Interfaces between water and vapor play a crucial role in atmospheric and environmental chemistry, for instance, in the uptake of aerosols or gases, such as carbon dioxide. Detailed knowledge of how water molecules organize close to the surface could help researchers understand and control such chemical processes. In Physical Review Letters, Yuki Nagata, at the Max Planck Institute for Polymer Research in Germany, and colleagues report that nuclear quantum effects play an unexpected role in determining the microscopic structure of water-vapor interfaces.
To probe the liquid-vapor interface, the authors applied a spectroscopic method called sum-frequency generation, whereby visible and midinfrared photons mix to generate photons at the sum of their frequencies. Being a second-order process (like second-harmonic generation), sum frequency is vanishingly small in isotropic media, such as bulk water, but yields detectable signals at the interface, where the symmetry is broken. By tuning the infrared pulses to the oxygen-hydrogen (O-H) vibrational resonance, the authors were able to selectively measure the vibrational spectra of molecules at the interface layer.
Guided by molecular dynamics simulations, Nagata et al. obtained a detailed picture of the interfacial water structure that revealed the average orientation of the O-H bond relative to the surface. By substituting hydrogen (H) with its heavier deuterium isotope (D), the authors were able to pinpoint the role played by quantum effects, which are particularly pronounced due to the low masses of the nuclei. The authors found that, while the interfacial water structures of and are indistinguishable, for HDO molecules, the OD bonds preferably orient down toward the bulk water, while the OH bond tends to orient up toward the vapor. This canting of the water molecule appears to result from the different zero-point energies of hydrogen and deuterium. – Matteo Rini