Spectral differentiation of trace concentrations of NO_2 from NO by laser photofragmentation with fragment ionization at 226 and 452 nm: quantitative analysis of NO–NO_2 mixtures

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Abstract
Laser-induced photofragmentation with fragment ionization is used to detect and spectrally differentiate trace concentrations of NO2 from NO in NO–NO2 mixtures. A laser operating near 226 or 452 nm ionizes the target molecules, and the resulting electrons are collected with miniature electrodes. NO is detected by (1 + 1) resonance-enhanced multiphoton ionization by means of its A 2Σ+ ← X 2Π (0, 0) transitions near 226 nm, whereas NO2 is detected near 226 nm by laser photofragmentation with subsequent NO fragment ionization by means of both its A 2Σ+ ← X 2Π (0, 0) and (1, 1) transitions. The NO fragment generated from the photolysis of NO2 is produced rovibrationally excited with a significant population in the first vibrational level of the ground electronic state (X 2Π, v″ = 1). In contrast, ambient NO has a room-temperature, Boltzmann population distribution favoring the lowest ground vibrational level (X 2Π, v″ = 0). Thus discrimination is possible when the internal energy distributions of both fragment NO and ambient NO are probed. We also demonstrate this approach using visible radiation, further simplifying the experimental apparatus because frequency doubling of the laser radiation is not required. We measured up to three decades of NO–NO2 mixtures with limits of detection (signal-to-noise ratio of 3) in the low parts per billion for both NO and NO2 for a 10-s integration time using both ultraviolet or visible radiation.