Two-Dimensional Infrared Spectroscopy of Supercooled Water

We present two-dimensional infrared (2D IR) spectra of the OD stretch vibration of isotope diluted water (HOD/H20) from ambient conditions (293 K) down to the metastable supercooled regime (260 K). We observe that spectral diffusion slows down from 700 fs to 2.6 ps as we lower the temperature. A comparison between measurements performed at the magic angle with those at parallel polarization shows that the 2D IR line shape is affected by the frequency-dependent anisotropy decay in the case of parallel polarization, altering the extracted correlation decay. A fit within the framework of an Arrhenius law reveals an activation energy of E-a = 6.2 +/- 0.2 kcal/mol and a pre-exponential factor of 1/A = 0.02 +/- 0.01 fs. Alternatively, a power law fit results in an exponent gamma = 2.2 and a singularity temperature T-s = 221 K. We tentatively conclude that the power law provides the better physical picture to describe the dynamics of liquid water around the freezing point.