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 approximately 5 x 10(-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.