The concept of two stochastic processes in liquid water and analytical theory of the complex permittivity in the wavenumber range 0-1000 cm(-1)

The linear dielectric response of liquid H2O is investigated in terms of tw o stochastic processes, which govern the wideband spectrum of water. These processes differ in the molecular motion and involve different dipole momen ts. One process concerns a single-molecule reorientation of a dipole with a moment mu (or) in an intermolecular potential well, of which the lifetime tau or approximate to 0.3 ps. The second process is a damped bending vibrat ion of a dipole with a small (mu (vib)/mu (or) approximate to 0.15) moment mu (vib) about the direction of the H-bond. This process is characterised b y shorter (tau (vib) approximate to 0.1 ps) lifetime and is accompanied by a stretching vibration of the H-bonded molecules. The key aspect of the pre sented work is that we show that such vibration is partially responsible : (i) for the narrow translational absorption band arising in the water near the frequency v(tr) similar to 200 cm(-1), and (ii) for the dielectric rela xation in the submillimetre wavelength range, which is characterised by a f ast relaxation time tau (2) approximate to 0.3 ps. The first process (reori entation) is mostly responsible for the main (librational) band arising in water around frequency v(lib) = 600 cm(-1) and for the main Debye relaxatio n in the microwave region. The spectra are described in the analytical form for the composite hybrid-cosine squared (HYB-CS) model. In the hybrid mode l the dielectric behaviour of a polar molecule reorienting in a rectangular well is considered. In the cosine squared model such behaviour is found fo r a dipole oscillating about the H-bond direction. The calculated spectra o f the complex permittivity and absorption coefficient agree satisfactorily with the measured water spectrum for a temperature of 300 K. The typical sp atial and time scales of the molecular events are estimated.