Absolute third-order rate constants for the recombination reactions between alkali-metal and iodine atoms and the measurement for Rb + I + He

Abstract

We present a direct measurement of the absolute third-order rate constant for the reaction Rb + I + He → RBI + He (1) which is considered within the context of the recombination of alkali-metal atoms and atomic iodine in general. I(5 2P3/2) was generated by the repetitive pulsed irradiation of RbI in a flow system with He, and monitored by time-resolved atomic resonance fluorescence in the vacuum u.v. at λ= 178.3 nm {I[5p4 6s(2P3/2)]–I[5p5(2P3/2)]} using photon counting. Rb(52S1/2) was monitored in the steady mode by atomic resonance fluorescence at λ= 421 nm [Rb(6 2PJ)–Rb(52S1/2)] using phase-sensitive detection. These measurements yield the value of k1(T= 540 K)=(3.34 ± 0.67)× 10–31 cm6 atom–2 s–1. We also report an estimate of the rate constant for the reaction I(5 2P3/2)+ Rb2→ RbI + I (3) of k3(T= 540 K)= 4.1 × 10–10 cm3 molecule–1 s–1, in accord with previous measurements for Br + K2 derived from molecular beams. The recombination reaction was modelled using trajectory calculations with Monte Carlo averaging, coupled with the Landau–Zener formalism for the covalent and ionic surfaces, the latter being constructed using the Rittner potential. The calculated result was in good accord with the experimentally measured result and further employed for extrapolation of k1 to T= 2400 K. Detailed theoretical consideration of the ‘fall-off’ region indicated that the present measurements have been carried out in the region of third-order kinetics. Direct experimental measurements of the recombination between Cs, Rb, K + I + He have now been reported by the present method, together with calculations of these rate constants that are in good accord with experiment. The analogous reactions for the lighter elements, Na and Li, lie beyond the temperature range accessible by the present experimental system, and hence for these we report analogous trajectory calculations only. The overall body of data may be summarised as follows: [graphic omitted] This set of rate constants emphasises the small temperature dependences for all five recombination processes, a direct result of the recombination dynamics occurring on an ionic surface in each case.