Kinetics of formation of electronically-excited Cl2, BrCl and Br2 in the recombination of ground-state halogen atoms

Abstract

Chlorine atoms, Cl 3p⁵²P_J, were produced in a flow of argon carrier gas at total pressures between 0.5 and 5 Torr; bromine atoms, Br 4p⁵²P, were formed by the rapid reaction of Cl atoms with Br₂: Cl + Br₂→ BrCl + Br. The kinetics of the following chemiluminescent reactions (the “halogen afterglows”) were investigated: Cl + Cl + M → Cl₂B³Π(0⁺_u)+ M, Br + Cl + M → BrCl B³Π(0⁺)+ M, Br + Br + M → Br₂A³Π(1_u), B³Π(0⁺_u)+ M. Atomic resonance absorption in the vacuum ultraviolet was used to measure Cl and Br atom concentrations directly. Chemiluminescence intensities were measured at lower concentrations (e.g., 4 × 10¹³[Cl] 5 × 10¹² cm^–3) than in previous work. Under these conditions, the kinetics were simplified by elimination or reduction of quenching by halogen atoms of the excited states of the diatomic halogen. Quantum efficiencies Φ for formation of excited state halogen molecules by atom recombination were estimated as follows: Cl + Cl + M, Φ= 0.043; Br + Cl + M, Φ= 0.038; Br + Br + M, Φ= 0.059. Photon emission rates and steady-state concentrations of excited Cl₂ and BrCl were determined.

Independent work on laser-induced fluorescence, involving defined quantum states of Cl₂(B) and BrCl (B), has been used to formulate a model for the halogen afterglows. Overall third-order recombination is believed to occur by consecutive bimolecular steps: (i) two-body recombination regarded as inverse predissociation, followed by (ii) vibrational or rotational energy relaxation, leading to stabilisation of the newly-formed excited Cl₂(or BrCl).