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
2000
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 A2Σ+ ← X2Π (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 A2Σ+ ← X2Π (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 (X2Π, v″ = 1). In
contrast, ambient NO has a room-temperature, Boltzmann population
distribution favoring the lowest ground vibrational level (X2Π, 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.
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