Low-frequency Raman spectrum of supercooled water

Abstract

We report measurements of the Raman spectrum of supercooled water in the hindered translational region (20–400 cm⁻¹) down to a temperature of −20 °C. The spectra are analyzed after correcting for the effects of Boltzmann factor and harmonic oscillator coupling, i.e., in the reduced R(ν̄) representation of Shuker and Gammon. Spectral deconvolution shows that in addition to the previously observed 0‐0‐0 bending mode (≂60 cm⁻¹) and the 0‐0 stretching mode (≂190 cm⁻¹), there is a weak feature at 260 cm⁻¹ whose intensity increases by almost an order of magnitude as temperature decreases from 40 to −20 °C. A plausible interpretation of the 260 cm⁻¹ band is that it is analogous to the 310 cm⁻¹ band seen in ice I and probably arises because of differing electrostatic interactions in different configurations of coupled H bonds of neighboring H₂O molecules. The 0‐0 stretching band at 190 cm⁻¹ changes in many respects as temperature decreases from 40 to −20 °C: (i) Its peak intensity increases almost four times; (ii) integrated intensity increases three times; (iii) bandwidth decreases about 30%; and (iv) peak maximum increases linearly from 176 cm⁻¹ at 40 °C to 202 cm⁻¹ at −20 °C. In contrast, the 0‐0‐0 bending at 60 cm⁻¹ is quite insensitive to changes in temperature. The increase in the intensity of the 190 and 260 cm⁻¹ bands is consistent with the idea that four‐coordinated H₂O molecules contribute directly to these spectral features and the fraction of such molecules increases with decreasing temperature. This effect on intensity is further enhanced by the strong coupling of the motion of a few four‐coordinated water molecules, as seen in the small‐angle x‐ray scattering data of Bosio et al. We also observe a limiting value to the width of the 190 cm⁻¹ band at temperatures below the melting point, suggesting that the local structure of supercooled water is approaching some limiting structure.