A study of laser-induced fluorescence back-scattered amplified stimulated emission of bulk vapors

A method for the stand-off detection of vapors based on the observation of laser-induced fluorescence back-scattered amplified stimulated emission (LIF-BASE) is described. LIF-BASE 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 stand-off distances is readily achieved. Molecules of differing vapor pressures, quantum yields of fluorescence, and fluorescence lifetimes were examined under adjustable experimental conditions. Excitation energy, laser fluence, analyte concentration, detection angle, and stand-off distance were among the factors studied in order to determine optimal parameters for ASE generation from aromatic hydrocarbon, ketone, nitrogen oxide, and nitroaromatic vapors.;Materials of environmental, health, and security importance have been identified with a stand-off LIF-BASE detection technique developed in our laboratory. LIF-BASE has been shown to detect selectively naphthalene, benzene, acetone, nitric oxide (NO), and NO photofragmentation products of nitrogen dioxide (NO2), nitrotoluene, and nitrobenzene in the vapor phase with sensitivity in the part per million (ppm) to part per billion (ppb) concentration range. It was determined in this study that a high powered pulsed laser system can generate ASE from low concentration vapors for detection at stand-off distances in the back-scattered direction.;The major motivation of this work was to develop and optimize a remote optical-detection method for energetic and hazardous vapors from stand-off distances. The intrinsically low vapor pressure of most primary explosives is a significant challenge, but our new strategy increases the intensity of the back-scattered signal. Trace detection of explosives has been accomplished using photofragmentation (PF)-LIF detection schemes using a single UV laser pulse to fragment the analyte and subsequently identify and quantify NO fragments via emission intensity measurements. Due to the short excited-state lifetime of NO in the presence of oxygen, detection of nitric oxide under ambient conditions is difficult but possible with LIF-BASE. The detection of nitrated-toluene and -benzene vapors via fluorescent NO and NO2 photofragments has been demonstrated in the ppb range.;Photofragmentation of nitroaromatic vapors generate vibrationally hot NO and results in emission spectra that differ from thermally relaxed NO vapor. In addition, the NO fragmentation products exist in an environment consisting of the parent nitroaromatic molecule and additional organic photofragments. Collisions between NO and surrounding molecules result in vibrational deactivation of excited vibrational states. Collisional deactivation kinetics was measured by monitoring the emission of NO fragments as a function of time. Identification of energetic and explosive materials can be monitored by observing the emission of NO* alone. The LIF-BASE technique can therefore be used to distinguish among the different nitro-aromatic precursors in the presence of ambient NO and facilitate the identification of the bulk vapor of these analytes.