The UV–Vis spectrum of the ClCO radical in the catalytic cycle of Cl-initiated CO oxidation

In Venus's mesosphere, the observation/model discrepancy of molecular oxygen, O₂, abundance has been a long-standing puzzle. Chlorine atoms have been proposed as a catalyst to oxidize carbon monoxide through the formation of chloroformyl radicals (ClCO), removing O₂ and ultimately generating CO₂. However, relevant kinetic studies of this catalytic cycle are scarce and highly uncertain. In this work, we report the spectrum of the ClCO radical between 210-520 nm using a multipass UV-Vis spectrometer coupled to a pulsed-laser photolysis flow reactor at 236-294 K temperature and 50-491 Torr pressure ranges. High-level ab initio calculations were performed to simulate the observed spectrum and to investigate the electronic structure. In addition, we observed the formation of molecular chlorine, Cl₂, and phosgene, Cl₂CO, suggesting that both the terminal chlorine and the central carbon in the ClCO radical are reactive towards chlorine atoms. Most importantly, the reported spectrum will enable future measurements of essential kinetic parameters related to ClCO radicals, which are important in regulating the O₂ abundance in Venus's mesosphere.