Probing the water-ice phase transition using spectroscopy of dissolved ions

Liquids are complicated. They lack the long-range order and symmetries of solids, but their atoms interact much more strongly than in gases. Therefore, their dynamics and phase transitions are difficult to model and measure. However, atomic ions in solution are sensitive to the local liquid environment. Perturbations to the motion or structure of dissolved ions due to the solvent can be measured using spectroscopy.

Here we use the Eu^{3+ 7}F₀ — ⁵D₀ optical transition as a tracer to study the environment of the ions. We have observed this transition, which is forbidden in a spherically-symmetric environment, to become allowed in water and ice due to local symmetry-breaking. We will discuss our measurements of the line strength and spectral properties of this europium ion transition as a means of understanding the water-ice phase change.