Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential.

The orientational distribution of free O-H (O-D) groups at the H_{2}O- (D_{2}O-)air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be ∼63°, substantially larger than previous estimates of 30°-40°. This discrepancy can be traced to erroneously assumed Gaussian or stepwise orientational distributions of free O-H groups. Instead, the MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface.