Abstract

A new method for detecting trace vapors of NO<sub>2</sub>-containing compounds near atmospheric conditions has been demonstrated with the use of one-color-laser photofragmentation/ionization spectrometry. An ArF laser is employed to both photolytically fragment the target molecules in a collision-free environment and ionize the characteristic NO fragments. The production of NO is hypothesized to result from a combination of two NO<sub>2</sub> unimolecular fragmentation pathways, one yielding NO in its <i>X</i><sup>2</sup>Π electronic ground state and the other in its <i>A</i><sup>2</sup>Σ<sup>+</sup> excited state. Ionization of ground-state NO molecules is accomplished by resonance-enhanced multiphoton ionization processes via its <i>A</i><sup>2</sup>Σ<sup>+</sup> ← <i>X</i><sup>2</sup>Π (3, 0), <i>B</i><sup>2</sup>Π ← <i>X</i><sup>2</sup>Π (7, 0) and/or <i>D</i><sup>2</sup>Σ<sup>+</sup> ← <i>X</i><sup>2</sup>Π (0, 1) bands at 193 nm. The analytical utility of this method is demonstrated in a molecular beam time-of-flight apparatus. Limits of detection range from the parts-per-million (ppm) to parts-per-billion (ppb) level for NO, NO<sub>2</sub>, CH<sub>3</sub>NO<sub>2</sub>, dimethylnitramine (DMNA), <i>ortho-</i> and <i>meta</i>-nitrotoluene, nitrobenzene, and trinitrotoluene (TNT). Under effusive beam experimental conditions, discrimination between structural isomers, <i>ortho</i>-nitrotoluene and <i>meta</i>-nitrotoluene, has been demonstrated with the use of their characteristic photofragmentation/ionization mass spectra.

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