Journal of Molecular Liquids 112 (2004) 125–135 0167-7322/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2003.12.014 Far infrared spectroscopy of water at different temperatures: GHz to THz dielectric spectroscopy of water J.K. Vij*, D.R.J. Simpson, O.E. Panarina Laboratory of Advanced Electronic Materials, Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland Received 2 July 2003; accepted 15 November 2003 Abstract The power absorption coefficient and refractive index of water at temperatures of 4, 8.9, 30 and 50 8C have been measured in the wavenumber range 50–220 cm . The power absorption coefficient profile is observed to increase with increase in temperatures y1 from 4 to 8.9 and then to 30 8C. This is followed by a decrease in the profile at 50 8C. 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 ) at 25 8C, with those compiled from the literature and y1 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 8C 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 . The lowest frequency process is pure Debye and interpreted as arising from the activation of the water molecule, y1 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 band due the y1 hydrogen bond bending andyor from a weak 30 cm band reported in the literature. The resonant process centred at 175 cm y1 y1 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.  2003 Elsevier B.V. All rights reserved. Keywords: Water; Spectroscopy; Infrared; Dielectric spectroscopy 1. Introduction It is known from most works in the literature that water absorbs heavily in the far infrared (submillimetre wave region). However, there exists some controversy w1,2x as to the presence of a peak in the absorption coefficient centred at ;30 cm (0.9 THz) and if it y1 exists, then whether it is a resonance process or a second relaxation process due to the free water molecules within the liquid quasi-lattice. If the peak corresponds to a relaxation process, the corresponding relaxation time is ;176 fs. This peak was seen in the absorption coeffi- *Corresponding author. Tel.: q353-1608-1431; fax: q353-1677- 2442. E-mail address: jvij@tcd.ie (J.K. Vij). cient data given by one of the authors w1x. The second aspect is whether the peak centred at ;190 cm is y1 due to the s stretch of the hydrogen bond alone or in some way also contributed by the translational–rotation- al motion of water molecules w3x. The answers to these questions can be given if both the refractive index and the absorption coefficient of water are measured as a function of temperature at far infrared frequencies and the data combined with that in the literature re-analysed in terms of the various processes. The data on the complex permittivity of water at far infrared frequencies are also of technological interest since the dielectric properties of water play a key role in the computations of atmospheric propagation and for remote sensing based on the effects caused by the rain droplets and droplets suspended in fogs and clouds.