Measurement of the absolute third-order rate constant for the reaction between Cs + OH + He determined by time-resolved molecular resonance-fluorescence spectroscopy, OH(A2Σ+–X2Π), coupled with steady atomic resonance fluorescence, Cs(7 2PJ–6 2S1/2)

Abstract

We present the first direct measurement of the absolute third-order rate constant (k₁) for the reaction Cs + OH + He → CsOH + He (T= 481 K) completing the series of analogous studies with the present experimental system we have also described for Na, K and Rb. OH(X²Π), generated by the pulsed irradiation of H₂O vapour through CaF₂ optics in a slow-flow system, kinetically equivalent to a static system, was monitored by time-resolved resonance fluorescence at λ= 307 nm [OH(A²Σ⁺–X²Π), (0, 0)] in the presence of an excess of atomic caesium derived from a heat-pipe oven and helium buffer gas. The atomic caesium was monitored in the steady mode using atomic resonance fluorescence of the unresolved Rydberg doublet at λ= 457 nm [Cs(7²P_J– 6²S_1/2)] coupled with phase-sensitive detection. The following value of k₁ was obtained: k₁(481 K)=(10.0 ± 1.5)× 10^–31 cm⁶ molecule^–2 s^–1. This result was extrapolated to 2000 K using the unimolecular reaction rate theory by Tröe, including the quantitative effects of hindered rotation on the low-frequency bending modes of CsOH. We have employed the same formalism to calculate the rate constant for the reaction between Li + OH + He at 500 and 2000 K. The resulting calculated temperature dependences of the third-order rate constants (k_R) for the reaction X + OH + He → XOH + He are thus compared: [graphic omitted]. Extrapolation of the rate data to flame conditions is considered with particular emphasis on the role of H₂O as a third body in recombination reactions of this type. It is concluded that a reassessment of the standard collisional deactivation efficiency, β_c, for H₂O at elevated temperatures can be used to reconcile former differences between recombination rate data extrapolated from measurements of the present type with those derived from modelling on flames.