Standoff Detection of Solid Materials Using Laser Induced Fluorescence of Laser-Produced Plasmas

Selective and sensitive detection technologies for rapid identification of substances at standoff distances are ultimately important for a large number of applications. For example, standoff, rapid detection is the most desired capability for an analytical tool for identifying dangerous substances in the field such as high-energy explosives and radioactive nuclear materials. Currently extensive efforts are going on to use emission spectroscopy of laser-produced plasma (LPP) or Laser-induced breakdown spectroscopy (LIBS) for standoff detection. However, the sensitivity of LIBS is poorer compared to sampling based detection techniques (eg. mass-spectroscopy). In addition to this, the spectral resolution available for all emission based methods are limited to instrumental broadening of the detection system. Laser-induced fluorescence (LIF) of LPPs can address these limitations.1 In LIF, a second laser is used to excite the lower-level or ground state population which exists during the entire lifespan of LPP even when the plasma is cooler in comparison with LIBS where hotter conditions are prerequisite for thermal excitation. By monitoring the plasma conditions at later times of its evolution there is a significant reduction in line broadening mechanisms in the plasma. The instrumental broadening with the LIF method is dictated by the linewidth of the probe laser, which for CW laser sources is very small ( a few femtometers).