Three-dimensional analysis of laser induced plasmas in single and double pulse configuration

In this work we studied the morphology of plasmas induced by single and double laser pulses, with the purpose to improve the understanding of the formation and evolution of the plume in the two cases and the reasons of the increase of sensitivity and detection limits in the double pulse configuration. Single and double pulse laser-induced breakdown spectroscopy experiments were carried out on a brass sample in air. Spectrally, temporally and spatially resolved measurements have been performed on the plume and a deconvolution algorithm has been applied on the line-of-sight integrated spectra in order to separate the spectral information coming from different regions of the plasma. While in single pulse configuration only the major elements in the matrix were observed, in double pulse case also the minor elements emission lines were measurable. The values of line emission intensity, plasma temperature and electron density have been evaluated at different positions in the plume in both the configurations. Appreciable differences in plume dimensions (three times wider in double pulse configuration), and electron density values (two times higher in single pulse case), have been found while the maximum temperature in the plasma core was similar in the two configurations. Beside the spectroscopic measurements, we used the shadowgraphic technique to observe the evolution of the plume in single and double pulse configuration. In double pulse configuration, the expansion of the plume induced by the second pulse is sensibly faster than the one induced by the first pulse and, in double pulse case, the plume tends to fill the region encompassed by the shock wave formed by the first pulse; on the other hand no shock wave produced by the second pulse was visible. The results obtained by the two different approaches were compared and an explanation of the physical processes occurring is proposed. (C) 2004 Elsevier B.V. All rights reserved.