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In order to determine the efficiency with which triple collisions bring about the recombination of free OH radicals, the rate constant for the reaction must be known in absolute units. Consequently it is necessary to measure in absolute units the ``standard concentration'' on which is based the rate constant previously determined spectroscopically by Frost and Oldenberg. For this purpose the intensities of the OH absorption lines in their ``standard discharge'' were measured and combined with the f values recently published. Because of the very small width of the OH absorption lines in the discharge, Fabry-Pérot plates had to be used in order to produce absorption spectra with a resolution sufficiently high for suitable intensity measurements. It was found that the efficiency of triple collisions is higher than unity if the number of such collisions is computed from gas kinetic data. This result indicates that OH forms collision complexes which have a lifetime much greater than the duration of bimolecular collisions between stable molecules.
Oldenberg et al. (1939) studied this question.