US Government As Represented by the Secretary of the Army

Josef B Simeonsson

Rosario C Sausa

George W Lemire

A method and apparatus is provided for real-time detection of trace amounts of atmospheric and tropospheric halogen-containing compounds. The present invention can be used either for in situ detection or remote sensing. A laser is employed to induce photofragmentation of the halogenated molecules and facilitate detection of the characteristic atom fragment by stimulated emission spectrometry. For brominated compounds, the output of a single laser is tuned to the strong two-photon 4p.sup.4 5p .sup.4 D.degree..sub.3/2 .rarw.4p.sup.5 2 P.degree..sub.3/2 transition of Br at 260.634 nm. The Br atoms are subsequently detected by stimulated emission (SE) via the 4p.sup.4 5p .sup.4 D.degree..sub.3/2 .fwdarw.4p.sup.4 5s .sup.4 P.sub.5/2 transition at 844 nm. For comparison, the laser-induced fluorescence (LIF) signal at the same wavelength is also monitored. The SE signal is distinct from the fluorescence in that it is coherent, bidirectional, and propagates coaxially with the laser beam. As a result of photophysical amplification, it is approximately two orders of magnitude greater than the LIF signal. The present invention has been demonstrated for trace detection of CHBr.sub.3 and CHClBr.sub.2 in N.sub.2 or air at total gas pressures of 1-760 Torr. Detection limits are in the parts-per-million range. The absorption cross section of the two-photon 4p.sup.4 5p .sup.4 D.degree..sub.3/2 .rarw.4p.sup.5 2 P.degree..sub.3/2 transition is estimated to be 3.8.times.10.sup.-43 cm.sup.4.

Detector of halogenated compounds based on laser

A method for the ultra-sensitive detection of atmospheric and surface adsorbed nitrocompounds such as propellants, explosives, and nitro-pollutants employs an excimer (ArF) laser operating at or near 193 nm to photofragment the target molecule and facilitate the detection of the characteristic NO fragment, formed from the dissociation of NO.sub.2, by resonance-enhanced multiphoton ionization (REMPI) and/or laser induced fluorescence (LIF) via its A-X, B-X, C-X, and/or D-X electronic transitions. In addition to NO, the detection of other fragments, such as C, CH, C.sub.2, NH and OH, by their ionization and/or prompt emission resulting from 193 nm excitation enhances the analyte identification. The analytical utility of this apparatus and method has been demonstrated on a number of compounds including dimethylnitramine (DMNA), nitromethane, nitrobenzene, orthonitrotoluene, para-nitrotoluene and trinitrotoluene (TNT). With the present system, limits of detection in the tens to hundreds of parts-per-billion (ppb) range have been achieved in the gas phase for all of these compounds, using only 1 mJ of laser energy at 193 nm.

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Iowa City, IA, United States of America

Josef B Simeonsson