Time-dependent single and double pulse laser-induced breakdown spectroscopy of chromium in liquid

A study of aqueous solutions of chromium using single and double pulse laser-induced breakdown spectroscopy (LIBS) is presented. Three atomic emission lines show enhancement in emission under dual pulse LIBS excitation. The temporal evolution of line emission indicates that a shock wave front produced by the first laser pulse plays an important role in determining the decay rate of intensity by excitation transfer in single pulse LIBS and by plasma confinement in double pulse LIBS. The ratio of emission in dual pulse LIBS to single pulse LIBS with time shows a linear increase followed by the onset of saturation. A theoretical calculation of the enhancement is found to be in qualitative agreement with the experimental results, suggesting that material ablation in dual pulse LIBS should be > or = 3.5 times that of single pulse LIBS. There is indication that the increase in ablation and subsequent enhancement in emission may be due to the rarefied gas density inside the region enclosed by the shock wave produced by the first laser pulse. The limit of detection of Cr in aqueous solution has been improved by an order of magnitude with double pulse LIBS.