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(62PJ–52S1/2)

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 2PJ–5 2S1/2), 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 Troe 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 (kR): [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.