Abstract：Liquid water is the chemical backbone of most biochemical processes on earth and is therefore essential for the metabolism of living beings. As a result it is a key topic across a wide range of scientific disciplines. The physicochemical properties of water are considered to be the result of hydrogen bond derived structures. However, it is still difficult to quantitatively assign the physicochemical properties of water molecules with the hydrogen bond structure in order to form a complete theory of liquid water molecular structure. Raman spectroscopy is one of the main methods to characterize the molecular structure of liquid water because of its fast and non-destructive advantages. At present, Raman spectroscopy of water molecule is mainly concerned with its high frequency vibration modes. Wide low-frequency Raman modes in liquid water are the result of hydrogen bonds and their local structural effects, which contain characteristic information that high-frequency peaks cannot be characterized. The ultra-low-frequency Raman spectroscopy can reveal many key details of water molecules (super-) structure at high temperature. Therefore, this article provides novel precision temperature-dependent ultra-low frequency Raman spectra of water molecules for the first time. All four translational characteristic modes predicted by theory have been experimentally detected, including the bending mode (51.7 cm-1), torsional mode (81.4 cm-1), symmetrical (154.0 cm-1) and asymmetrical stretching mode (188.6 cm-1) , an additional translational-rotational coupled characteristic mode is found at 225.2 cm-1. All feature modules are accurately assigned. From the spectroscopy results, first of all, when the temperature rises from 0 to 400 ℃, due to hydrogen bond breaking and the rapid average structure correlation length (SLG) decreasing, the frequencies of all four ultra-low frequency characteristic modes shift significantly blue with the increase of temperature. Secondly, the strength of the stretching modes drops obviously between 100 and 200 ℃. While The strength of bending mode increases in turn from high frequency to low frequency with the decrease of stretching modes, which has never been involved in theoretical research. Finally, the the Stokes/Anti-Stokes Ratio (RS/AS) increases rapidly from 1.1 to 1.3 at the temperature range 150～170 ℃ (about 2 kbar) and display a linear temperature dependence when the temperature is above 170 ℃. In general, the effects of temperature on the structure of water molecule, especially on hydrogen bond derivation, are obtained by studying the blue shift of resonance modes, the change of intensity and the Stokes/anti-Stokes ratio. This study provides a deep insight into the analysis of hydrogen-bond derived properties and a new experimental spectroscopy method for understanding the structure of water molecule in depth and comprehensively.
Key words：Ultra-low frequency Raman spectroscopy; Water molecules; Hydrogen bonds; High temperature
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