In summary, we have found that clusters of the (I 2 O 5 ) n (IO - 3 ) family photofragment by loss of O, I 2 O 5 , and I 2 O 4 . Clusters of increasing size appear to absorb progressively more in the atmospherically relevant region of the UV as they grow, with the larger clusters studied here absorbing up to 340 nm. We also identified the presence of I 3 O - 8 in all of the clusters studied using IR spectroscopy, and ultimately identified the cluster chromophore to be I For spectra displayed in Figure , UV light was generated through sum frequency generation (SFG) of the signal from a similar tunable OPO and the 1064 nm pump light produced by another Continuum Surelite EX laser. Ions were mass selected in the TOF and irradiated at at the space focus of the linear stage of the TOF. A reflectron was used to mass separate the photofragments from the non-fragmented ions.