Abstract

Photofragmentation (PF) and subsequent nitric oxide (NO) laser-induced fluorescence (LIF) is being developed to measure the concentration of energetic materials (EM’s) in soil and other media. Laser radiation near 226 nm photodissociates gas-phase EM to NO₂, which predissociates into NO that gives an intense luminescence. The EM concentration is inferred from the intensity of the NO fluorescence. We have studied the factors that affect the PF–LIF signal intensity, including the effect of buffer gas on the LIF spectrum of pure NO, the effect of 2,4,6-trinitrotoluene (TNT) pressure on the PF–LIF spectrum, the effect of buffer-gas pressure on the PF–LIF signal intensity of pure TNT, and the effect of temperature on the PF–LIF spectra of pure TNT and of TNT in simulated soil. Heating of the TNT sample above 343 K was found to increase the magnitude of the PF-LIF signal intensity significantly, but also was found to cause physical and chemical changes in the TNT sample. The effects of heating and evacuating on the TNT sample were investigated. TNT concentration calibration curves were obtained for TNT in simulated soil mixtures. The limit of detection of TNT in soil was estimated to be 40 parts in 10⁹.

© 1996 Optical Society of America

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