Detection of NO and NO(2) by (2 + 2) resonance-enhanced multiphoton ionization and photoacoustic spectroscopy near 454 nm.

Trace concentrations of NO and NO2 are detected with a dye laser operating near 454 nm. NO is detected by a (2 + 2) resonance-enhanced multiphoton ionization process by means of NO A2∑+–X2Π(0, 0) transitions with miniature electrodes, and NO2 is detected by a one-photon absorption photoacoustic process by means of NO2A˜′2B1(0,8,0)−X˜2A1(0,0,0) transitions with a miniature microphone. Rotationally resolved excitation spectra show that the spectral resolution is sufficiently high to identify these species at 1 atm. The technique’s analytical merits are evaluated as functions of concentration, pressure, and laser intensities. Low laser intensities favor NO2 photoacoustic detection whereas high laser intensities favor NO ionization. Limits of detection (signal-to-noise ratio 3) of 160 parts in 109 for NO and 400 parts in 109 for NO2 are determined at 1 atm for a 10-s integration time. Signal response and noise analyses show that three decades of NO/NO2 mixtures can be measured with a computational relative error in concentration that is three times the relative error in measuring the NO and NO signals.2