Reversible structural transformations in supercooled liquid water from 135 to 245 K

Water has many anomalous properties compared to "simple" liquids, and these anomalies are typically enhanced in supercooled water. While numerous models have been proposed, including the liquid-liquid critical point, the singularity-free scenario, and the stability limit conjecture, a molecular-level understanding remains elusive.The main difficulty in determining which, if any, of these models is correct is the limited amount of data in the relevant temperature and pressure ranges. For water at ambient pressures, which is the focus of this work, data is largely missing from 160 - 232 K due to rapid crystallization. Whether rapid crystallization is just an experimental obstacle, or a fundamental problem signaling the inability of water to thermally equilibrate prior to crystallization is also a major unanswered question. Here, we investigate the structural transformations of transiently-heated, supercooled water with nanosecond time resolution using infrared vibrational spectroscopy. The experiments demonstrate three key results. First, water's structure relaxes from its initial configuration to a "steady-state" configuration prior to the onset of crystallization over a wide temperature range. Second, water's steady-state structure can be reproduced by a linear combination of two, temperature-independent structures that correspond to a "high-temperature liquid" and a "low-temperature liquid." Third, the observed structural changes are reversible over the full temperature range. Taken together, these results show that supercooled water can equilibrate prior to crystallization for temperatures from the homogeneous nucleation temperature down to the glass transition temperature. Second, the results provide support for the hypothesis that supercooled water can be described as a mixture of two, structurally-distinct, interconvertible liquids from 135 K to 245 K.