Photochemistry of halide ion-molecule clusters: Dipole bound excited states and the case for asymmetric solvation

We have demonstrated that the long-range interaction between a photoejected electron and the cluster core can be used to deduce the arrangement of solvent molecules around halide anions. The spectra reveal that when a second solvent molecule binds to a monosolvated anion, it occupies an asymmetric position (i.e. X-·M·M, with the outer M in the second solvation shell). The neutral frameworks of these asymmetric ionic forms display very large dipole moments, leading to the formation of dipole-bound excited states just below the vertical electron detachment energies of the anions. Asymmetric solvation is generally observed for polar solvent molecules with C3v, symmetry. Photoexcitation of the complexes to their dipole-bound excited states releases the solvent from the halogen atom as a ground-state, dipole-bound anion, providing an efficient means for studying these fragile anions. For the X-·M2 clusters, the solvents are released with retention of the high dipole geometry of the core complex and the excess electron can be field stripped as the final step in an efficient "photosynthetic" route to high-energy isomers of neutral vdW complexes.