We investigated solvation structures of I(-) on and below a surface of an aqueous solution by photodetachment spectroscopy. An aqueous solution of an alkali halide was introduced to the vacuum as a continuous liquid flow (liquid beam), and the liquid beam was irradiated with a UV laser pulse. The intensity of electrons emitted from the surface by the laser excitation was measured as a function of wavelength (photodetachment spectroscopy), and we obtained absorption spectrum of I(-) on and below the solution surface. From the absorption spectrum, we found that I(-) starts to appear on the solution surface as the bulk NaI concentration increases. Similar concentration dependence was observed for the KI solution. We also found that I(-) located inside the solution is pushed to the surface, when NaCl is added to the solution. These changes are explained in terms of the difference in the polarizability of halide ions.

We investigated solvation structures of I− and Na+ on an aqueous solution surface by photodetachment spectroscopy and mass spectrometry. An aqueous solution of NaI was introduced into the vacuum as a continuous liquid flow (liquid beam), and the liquid beam was irradiated with a UV laser pulse. The abundance of electrons emitted by the laser excitation was measured as a function of wavelength (photodetachment spectroscopy). For a concentrated aqueous solution of NaI, we observe an absorption peak at longer wavelengths than the charge-transfer-to-solvent band of I− in solution. This feature is assigned to the photoabsorption of I− at the surface. This finding indicates that when the concentration of NaI is high (>1.0 M), I− exists on the solution surface. The identity of the ion clusters ejected from the liquid beam following selective laser excitation of I− on the surface or I− inside the solution was revealed by mass spectrometry. The mass spectra show that Na rich clusters are formed when I− inside the solution is excited, whereas Na rich clusters are hardly formed by the excitation of surface I−. These findings lead us to conclude that Na+ does not exist on the surface of the NaI aqueous solution.