Far infrared spectroscopy of water at different temperatures: GHz to THz dielectric spectroscopy of water

Abstract

The power absorption coefficient and refractive index of water at temperatures of 4, 8.9, 30 and 50 °C have been measured in the wavenumber range 50–220 cm −1. The power absorption coefficient profile is observed to increase with increase in temperatures from 4 to 8.9 and then to 30 °C. This is followed by a decrease in the profile at 50 °C. These results show that the absorbance arises from both the dipolar orientational and the translational stretch of the hydrogen bond. The dielectric loss spectrum in the frequency range 1 GHz to 7 THz wavenumber range (0.03 to 233.3 cm −1) at 25 °C, with those compiled from the literature and combined with these results, is reanalysed in terms of the three relaxation processes and the fourth resonant process. The relaxation times at a temperature of 25 °C are found to be 8.31, 1.0 and 0.10 ps. For the resonant process, the dielectric loss peak is centred at 175 cm −1. The lowest frequency process is pure Debye and interpreted as arising from the activation of the water molecule, from one of the four sites surrounding a central molecule, to a neighbouring unoccupied site. The relaxation process is perfect Debye because the activation has the same barrier for each of the four sites. The second process follows Davidson–Cole distribution and is interpreted as arising from the rotation of single water molecules that are not hydrogen-bonded at a given instant of time. The fastest relaxation process is associated with the intermolecular energy transfer or the energy dissipation through the interaction between the O–H stretch modes. This process may well arise partly from the weak 60 cm −1 band due the hydrogen bond bending and/or from a weak 30 cm −1 band reported in the literature. The resonant process centred at 175 cm −1 arises from the translational modes arising from the stretching of the hydrogen bonds and this involves fluctuations both in the dipole moment and the polarizability as the band is seen in the Raman spectra too.