Heterodyne-detected electronic sum frequency generation: “Up” versus “down” alignment of interfacial molecules

Analysis of anisotropic local field in sum frequency generation spectroscopy with the charge response kernel water model

A new flexible and polarizable water model based on the charge response kernel (CRK) theory is developed for the analysis of sum frequency generation (SFG) spectroscopy. The CRK model well describes several bulk water properties and SFG spectrum by molecular dynamics (MD) calculations. While the flexible and polarizable MD simulation generally adopts the short-range damping of intermolecular interaction, it is found that the same procedure is not adequate for the calculation of transition dipole in strongly hydrogen bonding environment. Accordingly, the improved calculation of the nonlinear susceptibility of water surface results in the positive imaginary part in the 3000-3200 cm(-1) region, which is consistent with recent phase-sensitive experiments. The mechanism of the positive region is attributed to the anisotropic local field effect induced by the orientational correlation of surface water.

Direct evidence for orientational flip-flop of water molecules at charged interfaces: a heterodyne-detected vibrational sum frequency generation study

Complex chi(2) spectra of air/water interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the first time. In contrast to the neat water surface, the signs of chi(2) for two broad OH bands are the same in the presence of the charged surfactants. The obtained chi(2) spectra clearly showed flip-flop of the interfacial water molecules which is induced by the opposite charge of the head group of the surfactants. With the sign of beta(2) theoretically obtained, the absolute orientation, i.e., up/down orientation, of water molecules at the charged aqueous surfaces was uniquely determined by the relation between the sign of chi(2) and the molecular orientation angle. Water molecules orient with their hydrogen up at the negatively charged aqueous interface whereas their oxygen up at the positively charged aqueous interface.