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(72PJ–62S1/2)

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(72PJ– 62S1/2)] 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 Troe, 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 (kR) 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, βc, 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.