Temporal follow up of the LTE conditions in aluminum laser induced plasma at different laser energies

In order to analyze the emission spectrum of a laser-induced plasma for obtaining quantitative information on the abundance of the species present in the plasma it is necessary to study the local thermodynamic equilibrium (LTE) conditions in the plasma and determine the best conditions at which they are satisfied. In the present work Nd:YAG laser light pulses (lambda= 1064 nm, 6 ns) of different energies (25, 50, 75 and 100 mJ) are focused using a quartz lens (focal length 10 cm) onto certified aluminum alloy samples in air under atmospheric pressure. The emitted spectra are collected and analyzed using an echelle spectrometer coupled with an intensified charge coupled device camera. The temporal history of the plasma is obtained by recording the emission features at predetermined delays and at a fixed gate width (2500 ns). For each spectrum both electron density and excitation temperature are calculated for each delay time and laser pulse energy; we found that the values of the electron density are decreasing from 1018 to 1017 cm-3. The corresponding excitation temperatures were between 30000 and 4000 K depending on the laser pulse energy and the sample used. The LTE conditions were followed up for the different delays and different energies to determine the temporal range in which they are satisfied. It has been found that in the cases of 25- and 50-mJ laser energies, the LTE conditions were satisfied in the chosen delay range (500-5000 ns). On the other hand, for higher laser energies, the LTE conditions were critical at delay times less than 1500 ns and are satisfied for longer delays.