Standoff detection of nitrotoluenes using 213-nm amplified spontaneous emission from nitric oxide

A method of standoff detection based on the observation of laser-induced fluorescence–amplified spontaneous emission (LIF-ASE) is described. LIF-ASE generates uniaxial intensity distributions of the observed fluorescence with the majority of intensity propagating along the excitation axis in both the forward and backward directions. The detection of bulk vapor at significant standoff distances is readily achieved. This method was used to detect NO directly and as a photoproduct after 213-nm excitation of 2-, 3-, and 4-nitrotoluene. The NO LIF-ASE spectra were studied as a function of buffer gas. These studies showed that the emission from different vibrational states was dependent upon the buffer gas used, suggesting that the populations of vibrational states were influenced by the environment. A similar sensitivity of the vibrational populations was observed when the different nitroaromatic precursors were used in nitrogen buffer gas. Such sensitivity to environmental influences can be used to distinguish among the different nitroaromatic precursors and facilitate the identification of the bulk vapor of these analytes.