Lowhyphen;frequency Raman spectrum of supercooled water

摘要

We report measurements of the Raman spectrum of supercooled water in the hindered translational region (20ndash;400 cmminus;1) down to a temperature of minus;20thinsp;deg;C. The spectra are analyzed after correcting for the effects of Boltzmann factor and harmonic oscillator coupling, i.e., in the reducedR(ngr;macr;) representation of Shuker and Gammon. Spectral deconvolution shows that in addition to the previously observed 0hyphen;0hyphen;0 bending mode (bartil;60 cmminus;1) and the 0hyphen;0 stretching mode (bartil;190 cmminus;1), there is a weak feature at 260 cmminus;1whose intensity increases by almost an order of magnitude as temperature decreases from 40 to minus;20thinsp;deg;C. A plausible interpretation of the 260 cmminus;1band is that it is analogous to the 310 cmminus;1band seen in ice I and probably arises because of differing electrostatic interactions in different configurations of coupled H bonds of neighboring H2O molecules. The 0hyphen;0 stretching band at 190 cmminus;1changes in many respects as temperature decreases from 40 to minus;20thinsp;deg;C: (i) Its peak intensity increases almost four times; (ii) integrated intensity increases three times; (iii) bandwidth decreases about 30percnt;; and (iv) peak maximum increases linearly from 176 cmminus;1at 40thinsp;deg;C to 202 cmminus;1at minus;20thinsp;deg;C. In contrast, the 0hyphen;0hyphen;0 bending at 60 cmminus;1is quite insensitive to changes in temperature. The increase in the intensity of the 190 and 260 cmminus;1bands is consistent with the idea that fourhyphen;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 fourhyphen;coordinated water molecules, as seen in the smallhyphen;angle xhyphen;ray scattering data of Bosioet al.We also observe a limiting value to the width of the 190 cmminus;1band at temperatures below the melting point, suggesting that the local structure of supercooled water is approaching some limiting structure.