Wideband (from 0 to 1000 cm−1) dielectric/FIRspectra of ordinary and heavy water: calculation in terms of the composite hat curved–harmonic oscillator model

Abstract

This paper presents a fundamental development of recent work by the authors (V. I. Gaiduk and J. K. Vij, Phys. Chem. Chem. Phys., 2001, 3, 5173), where a linear dielectric response of liquid H₂O was investigated in terms of two processes characterised by two correlation times. One process concerns reorientation of a single-molecule and the second one concerns a damped vibration of the H-bonded molecules. In this work, to study the reorientation process a new hat curved (HC) model suggested by Gaiduk et al. (V. I. Gaiduk, B. M. Tseitlin and D. S. F. Crothers, J. Phys. C, submitted) is used instead of the rectangular-well (hybrid) model. In the hat curved model a hat-like intermolecular potential comprises a flat bottom and parabolic walls followed by a constant potential. For the studies of the vibration process the harmonic oscillator (HO) model based on application of the parabolic potential is applied instead of the cosine squared potential model. The HO model describes the dielectric response of two charged particles with charges ±δq_vib performing damped stretching vibrations along the H-bond direction; the existence of an effective non-rigid dipole formed by two water molecules is postulated. The charge δq_vib is shown to be commensurable with charge e of an electron. The complex permittivity ε* and absorption coefficient α are calculated in the range from 0 to 1000 cm⁻¹ for water H₂O at temperatures of 22.2 and 27 °C and for D₂O at 22.2 °C. The composite hat curved–harmonic oscillator (HC-HO) model is shown to avoid the drawbacks of the previously used hybrid-cosine squared potential model. The new composite HC-HO model allows a natural explanation, on a molecular basis, of the difference between the far-infrared spectra of liquid H₂O and D₂O. An interpretation is given of the well-known results of Liebe et al. (H. J. Liebe, G. A. Hufford and T. Manabe, Int. J. Infrared Millimeter Waves, 1991, 12, 659) based on an empirical double Debye/double Lorentz representation of the complex permittivity.