A direct kinetic study of the reaction K + OH + He → KOH + He by time-resolved molecular resonance-fluorescence spectroscopy, OH(A2∑+–X2Π), coupled with steady atomic fluorescence spectroscopy, K(52PJ–42S1/2)

Abstract

We present the first direct measurement of the absolute rate constant for the third-order reaction K + OH + He → KOH + He. A complex experimental system is described in which a heat-pipe oven, from which atomic potassium is generated, is coupled to a high-temperature reactor for time-resolved resonance-fluorescence measurements on OH following pulsed irradiation. The apparatus is a slow-flow system kinetically equivalent to a static system. Atomic potassium is monitored in the steady mode using resonance fluorescence of the Rydberg transition at λ= 404 nm K[(5 ²P_J)–(4 ²S_1/2)] coupled with phase-sensitive detection. Ground-state OH(X²Π), generated by the repetitively pulsed irradiation of water vapour, is monitored in the time-resolved mode at λ= 307 nm [OH(A²∑⁺–X²Π), (0, 0)] using molecular resonance-fluorescence measurements following optical excitation with pre-trigger photomultiplier gating, photon counting and signal averaging. Thus the decay of OH(X²Π) as a function of time is studied both as a function of [K(4²S_1/2)] and [He], yielding the absolute rate constant k₃(K + OH + He)=(8.8 ± 1.8)× 10^–31 cm⁶ molecule^–2 s^–1(T= 530 K). A full account is given of the isolation of this reaction by the use of a ‘chemical window’ through the control of temperature and the effective elimination of the reaction between K₂+ OH, and the use of He as the third body which demonstrates negligible collisional quenching efficiency with OH(A²∑⁺, v′= 0). We also present a detailed extrapolation of the rate data to the environment of flames using unimolecular-reaction-rate theory developed by Tröe and coworkers for dissociation reactions. The agreement between the present results for the isolated reaction extrapolated to flame temperatures and measurements on flames (1800 < T/K < 2200), in which k₃(K + OH + M)(where M stands for the burnt gases of a fuel-rich flame) has been extracted by modelling the complex coupled equilibria, is considered highly satisfactory. Quantitative account of the modelling of the dissociation of KOH is presented in the paper.