Photoelectron spectroscopy of Cl^-, Br^-, and I ^- solvated in water clusters

We present the photoelectron-spectra of Cl^- ,Br^-, and I^-, solvated in water clusters-(H₂O)_n, where n is 1-7, 1-16, and 1-60, respectively, taken with 7.1 eV photon energy. The vertical binding energies of the solvated anions are used to extract the solvent electrostatic stabilization energies of the anion. The photoelectron spectra of the solvated I^- indicate the formation of the first solvation layer with a coordination number of six. Ab initio calculations support solvation shell closure at n=6. This conclusion is not born-out by current molecular dynamics calculations. These calculations favor structures with a surface solvated anion (coordination number of 3-4) and reproduce (within 0.2 eV) our vertical binding energies. The fitting of the experimental binding energies of large I^-(H₂O)_n to the models of classical electrostatic solvation is consistent with surface solvation. In the size range n=34-40 we have detected special cluster structures, with very low electrostatic stabilization.