Hydrogen order at the surface of ice I revealed by vibrational spectroscopy.

Among numerous crystalline phases of ice, the I phase is the most stable above 72 K at atmospheric pressure. It is well established that the orientations of water molecules in the bulk of ice I are statistical without long-range order. However, the orientational order of water at the surface of ice I has been enigmatic. Here we show that the surface of ice I at 100 K has hydrogen order with the OH group pointing upward to the air ("H-up" orientation). We applied nonlinear optical spectroscopy and theoretical modeling to the surface of isotopically pure and diluted ice I and observed OH stretch vibrational signatures attributed to H-up ordering. Furthermore, we found that this hydrogen order takes place despite a more inhomogeneous microenvironment at the surface than in the bulk. Our results suggest the prominent role of the surface to allow the reorientation of water molecules for hydrogen ordering that is virtually prohibited in the bulk.