Quantitative Measurements of Gas Phase O and N Atom Concentrations by ESR

The general theory relating the measured integrated intensities of ESR absorption lines to the concentrations of odd electron species in the gas phase is reviewed and discussed in some detail. It is then applied in the form required for determining the absolute concentrations of O, N, and H atoms using O₂ as a calibrating gas as first suggested by Krongelb and Strandberg. Experimental data are presented which verify the theoretical predictions of the intensities of various O₂ lines. The determination of absolute O and N concentrations by ESR is shown to be a reliable experimental technique by comparison with the independent results of titration with NO₂ and NO, respectively. Agreement of the two techniques strongly supports the validity of both. The O–NO₂ titration curves enable a value for the rate constant of $$\mathrm{O}+\mathrm{NO}+{\mathrm{O}}_2\to {\displaystyle {lim}_{k_6}}{\mathrm{NO}}_2+{\mathrm{O}}_2$$ to be evaluated at 300°K as 2.9×10¹⁶ cm⁶ mole^—2 sec^—1 in agreement with other reported values.