Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy. Nature 474

The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The airwater interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3 ångströms (1 Å 5 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers The vibrational spectroscopy of aqueous interfaces has progressed significantly in recent years with the development of surface-selective spectroscopic techniques The intramolecular and intermolecular vibrational coupling between the OH transition dipoles of the same molecule or between neighbours affect the spectral lineshapes of the water OH-stretch band We overcome these challenges by using the heterodyne-detected SFG technique 10 , which uses interference of the signal with a reference beam to (1) amplify the SFG signal, considerably enhancing the sensitivity, (2) make it linear with the number of chromophores, an