INVESTIGATION OF THE TEMPERATURE-DEPENDENCE OF DIELECTRIC-RELAXATION IN LIQUID WATER BY THZ REFLECTION SPECTROSCOPY AND MOLECULAR-DYNAMICS SIMULATION

We report measurements of the real and imaginary part of the dielectri c constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been o btained with the use of THz time domain reflection spectroscopy, utili zing ultrashort electromagnetic pulses generated from a photoconductiv e antenna driven by femtosecond laser pulses. A Debye model with an ad ditional relaxation time is used to fit the frequency dependence of th e complex dielectric constants, We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding t ime correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye-Stoke-Einstein relation between the viscosity and the Debye time, and a model stating that it s temperature dependence can be extrapolated from a singularity of liq uid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results w ith too large statistical error for establishing a relation for the te mperature dependence. (C) 1997 American Institute of Physics.