| Abstract
| - The (CO2)n- clusters are thought to accommodate the excess electron by forming a localized molecular anion,or “core ion”, solvated by the remaining, largely neutral CO2 molecules. Earlier studies interpreteddiscontinuities in the (CO2)n- photoelectron spectra to indicate that both the CO2- and C2O4- species werepresent in a size-dependent fashion. Here we use vibrational predissociation spectroscopy to unambiguouslyestablish the molecular structures of the core ions in the 2 ≤ n ≤ 17 size range. Spectra are reported in the2300−3800 cm-1 region, which allows us to independently monitor the contribution of each ion through itscharacteristic overtone and combination bands. These signature bands are observed to be essentially intact inthe larger clusters, establishing that the CO2- and C2O4- molecular ions are indeed the only electronaccommodation modes at play. The size dependence of the core ion suggested in earlier analyses of thephotoelectron spectra is largely confirmed, although both species are present over a range of clusters near theexpected critical cluster sizes, as opposed to the prompt changes inferred earlier. Perturbations in the bandsassociated with the nominally neutral CO2 “solvent” molecules are correlated with the changes in the molecularstructure of the core ion. These observations are discussed in the context of a diabatic model for electrondelocalization over the CO2 dimer. In this picture, the driving force leading to the transient formation of themonomer ion is traced to the solvent asymmetry inherent in an incomplete coordination shell.
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