Measurement of the absolute third-order rate constant for the reaction between Cs + I + He by time-resolved atomic resonance fluorescence monitoring of iodine atoms in the vacuum ultraviolet region {I[6s(2PJ)]–I[5p5(2P3/2)]} coupled with steady atomic resonance fluorescence on atomic caesium [Cs(72PJ)– Cs(62S1/2)] in the visible region

We present a direct kinetic measurement of the absolute third-order rate constant for the reaction Cs + I + He → CsI + He. (1) I(52P3/2) was generated by the repetitive pulsed irradiation of CsI produced in a flow system from the reaction of CH3I in the presence of excess atomic caesium derived from a heat-pipe oven. The ground-state iodine atom was monitored by time-resolved atomic resonance fluorescence at λ= 178.3 nm {I[5p46s(2P3/2)]–I[5p5(2P3/2)]} using photon counting in the vaccum ultraviolet region. Cs(62S1/2) was monitored by steady atomic resonance fluorescence of the Rydberg doublet at λ= 457 nm [Cs(72PJ)–Cs(62S1/2)] using phase-sensitive detection. We report a measured absolute third-order rate constant of k1(T= 491 K)=(7.9 ± 1.2)× 10–31 cm6 atom–2 s–1, which we believe to be the first measurement of its kind. We also report an estimate of the rate constant for the reaction I + Cs2→ CsI + Cs (3) of k3(T= 491 K)≈ 2 × 10–10 cm3 molecule–1 s–1, which is found to be in accord with previous kinetic measurements on I + Na2 and Br + K2. The crossing between the covalent and the ionic surface, described by the Rittner potential function, is found to take place at 17.3 A, and hence the recombination is governed by the large impact parameters involved on collision. Consideration of the splitting between the two surfaces, coupled with the Landau–Zener formalism and Monte Carlo calculations of trajectories on those surfaces, yields a good quantitative account of the observed kinetic behaviour, not only of the present recombination measurements but also of shock-tube data that have been reported for the dissociation of CsI at elevated temperatures (2400 K). In view of this, despite the clear third-order kinetic behaviour observed here for reaction (1), detailed theoretical consideration of the ‘fall-off’ pressure regime found to satisfy both the present measurements and the shock-tube data leads to the preferred value of k1(T = 491 K)= 9.1 × 10–31 cm6 atom–2 s–1(±25%). The combination of the recombination and dissociation-rate measurements and the modelling calculations lead to the temperature dependence of the form k1(491 < T/K < 2400)= 4.1 × 10–30T–0.24 cm6 atom–2 s–1.