Photoinduced oxidation of sea salt halides by aromatic ketones: a source of halogenated radicals

The interactions between triplet state benzophenone and halide anion species (Cl < sup, Br < sup and I < sup have been studied by laser flash photolysis (at 355 nm) in aqueous solutions at room temperature. The decay of the triplet state of benzophenone was followed at 525 nm. Triplet lifetime measurements gave rate constants, k(q) (M-1 s < sup-1), close to diffusion controlled limit for iodide (similar to 8x10(9) M-1 s < sup-1), somewhat less for bromide (similar to 3x10(8) M-1 s < sup-1) and much lower for chloride (< 10(6) M-1 s < sup-1). The halide (X-) quenches the triplet state; the resulting product has a transient absorption at 355 nm and a lifetime much longer than that of the benzophenone triplet state, is formed. This transient absorption feature matches those of the corresponding radical anion (X-2 < sup). We therefore suggest that such reactive quenching is a photosensitized source of halogen in the atmosphere or the driving force for the chemical oxidation of the oceanic surface micro layer.