LIBS Plasma Enhancement for Standoff Detection Applications

摘要

We have used a simultaneous 10.6 micron 2laser pulse to enhance the Laser InducedBreakdown Spectroscopy (LIBS) emission from a 1.064 micron Nd:YAG laser induced plasma on a hardtarget. The enhancement factor was found to be one or two orders of magnitude, depending upon theemission lines observed and the target composition. The output energy of the 5 ns Nd:YAG laser pulsewas about 50 mJ and was focused to a 1 mm diameter spot to produce breakdown. The 2laser pulse(100 ns spike, 5 microsec tail) had a similar energy density on target (0.06 J/mm2). Timing overlap of thetwo laser pulses within 1 microsecond was important for enhancement to be observed. Enhancement of neutral atomic emission was usually on the order of 5-20X, while enhancement ofionized species tended to be higher, 10-200X. We attribute the increase in both the atmosphericcomponents and the +1 and +2 ionic emission to heating of the Nd:YAG plasma by the coincident 2laser. Such inverse bremsstrahlung absorption of 2laser radiation by the free lectrons of plasma is wellknown. We are conducting additional studies to better quantify the effects of laser beam mode, pulse-to-pulse jitter, temporal pulse shaping, and optimization of these parameters for different LIBS targetcompositions.