I. Trace Analysis of NO/NO2 Mixtures by Laser Photofragmentation/ Fragment Photoionization Spectrometry at Visible Wavelengths

摘要

Trace concentrations of NO and NO2 molecules are differentiated spectrally using a visible dye laser and a simple flow cell with a pair of miniature electrodes for ion detection. NO is detected near 452 nm by (2+2) resonance-enhanced multiphoton ionization via its A2SIGMA(+) - X2II (0,0) transitions, while NO2 is detected by laser photofragmentation with subsequent fragment NO ionization via the A2SIGMA(+) - X2II (0,0) and (1,1) transitions. Spectral differentiation is possible since the internal energy of the NO photofragment differs from that of 'ambient'; NO. Measurement of vibrationally excited NO via its A2SIGMA(+) - X2II (0,3) band is also demonstrated at 517 nm. Rotationally resolved spectra of NO and fragment NO are analyzed using a multiparameter computer program based on two-photon energy-level expressions and line strengths for A2SIGMA(+) - X2II transitions. Boltzmann analysis of the P12 + O22 branch of the (0,0) band reveals that the rotational temperature of fragment NO is approximately 500 K compared to room temperature NO. Limits of detection (S/N = 3) of NO and NO2 are in the 20-40-ppbv range at 449.2, 450.7, and 452.6 nm for a 10-s integration time. The limit of detection of NO2 at 517.5 nm is 75 ppbv. The analytical utility of the technique for ambient air analysis is evaluated and discussed.