Polarized adsorption of H2O on NaCl(100) in air, observed by second harmonic generation

Abstract

We have observed a pronounced m-symmetry in the azimuthal dependence of optical second harmonic generation in reflection from NaCl(100) in air at room temperature, in spite of the fact that the C4v symmetry of this surface should not exhibit any anisotropy. The observed anisotropy builds up and then disappears again with time of exposure to ambient air. The high solubility of NaCl in water, together with the high polarity of the water molecule, suggests that the dipolar surface hyperpolarization, responsible for second harmonic generation, is caused by adsorbed H2O at a certain range of coverage. The m-symmetry then reflects a macroscopic polarization due to a partially parallel orientation of the adsorbed water molecules. In accord with tentative results of molecular dynamics calculations, this indicates polymerization of water on NaCl(100) at room temperature.