Solvation structure of iodide on an aqueous solution surface and relevant heterogeneous chemical reactions

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Specific interaction of ions with solvent molecules on a solution surface causes characteristic solvation structures of ions. As solvation structures of halogen ions relate to the heterogeneous chemical reaction of OH radicals in the atmosphere, it is very important to elucidate the solvation structures. First-principles molecular dynamics simulation suggests that iodide ions tend to locate near the surface, whereas chloride ions prefer to be located deep inside the surface.We measured absorption spectrum of iodide ions in the vicinity of the aqueous solution surface : When a NaI aqueous solution is irradiated with a UV light, I- is excited to the CTTS (charge transfer to solvent) state. Then, it is further excited by absorbing another photon and the electron is released from I-. As the electron escape depth from the solution surface is in the order of 1nm, we are able to detect electrons originating from I- located in the vicinity of the surface by the gas phase method. We measured a signal intensity of the electrons as a function of the wavelength of the laser, and found out that I- tends to be located inside the solution at the lower NaI concentration, starting to appear on the surface at the higher NaI concentration. We examined the solvation structures of Na^+ by detecting ions from the surface. It was found that Na^+ tends to be inside the solution regardless of the concentration of NaI.