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Low-temperature Raman study of water

Pure water was investigated in solid state at low temperatures by Raman spectroscopy using two spray-on techniques. In the first case, we have prepared solid samples by spraying gaseous matter directly on the cold, golden finger in vacuum. Starting matter was a) double-distilled and deionized H2O and b) commercial D2O. The common knowledge of water structure is that ordering of water molecules in such an instant solidification strongly depends on the temperature of the substrate [1]. Below some 120 K, we confirmed a vitreous phase, but above » 150 K, the matter solidified in a crystalline form [2-4]. Another spray-on technique performed here was argon matrix isolation. For that purpose, we first prepared a mixture of argon and water vapor on the vacuum line in nearly 100:1molar ratio. Then, gas mixture was solidified by spraying it on the cold finger in the same way as in the previous case of bulk water, but at temperatures 10 – 20 K.In the high-frequency region of bulk matter we observe for both, light- and heavy-water, well known, complicated, but much better resolved spectral features. Stretching region of both spectra are plotted with the scaling factor = √ MO-D / MO-H (harmonic approx.). Obviously, the agreement is perfect for upper pair of bands, but for other lower bands harmonic approximation is not so good. In the low frequency region we observe a true crystalline ice band, but sometimes together with a Bose band which is characteristic for a vitreous state. In the high-frequency region of matrix-isolated water, we have clearly observed bands of monomer species, probably dimeric bands, and broad feature characterizing bulk water. In the matrix- isolated heavy-water spectrum, we have noticed for the first time the presence of nitrogen molecule. Low frequency spectra of both water samples did not exhibit characteristics of crystalline structure, but Bose band was, as expected, very prominent. Our experience permits us to speculate that presence of atmospheric impurities causes disagreements between authors.

Raman; water; matrix isolation; spray-on technique

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