Measurement of the Absolute Third-Order Rate Constant for the Reaction between K+I+He by Time-Resolved Atomic Resonance Fluorescence Monitoring of Iodine Atoms in the Vacuum Ultraviolet(I(5p46s(2P3/2))-(I(5p46s(2P03/2))) Coupled with Steady Atomic Resonance Fluorescence ...

Article abstract of DOI:10.1021/j100275a030

The absolute third-order rate constant for the reaction K+I+He->KI+He has been measured directly by monitoring atomic iodine under pseudo-first-order conditions in the presence of excess atomic potassium, derived from a heat pipe oven, and helium.I(5²P_3/2) was generated by the repetitive pulsed irradiation of KI vapor and studied by time-resolved resonance fluorescence at λ=178.3 nm (I(5p⁴6s(²P⁰_3/2))) using photon counting and signal averaging.K(4²S_1/2 was monitored in steady mode by resonance fluorescence of the Rydberg transition at λ=404 nm (K(5²P_J)-K(4²S_1/2)) using phase-sensitive detection.We report the third-order rate constant of k₁(T=567 K)=(3.04+/-0.73)x10^-31 cm⁶ atom^-2 s^-1.We also report an estimate of the rate constant for the reaction I+K2-> KI+K of k₃(T=567 K)= ca. 5x10^-10 cm³ molecule^-1 s^-1, in accord with the reaction cross section reported for the analogous reaction of Br+K2 derived from molecular beams.The recombination reaction has been modeled by trajectory calculations over the appropiate potential surfaces, the large value of k₁ being governed by the ionic surface constructed on the basis of the Rittner potential function, which crosses the covalent surface 11.3 Angstroem, and by the large impact parameters for reaction.Detailed dynamical calculations on these surfaces employing Monte Carlo techniques have been found to account both for the present measurements and for high-temperature dissociation rate data for KI derived from shock tube measurements.These calculations, coupled with the present measurements at T=567 K, indicate a low negative temperature dependence of the form k₁(567 K-31T^-0.13 cm⁶ atom^-2 s^-1, consistent with previous measurements and calculations on the analogous recombination between Cs+I+He and emphasizi ng the dominance of the ionic surface.