Spectroscopic studies of UV lead plasmas produced by single and double-pulse laser excitation

Abstract

A spectroscopic study to compare single- and double-pulse laser-induced breakdown spectroscopy (LIBS) using two Q-switched Nd:YAG lasers emitting at 532 nm is reported. The two laser beams were combined in the same direction (collinear beam geometry) to focus on the Pb targets in the open air. Various parameters, such as incident laser irradiance, placement of the laser beam focus position relative to the illuminated surface and gate delay times (delay between the incident laser pulse and the ICCD camera), were used as variables to enhance the sensitivity of LIBS signals. Several atomic and ionic emission lines of Pb were registered in the 200–290 nm UV spectral domains. In order to study the temporal evolution of plasma parameters for single- and double-pulse laser (SP and DP) configurations, the observed profiles of neutral lead lines were used to extract the plasma temperature (Te) using Boltzmann plots, whereas electron number density (ne) was determined from the profile of the Stark-broadened line. In the case of the DP configuration, the intensity of the atomic Pb I signal at 280.2 nm was enhanced eightfold. The intensity enhancement could help the analytical performance of the LIBS technique in terms of improvement of sensitivity and reduction of the self-absorption effect. This study contributes to a better understanding of the LIBS plasma dynamics by observing the temporal evolution of various emission lines of Pb. The results demonstrate a faster decay of the continuum relative to the spectral lines and a slightly longer plasma life-time for the DP configuration as compared with the SP configuration. In order to avoid inhomogeneous effects in the plasma, sufficiently high laser intensity and short delay time are required. Special attention was paid to possible self-absorption of different transitions. The micro-craters generated by SP and DP laser ablation were also compared using an optical microscope.