Molecular halogen elimination from halogen-containing compounds in the atmosphere

Abstract

Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X₂ product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X₂ primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH₂Br₂, CF₂Br₂, CHBr₂Cl, and CHBr₃), dibromoethanes (1,1-C₂H₄Br₂ and 1,2-C₂H₄Br₂) and dibromoethylenes (1,1-C₂H₂Br₂ and 1,2-C₂H₂Br₂), diiodomethane (CH₂I₂), thionyl chloride (SOCl₂), and sulfuryl chloride (SO₂Cl₂), along with a brief discussion on acyl bromides (BrCOCOBr and CH₂BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the X₂ fragments have been characterized, especially for Br₂ and I₂. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation.