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.