Near-Infrared Spectroscopic Sensing of Hydrogen Order in Ice XIII

We identify hydrogen ordering in H₂O ices spectroscopically in the near-infrared (NIR) range (10 000-4000 cm⁻¹/1-2.5 μm) based on the example of ices V/XIII. Previously it was thought that hydrogen ordering can only be revealed on the basis of lattice phonons, i.e., intermolecular vibrations. Here we show differences in the overtone spectrum of the intramolecular OH-stretching vibration. This makes NIR spectroscopy the first remote sensing method that is sensitive to different orientations of the water dipoles within ice. As such it will allow for future observations of the hydrogen order of ices in space by the James-Webb Space Telescope or the JUICE mission.