Photochemical Mechanisms in Atmospherically Relevant Iodine Oxide Clusters

Atmospheric new particle formation events can be driven by iodine oxides or oxoacids via both neutral and ionic mechanisms. Photolysis of new particles likely plays a significant role in their growth mechanisms, but their spectra and photolysis mechanisms remain difficult to characterize. We recorded ultraviolet (UV) photodissociation spectra of (I₂O₅)_0-3(IO₃^-) clusters, observing loss of an O atom, I₂O₄, and (I₂O₅)_1,2 in the atmospherically relevant range of 300-340 nm. With increasing cluster size, the intensity of absorption red shifts and generally increases, suggesting particles photolyze more frequently as they grow. Estimates of the rates indicate that even relatively small clusters are likely to undergo photolysis under high-UV conditions. Vibrational spectra identify the covalent moiety I₃O₈^- as the likely chromophore, not IO₃^-. The I₂O₅ loss pathway competes with particle growth, while the slower O loss pathway likely produces ³O + ³(cluster) products that could drive subsequent intraparticle chemistry, particularly with co-adsorbed organic or amine species.