Quenching of excited hydroxyl (2Σ+, υ′ = 0) radicals in flames

An alternating current photoelectric device (compare the work of Boers et al.,(1)Alkemade,(2) Hooymayers et al.(3,4) and Hooymayers(5)) has been used for determining the yield factor p of resonance fluorescence for some specific rotational transitions of the (0,0) band of the hydroxyl 2Σ+ → 2Π-band system. Excitation of the selected rotational levels was achieved by irradiating the flame with a hollow cathode bismuth-lamp emitting the 3067·72 Å Bi-line. From the p-values measured in three different hydrogen flames at about 1500–1800°K and 1 atm pressure the specific effective cross-sections(4,5) S of the excited radical in collision with N2, O2 and H2O molecules as quenchers could be derived with an accuracy of about 18 per cent. Here the quenching cross section S is defined as π times the square of the distance between the centers of the colliding species. In contrast with the results of our quenching experiments on excited alkali-atoms, (4,5) it was found that the S-value for H2O as quencher (SH2O = 37 ± 6 Å2) exceeds the values for the other molecules mentioned (SO2 = 10 ± 2 Å2 and SN2 = 7 ± 1 Å2) by a factor of at least 4. Combining our results with other data obtained by Zeegers(6–8) at our laboratory in OH-chemiluminescence experiments, it appeared that at our flame conditions, (4,5) quenching of excited OH(2Σ, υ′ = 0) radicals by H2O molecules is partly (≈ per cent) achieved by dissociative quenching according to the reaction OH* + H2O → OH + OH + H