Quantitative ESR Measurements of Gas-Phase H and OH Concentrations in the H–NO2 Reaction

Abstract

An application of the intensity relations derived previously for determining absolute concentrations of H atoms and OH radicals in the gas phase by ESR spectroscopy is described. Magnetic-dipole transitions in O2 and electric-dipole transitions in NO are used as calibration standards for determining H and OH, respectively. The construction of a cylindrical TE011 cavity suitable for observing both types of transitions in a fast flow system is described. Results of measurements of H and OH concentrations during the ``titration'' reaction H+NO2→NO+OH are presented. The internal consistency of the theoretical OH intensity relations is demonstrated by experimental measurements on J=32 and J=52 OH lines. At reaction times of a few milliseconds, the H titration curve is shown to have an initial linear portion which extrapolates to an NO2/H ratio of unity. It then becomes curved as expected by the generation of an additional H from OH+OH→ lim k2H2O+O and OH+O→O2+H.The OH titration curves yield k2=1.2±0.3×1012 cm3 mole−1·sec−1 at 300°K in good agreement with the only other reported value. Finally, some qualitative observations on a striking generation of free electrons associated with the main H–NO2 reaction are noted.