Absolute measurements of absorption cross sections of HgBr2 and HgBr using CRDS-laser photolysis

The redox chemistry of mercury in the gaseous atmosphere is the simplest aspect of its complicated environmental cycling, yet there are huge uncertainties in the kinetics of this chemistry. Atmospheric modeling of mercury of this chemistry has the potential to constrain interpretations of observations of mercury fluxes in ecosystems; however, that potential cannot be realized without reliable experimental data on the kinetics of mercury oxidation. Unfortunately, reported rate constants vary by up to two orders of magnitude. One major challenge is that laboratory kinetic studies of Hg chemistry involve side reactions whose importance we cannot determine unless we know initial radical concentration, e.g., [HgBr]₀. Determining these concentrations requires knowledge of the UV absorption cross-sections of HgBr₂ and HgBr (30-50%). Unfortunately, reported cross-sections of HgBr₂ vary considerably, and there is no data on the cross sections for HgBr.
We used Cavity Ring Down Spectroscopy (CRDS) to monitor HgBr₂, HgBr, and Hg (0) in the wavelength range from 253.0 nm to 257.5 nm. HgBr is generated by laser photolysis of HgBr₂ at 266 nm. CRDS of 10 vibronic bands of the HgBr (D²Π_3/2 – X²Σ⁺) system have been measured. By controlling the experimental conditions, we determined the wavelength-dependent absorption cross-sections for HgBr₂ and HgBr at 10 different wavelengths at specific temperatures and pressures. These data are useful for accurately measuring rate constants of HgBr reactions.