Photoelectron imaging of hydrated carbon dioxide cluster anions

The effects of homogeneous and heterogeneous solvation on the electronic structure and photodetachment dynamics of hydrated carbon dioxide cluster anions are investigated using negative-ion photoelectron imaging spectroscopy. The experiments are conducted on mass-selected [(CO ₂) _n(H ₂O) _m] ^- cluster anions with n and m ranging up to 12 and 6, respectively, for selected clusters. Homogeneous solvation in (CO ₂) _n ^- has minimal effect on the photoelectron angular distributions, despite dimer-to-monomer anion core switching. Heterogeneous hydration, on the other hand, is found to have the marked effect of decreasing the photodetachment anisotropy. For example, in the [CO ₂(H ₂O) _m] ^- cluster anion series, the photoelectron anisotropy parameter falls to essentially zero with as few as 5-6 water molecules. The analysis of the data, supported by theoretical modeling, reveals that in the ground electronic state of the hydrated clusters the excess electron is localized on CO ₂, corresponding to a (CO ₂) _n ^-·(H ₂O) _m configuration for all cluster anions studied. The diminishing anisotropy in the photoelectron images of hydrated cluster anions is proposed to be attributable to photoinduced charge transfer to solvent, creating transient (CO ₂) _n·(H ₂O) _m ^- states that subsequently decay via autodetachment.