Solvation Structure of I- and Na+ on the Surface of NaI Aqueous Solution Studied by Photodetachment Spectroscopy in Combination with Mass Spectrometry

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.