Absolute third-order rate constant for the reaction between Rb + OH + He determined by time-resolved molecular resonance-fluorescence spectroscopy, OH (A2Σ+–X2Π), coupled with steady atomic resonance-fluorescence measurements, Rb(6 2PJ–5 2S1/2)

Abstract

We present the first direct measurements of the absolute third-order rate constant (k₂) for the reaction Rb + OH + He → RbOH + He. OH(X²Π) was generated from the repetitive pulsed irradiation of H₂O vapour through CaF₂ optics in the presence of an excess of atomic rubidium, derived from a heat-pipe oven, and helium buffer gas. The decay of ground-state hydroxyl radical was monitored by OH(A–X) resonance fluorescence following optical excitation in a flow system, kinetically equivalent to a static system, using signal averaging. Atomic rubidium was monitored in the steady mode via the Rydberg transition, Rb(6 ²P_J–5 ²S_1/2), at λ= 421 nm using phase-sensitive detection. The following value for k₂ was obtained: k₂=(8.8 ± 1.3)× 10^–31 cm⁶ molecule^–2 s^–1(T= 490 K). This was extrapolated to high temperatures (2000 K) using the unimolecular reaction rate theory developed by Tröe and incorporating, in a quantitatively detailed manner, the effects of hindered rotation on the low-frequency bending modes of RbOH. The extrapolations for the previous rate measurements we have reported for the analogous reactions involving Na and K have been reanalysed to include the effects of hindered rotation in the same manner, yielding the following forms for the recombination rate constants (k_R): [graphic omitted]. The rate data extrapolated to 2000 K are also considered in terms of the role of alkali-metal hydroxides and alkali-metal atoms in flame inhibition.