Photoemission from aqueous alkali-metal-iodide salt solutions using EUV synchrotron radiation

The valence band photoemission of aqueous alkali-metal halide solutions is studied for photon energies from 90 to 110 eV. A 6 mum diameter liquid microjet provides a free vacuum surface, allowing water molecules to evaporate without collisions, and hence enables the direct detection of photoelectrons originating from the liquid. The experiments were performed at the MBI undulator beamline of the synchrotron radiation facility BESSY. Here, we focus on the determination of electron binding energies of solvated anions and cations. The effect of different countercations (Li+, Na+, K+, and Cs+), and salt concentrations is systematically investigated. Electron binding energies of the solvated ions are found to differ considerably from those in the gas phase; contrary to intuition, the energies do not depend. on the salt concentration. Measured binding energies can be surprisingly well explained within a simple dielectric cavity model. For a NaI aqueous solution, negative surface excess is inferred from the evolution of the ion photoemission signal as a function of the salt concentration.