الفهرس 
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Abstract
Laser-induced breakdown spectroscopy (LIBS) has become a widely- adopted technique for the elemental analysis of materials, both for the laboratory as well as for in-situ applications, and remote analysis in hostile environments. In order to analyze the emission spectra of Laser Induced Plasmas (LIP) for obtaining quantitative information on the abundance of the species present in the plasma it is necessary to rely on a model, which describes the population of different quantum states of atoms and ions present. This made it necessary to study the Local Thermodynamic Equilibrium (L TE) conditions in the plasma and determine the best conditions at which it is satisfied. In the present work an Nd:YAG laser light pulse (6ns) of different energies (25, 50, 75 and 100mJ) are focused using a quartz lens of focal length (10 cm ) onto certified aluminium alloy samples. The emitted spectra are collected and analyzed using an echelle spectrometer coupled with an ICCD camera. The temporal history of the plasma is obtained by recording the emission features at predetermined delays. For each spectrum recorded, the peak intensity, the full width at half maximum FWHM and the center wavelength of some chosen spectral lines, are then determined. Both the electron density and the excitation temperature are calculated for each delay time and laser pulse energy where we found that the values of electron density are between 1018 to 1017 cm-3 while the corresponding excitation temperatures were between 30000 and 4000 K depending on the laser pulse energy and the sample used. The local thermodynamic equilibrium (LTE) conditions were followed up for the different delays and different energies to determine the temporal range in which it is satisfied. It has been found that in case of 25 and 50 mJ laser energies, the LTE conditions were not satisfied in the chosen delay range (500 - 5000 ns). On the other hand for higher laser energies, the L TE conditions were satisfied in the same chosen delay range.