Momentum distribution, vibrational dynamics and the potential of mean force in ice

TL;DR

This study investigates the anisotropic vibrational behavior of protons in ice using path integral and phonon calculations, revealing significant anharmonic effects and providing a theoretical prediction for directional momentum distributions.

Contribution

It introduces a detailed analysis of proton dynamics in ice, highlighting anisotropic quasi-harmonic potentials and the necessity of directional distributions for accurate characterization.

Findings

Protons in ice experience anisotropic quasi-harmonic potentials with three principal frequencies.

Anharmonic features are significant and of quantum origin, affecting spectroscopic interpretations.

Directional momentum distributions are predicted for future experimental verification.

Abstract

By analyzing the momentum distribution obtained from path integral and phonon calculations we find that the protons in hexagonal ice experience an anisotropic quasi-harmonic effective potential with three distinct principal frequencies that reflect molecular orientation. Due to the importance of anisotropy, anharmonic features of the environment cannot be extracted from existing experimental distributions that involve the spherical average. The full directional distribution is required, and we give a theoretical prediction for this quantity that could be verified in future experiments. Within the quasi-harmonic context, anharmonicity in the ground state dynamics of the proton is substantial and has quantal origin, a finding that impacts the interpretation of several spectroscopies.