Gaseous Detonations. XII. Rotational Temperatures of the Hydroxyl Free Radicals

Abstract

An apparatus is briefly described which permits the photography of the ultraviolet absorption spectrum of the hydroxyl free radicals in gaseous detonation waves with a time resolution of a few microseconds. Using two mixtures, 2H₂+O₂ and 2H₂+O₂+0.075 C₂H₂ at 20 mm pressure, changes were studied in the absorption spectra with the progress of chemical reactions in a detonation wave. Densitometry of the absorption lines in the R₂ branch of the O–O band in the ${}^2{\Sigma }^{+}{\to }^2\pi$ transition, combined with the use of the iso‐intensity method, showed that the ``rotational temperatures'' of the radicals in their ground electronic state are initially much higher than the ``translational temperatures'' calculated from the experimentally observed and elsehere reported density‐time curves in detonation waves of similar gas mixtures. After the completion of the reactions and the attainment of the Chapman‐Jouguet state the rotational temperature was found to be slightly lower than the calculated equilibrium temperature, which is attributed to systematic experimental errors. The observed initial lack of equilibration between the translational and rotational temperatures is interpreted to mean that the lifetime of hydroxyl radicals in partially reacted mixtures is comparable to their rotational relaxation time, i.e., is of the order of the time for ten collisions.