LIBS plasma enhancement for standoff detection applications

We have used a simultaneous 10.6 micron CO2 laser pulse to enhance the Laser Induced Breakdown Spectroscopy (LIBS) emission from a 1.064 micron Nd:YAG laser induced plasma on a hard target. The enhancement factor was found to be one or two orders of magnitude, depending upon the emission lines observed and the target composition. The output energy of the 5 ns Nd:YAG laser pulse was about 50 mJ and was focused to a 1 mm diameter spot to produce breakdown. The CO2 laser pulse (100 ns spike, 5 microsec tail) had a similar energy density on target (0.06 J/mm2). Timing overlap of the two 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 of ionized species tended to be higher, 10-200X. We attribute the increase in both the atmospheric components and the +1 and +2 ionic emission to heating of the Nd:YAG plasma by the coincident CO2 laser. Such inverse bremsstrahlung absorption of CO2 laser radiation by the free electrons of plasma is well known. 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 target compositions.