Surface photochemistry of bromoform on ice: cross section and competing reaction pathways

The 266 nm photodissociation of bromoform adsorbed on an amorphous solid water (ASW) layer has been investigated for the first time under well-defined ultrahigh vacuum conditions. Time-of-flight (TOF) measurements indicate direct release of gas-phase Br, CHBr₂, Br₂, and CHBr species, with potential implications for stratospheric chemistry. Furthermore, new, ice-surface-mediated C-C (C₂H₂Br₂) and C-O (CHBrO, CO) species are revealed in postirradiation temperature programmed desorption (TPD) and reflection absorption infrared (RAIR) spectra. A cross section of ~5X10^-20 cm² is determined for bromoform photodissociation at 266 nm based on the integrated area of both the TOF spectra of Br and Br₂ and the postirradiation TPD curves of CHBr₃. The involvement of the free, non-hydrogen-bonded water groups at the ASW surface in the formation of the photoproducts is evident from the RAIRS results.