Our latest work on the molecular structure of water at the interface to air is now published in Science Advances. This work by Alex and Louis Lehmann (Netz group, FU Berlin) probes the vibrational bending mode using our SFG/DFG technique. The analysis of the spectra confirms previous indications that the SFG signal of the bending mode is dominated by electric quadrupolar transitions but also reveals the desired purely dipolar spectrum for the first time. Comparison to calculated spectra from simulations shows almost perfect quantitative agreement. Furthermore, a detailed analysis of the depth-dependent nonlinear signals uncovers a pronounced layering in molecular arrangements of water molecules with alternating tilt and in particular molecular twist angles. These findings offer a revised picture of the structural particularity of interfacial water by showing that the common structural analysis in terms of “pointing up or down” of water molecules is largely insufficient.
Twisting water