Water bending mode at the water-vapor Interface probed by sum-frequency generation spectroscopy : A combined molecular dynamics simulation and experimental study

We present a combined molecular dynamics simulation and experimental study on the water bending mode at the water–vapor interface using sum-frequency generation (SFG) spectroscopy. The SFG spectrum simulated using an ab initio-based water model shows good agreement with the experimental data. The imaginary part of the SFG response shows a negative peak at 1650 cm^-1 and a positive peak at 1730 cm^-1. Our results reveal that these widely (80 cm^-1) separated peaks result from the interference of two closely spaced (29 cm^-1) peaks of opposite sign. The positive peak at 1689 cm^-1 originates from water with two donor hydrogen atoms with the HOH angular bisector pointing down toward the bulk, and the negative peak at 1660 cm^-1 from water with free O–H groups, pointing up. The small frequency difference of 29 cm^-1 indicates that the HOH bending mode frequency of interfacial water is relatively insensitive to the number of hydrogen bonds.