A representative snapshot of the MD simulations. Na + and OH are highlighted in blue and orange, respectively. Selected water molecules coordinated to the ions that are responsible for the observed changes in the HD-VSFG spectrum are also highlighted. The instantaneous air/liquid interface is depicted by a grey surface and the dashed lines represent the interfaces between pure water and bulk electrolyte solution. The H 2 OOH and H 2 ONa + water molecules that give rise to the changes in the VSFG spectra have opposite polarization and lie in an interfacial region depleted of ions. For clarity, H 2 OOH and H 2 ONa + water molecules giving rise to the changes in the VSFG spectra are marked with light-blue and orange areas, respectively.
