Calcium Hydroxide Matrix

In this paper, we investigate the self-absorption of Mg I-II emission lines on laser-induced breakdown spectroscopy (LIBS) experiments. We produced the plasmas in air at atmospheric pressure by focusing a Nd:YAG laser onto a pellet of finely powdered calcium hydroxide with a concentration of 625 ppm of Mg and recorded the intensity profiles of Mg lines at different delay times after the laser pulse from plasmas generated with different laser energies. We carried out the analysis of the line profiles within the framework of a homogeneous plasma in local thermodynamic equilibrium by a computer algorithm that calculates the emission spectra and matches them to the experimental measurements. It allowed evaluation and compensation of self-absorption by using the spectroscopic information saved on the optical thicknesses of the lines. After that, we performed the characterization of the plasma and correlated the self-absorption features with the different trends of the temperature, the electron density, and the Nl parameters obtained. At the early times of plasma evolution, we found that the fast expansion of the plume was dominant and the self-absorption effects are difficult to interpret within the current approach, supplying only qualitative information. On the other hand, at later times, the results obtained could be properly analyzed showing the practical usefulness of the method to provide valuable information for both basic and applied LIBS research.