Next, we sought to determine the nature of the chromophore responsible for UV absorption. We recorded vibrational spectra of the clusters (I 2 O 5 ) 1-3 (IO - 3 ) using cryogenic ion vibrational presdissociation spectroscopy (CIVP). CIVP spectroscopy allows the vibrational spectra of clusters with specific compositions to be recorded. Thus, it is possible to establish experimentally present structures by comparing quantum-chemically-computed vibrational spectra for a number of structures to the experimental spectra. The experimental spectra are shown in black, and the computed spectra for the lowest energy isomers are shown in grey in Figure . The approach to generate the computed spectra was validated previously for clusters of this type and is described in the Experimental Methods. The lowest energy optimized structures are shown alongside their corresponding computed spectra and their 0 K, zero-point-corrected internal energies. We explored previously reported structures of these cluster compositions as well as derivatives of ones we previously reported as a starting point for structure generation. Further candidate structures were developed using chemical intuition, and all were optimized to local minima. While this approach does not exhaustively explore the structural space available to these clusters, it is sufficient to identify the structural motif associated with the UV chromophore.