  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.
