Recombination of ground state halogen atoms. Part 3.—Radiative Recombination of Br2PAtoms

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Abstract
In the recombination of Br 2P atoms, the A3Π(1u) excited state of Br2 is formed directly, and gives rise to the A3Π(1u)→X1∑+ g chemiluminescent emission spectrum at wavelengths λ > 0.62 µm. The mechanism and kinetics of formation and decay of Br2A3Π(1u) excited molecules during atom recombination have been studied in a discharge-flow system with Br2 and Ar as third bodies (298–475 K), and with He as third body (298 K). Br atom concentrations were measured from the changes in continuum absorption by molecular Br2 at λ= 416 nm. For M = Br2, Ar and He, the integrated intensity over all vibrational levels, ∑Iλ, obeyed the kinetic equation, [Br]2/∑Iλ=α+β[Br], with αβ[Br] under most experimental conditions. For M = Br2, β was apparently independent of [Br2], whilst for M = Ar and He, β varied inversely with [M]. The results are interpreted in terms of a scheme in which Br2A3Π(1u) molecules are formed near the dissociation limit by recombination (1); the Br2(A) molecules formed are predominantly removed from the system by specific quenching (4) by Br 2P atoms: Br + Br + M [graphic omitted] Br2(A)+ M (1), Br2(A)+ Br [graphic omitted] Br2(X1∑+ g)+ Br (4), Br2(A) [graphic omitted] Br2(X1∑+ g)+hν. (3) As for the B—X emission from Cl atom recombination, there is no evidence for significant electronic quenching of Br2A state molecules. Values for the rate constants, k1,M, for recombination of Br to Br2(A) are reported at 298 K: k 1,Br2 = 10(14.50 ± 0.20) cm6 mol–2 s–1; k 1,Br2 /k1,Ar= 22; k1,He/k1,Ar= 1.1. Only a small fraction of overall recombination of Br 2P atoms (approx. 1 %) thus occurs via the excited A state. Arrhenius temperature coefficients for k 1,Br2 and k1,He are reported: k 1,Br2 =–(9.6± 3.0) kJ mol–1; k1,Ar=–(7.9± 3.0) kJ mol–1.

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