DETECTION OF NO AND NO2 BY (2-RESONANCE-ENHANCED MULTIPHOTON IONIZATION AND PHOTOACOUSTIC-SPECTROSCOPY NEAR 454 NM(2))

Trace concentrations of NO and NO2 are detected with a dye laser opera ting near 454 nm. NO is detected by a (2 + 2) resonance-enhanced multi photon ionization process by means of NO A (2) Sigma(+)-X (II)-I-2(0, 0) transitions with miniature electrodes, and NO2 is detected by a one -photon absorption photoacoustic process by means of NO2 (A) over tild e' B-2(1)(0, 8, 0)- (X) over tilde (2)A(1)(0, 0, 0) transitions with a miniature microphone. Rotationally resolved excitation spectra show t hat the spectral resolution is sufficiently high to identify these spe cies at 1 atm. The technique's analytical merits are evaluated as func tions of concentration, pressure, and laser intensities. Low laser int ensities favor NO2 photoacoustic detection whereas high laser intensit ies favor NO ionization. Limits of detection (signal-to-noise ratio 3) of 160 parts in 10(9) for NO and 400 parts in 10(9) for NO2 are deter mined 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 wi th a computational relative error in concentration that is three times the relative error in measuring the NO and NO2 signals. (C) 1996 Opti cal Society of America