Detector of halogenated compounds based on laser photofragmentation/fragment stimulated emission

摘要

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 4p45p 4D DEG 3/2<-4p5 2P DEG 3/2 transition of Br at 260.634 nm. The Br atoms are subsequently detected by stimulated emission (SE) via the 4p45p 4D DEG 3/2->4p45s 4P5/2 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 CHBr3 and CHClBr2 in N2 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 4p45p 4D DEG 3/2<-4p5 2P DEG 3/2 transition is estimated to be 3.8x10-43 cm4.