Proton order in the ice crystal surface

Area of Science:

• Surface science
• Computational chemistry
• Physical chemistry

Background:

• The atomic-level structure of pristine ice surfaces at low temperatures remains poorly understood.
• Understanding ice surface physics is crucial for terrestrial and planetary chemistry, particularly regarding adsorbate interactions.

Purpose of the Study:

• To investigate the atomic-level structure of dangling hydrogen (H) and oxygen (O) atoms at the basal ice surface.
• To determine the preferred arrangement of these dangling atoms and their implications for adsorbate binding.

Main Methods:

• Extensive computer simulations using two different empirical potentials.
• Free energy calculations to compare different surface structures.

Main Results:

• Simulations indicate a significant free energy preference for a striped phase with alternating rows of dangling H and dangling O atoms.
• This striped phase model aligns with previously unexplained experimental helium diffraction data.
• The striped model improves agreement with experimental ppp-polarized sum frequency generation spectra compared to a disordered model.

Conclusions:

• The basal ice surface exhibits a striped phase of dangling atoms, not the expected disordered pattern.
• This finding provides a more accurate model for ice surface structure and adsorbate interactions.
• The striped phase has implications for understanding chemical processes on ice in various environments.