Spectroscopic evidence for the specific $Na+$ and $K+$ interactions with the hydrogen-bonded water molecules at the electrolyte aqueous solution surfaces

Sum frequency generation vibrational spectra of the water molecules at the NaF and KF aqueous solution surfaces showed significantly different spectral features and different concentration dependence. This result is the first direct observation of the cation effects of the simple alkali cations, which have been believed to be depleted from the aqueous surface, on the hydrogen bonding structure of the water molecules at the electrolyte solution surfaces. These observations may provide important clues to understand the fundamental phenomenon of ions at the air/water interface.