Determination of the absolute third-order rate constant for the reaction between Na + OH + He by time-resolved molecular resonance-fluorescence spectroscopy, OH(A2∑+–X2Π), coupled with steady atomic fluorescence spectroscopy, Na(3 2PJ– 32S1/2)

Abstract

We present the first direct measurement of the absolute third-order rate constant for the reaction Na + OH + He → NaOH + He. OH(X²Π) was generated by the repetitive pulsed irradiation of H₂O vapour through CaF₂ optics in the presence of He and an excess of atomic sodium derived from a heat-pipe oven and comprising part of a slow-flow system, kinetically equivalent to a static system. Ground-state OH was monitored by time-resolved molecular resonance fluorescence at λ= 307 nm [OH(A²∑⁺–X²Π), (0,0)] following optical excitation using pre-trigger photomultiplier gating, photon counting and signal averaging. Na(3 ²S_1/2) was monitored in the steady mode using atomic resonance fluorescence at λ= 589 nm [Na(3 ²P_J)–Na(3 ²S_1/2)] coupled with phase-sensitive detection. The following absolute third-order rate constant (T= 653 K) was obtained: k₁(Na + OH + He)=(1.07 ± 0.21)× 10^–30 cm⁶ molecule^–2 s^–1. Quantitative extrapolation to high temperatures using the unimolecular reaction rate theory developed by Tröe yields the result k₁(Na + OH + He)=(4.7 ± 1.0)× 10^–26T^–1.65 cm⁶ molecule^–2 s^–1. Furthermore, modifications to these calculations for flame conditions (M) indicate that k₁(Na + OH + M) lies in the range (1–5)× 10^–31 cm⁶ molecule^–2 s^–1 at 2000 K. This is significantly different to the values of k₁ derived from modelling in flames. The role of Na + OH + M in flames seeded with atomic sodium is considered in this paper.