| Abstract
| - Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probingof the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electronbinding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electroniclevels of the solute could be observed. Electron binding energies of a given aqueous ion are found to beindependent of the counterion and the salt concentration. The experimental results are complemented byab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions inthe aqueous phase. The solvent effect was accounted for in the electronic structure calculations in twoways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classicalmolecular dynamics simulations and a fractional charge representation of solvent molecules give goodresults for halide ions. The electron binding energies of alkali cations computed with this approach tend tobe overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabaticbinding energies, performs well for the alkali cations but fails for the halides. Photon energies in theexperiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers ofthe liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structuresand chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well asfundamental bulk solvation properties.
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