Developmental studies for remote and in-situ detection of halogenated compounds by laser-induced photofragmentation/fragment detection spectrometry

The analytical capabilities of a one-color, laser-induced photofragmentation/fragment detection technique for the remote and in situ detection of halogenated compounds is reported. For brominated compounds, 260.634-nm laser radiation is used to fragment the target compound and also excite the characteristic Br atom photofragment by means of its two-photon 4p⁴5p ⁴D_3/2⁰←4p^{5 2}P_3/2⁰ transition, whose absorption cross section is estimated to be 3.8×10^-43 m⁴. Detection is accomplished by (2+1) resonance-enhanced multiphoton ionization, as well as, laser-induced fluorescence and stimulated emission from the 4p⁴5p ⁴D_3/2⁰→4p⁴5s⁴ P_5/2.3/2 transitions at 844 and 751 nm, respectively. The SE signal is distinct from the LIF signal in that it is coherent, bidirectional, and propagates coaxially with the laser beam. It is also approximately two orders of magnitude greater than the LIF signal at 844 nm because of photophysical amplification. The technique has been demonstrated for CH₃Br, CHClBr₂, and CHBr₃ at total gas pressures of 1-760 Torr with detection limits in the parts-per-billion