Femtosecond laser-induced breakdown spectroscopy of aluminum plasma under the vacuum & air environments at various laser irradiances

Abstract

The effect of environmental conditions and irradiances on femtosecond laser-induced breakdown spectroscopy of Aluminum (Al) plasma has been investigated. For this purpose, a femtosecond laser (1030 nm, 220 fs) was employed to irradiate the Al targets under air and vacuum environments at different laser irradiances ranging from 1.8 TWcm⁻² to 5.4 TWcm⁻². LIBS analysis shows that optical intensity of emission spectra and plasma parameters (electron temperature “T_e” and electron number density “n_e”) are increased with increasing the laser irradiances of Al plasma under both environments. The electron temperature is evaluated by using both Boltzmann and Saha Boltzmann distributions, whereas electron density is evaluated by using Stark broadening. With increasing laser irradiances, the values of T_e of Al plasma increases from 8061 K to 8745 K under vacuum environment, whereas, for air T_e increases from 8203 K to 8874 K. In case of Saha Boltzmann plot, the values of T_e under vacuum vary from 8467 K to 8949 K and in air environment the estimated values of T_e increases from the 8600 K to 9027 K. Likewise, the values of n_e of Al plasma vary from 8.52 x 10¹⁸ cm⁻³ to 9.22 x 10¹⁸ cm⁻³ under vacuum environmental condition, while in case of air, the values of n_e vary from 8.72 x 10¹⁸ cm⁻³ to 9.38 x 10¹⁸ cm⁻³. Slightly higher values of Al plasma parameters in air as compared to vacuum are explainable on the basis of confinement effect offered by 760 Torr atmospheric pressure. The air environment restricts free expansion of Al plasma that results into enhanced rate of collisional excitation, recombination with increased life time of plasma that slows down the plasma plume. The laser-target interaction process of air also causes exothermic reactions of reactive gases (H₂, O₂, CO₂ etc) which are responsible for enhanced energy coupling to target in addition to laser energy deposition which in turn increases ablation rate and evaporation of the target. The increasing trends of emission intensity and plasma parameters with the increasing laser irradiances are attributed to enhanced mass ablation rate due to more energy deposition that increases the excitation and de-excitation within the plasma. The higher values of Al plasma parameters under certain environments make it more beneficial for various applications including surface structuring, electron ion implantation, pulse laser deposition of thin films and other industrial applications.