Incoherent inelastic neutron scattering from liquid water: A theoretical investigation

Basic Molecular Mechanisms Underlying Complex Permittivity of Water and Ice

Four dielectric-loss frequency dependences epsilon''j(nu), j = 1-4, which constitute a basis for underlying far-infrared (FIR) spectra of water and ice, are briefly analyzed. The relevant molecular mechanisms are (a) free-like libration of a permanent dipole in a hat potential, (b) elastic vibration of a nonrigid dipole along the H-bond (HB), (c) elastic reorientation of a permanent dipole around this bond, and (d) vibration of HB water molecules transverse to the HB direction. A semiphenomenological (SP) approach, based on analytical calculation of the spectrum of autocorrelation function, is applied. The total loss curve epsilon''(nu), accounting for these mechanisms and presented for liquid water at 27 and 81.4 degrees C, for supercooled water at -5.6 degrees C, and for ice at -7 degrees C, demonstrates a very good agreement with experimental spectra. A simple formula is proposed for an "association factor" zeta relating the dipole moments mu(perpendicular) and mu(q) of transversally and longitudinally (with respect to HB direction) vibrating molecules. The parametrization of the model yields for T < 300 K a sharp increase of this factor zeta with a decrease of temperature.