Recombination of ground state halogen atoms. Part 3.—Radiative Recombination of Br2P Atoms

Abstract

In the recombination of Br ²P atoms, the A³Π(1_u) excited state of Br₂ is formed directly, and gives rise to the A³Π(1_u)→X¹∑⁺_g chemiluminescent emission spectrum at wavelengths λ > 0.62 µm. The mechanism and kinetics of formation and decay of Br₂A³Π(1_u) 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, ∑I_λ, obeyed the kinetic equation, [Br]²/∑I_λ=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(1_u) 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, k_1,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/k_1,Ar= 22; k_1,He/k_1,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 k_1,He are reported: k_1,Br2=–(9_.6± 3_.0) kJ mol^–1; k_1,Ar=–(7_.9± 3_.0) kJ mol^–1.