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Detection of Ice-Like Clusters Spectral Component in OH Band of Water at the Temperature 13 - 99 °С


S.M. Pershin, T.G. Adiks, V.A. Lukyanchenko, R.R. Nigmatullin, A.A. Potapov

The temporal (in the interval 13-99 °С) evolution of OH band of Raman scattering on the valence OH stretching mode of water molecules is studied by new method. In the frame of new approach the measured spectrum is divided on two parts: the smoothed and generalized mean value and the remnant modulation spectrum of the band envelope. The comparison of the new method with suggested earlier approach (based on the recognition of an object by means of radiolocation impulses obtained from a shaggy surface and the subsequent analysis of the recorded impulse with the help of gaussian and non-gaussian statistics) shows that the introduction of the statistics of the factional moments satisfies to more «soft» condition. It makes useless a priori requirement related to the statistics of the signal or its component. The application of new approach allows us to discover that the fluctuated part of OH band (determined by the modulation of envelope) contains two dominant components at ~3200 and ~3450 cm-1 The comparison of these frequencies with frequencies of the large water clusters measured earlier in molecular beams showed that they correspond to hexameric (ice-like) and tetrameric molecular complexes. In the first time to the best of our knowledge it was established that the increasing of water temperature up to the boiling one (99 °С) leads to decreasing of amplitudes of ice-like components and at the same time leads to increasing the amplitudes of tetrameric complexes without noticible frequency changes of the latter ones. At this process the gravity center of the generalized mean value of OH band is shifting to the high-frequency region with coefficient ~1 cm-1deg-1. The discovered peculiarities can be interpreted as spectral confirmation of existence of stable hydrogen-bonded ice-like molecular clusters in liquid water. Their spectral contribution decreases with increasing of temperature (up to 99 С) but remains significant. We formulated the conclusive hypothesis that the discovered spectral components at ~3200 and ~3450 cm-1 reflect the evolution of para-/ortho spin-isomer of a molecule Н2О in water. This ratio (before boiling) takes the equilibrium value 1:3 as molecules in gas. Further, according to our data this ratio in the region close to 0°С accepts the value 1:1 and after crystallization it is inverted to the value ~ (3:1).

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