Molecular-dynamics study of incoherent quasielastic neutron-scattering spectra of supercooled water

Abstract

A molecular-dynamics simulation of extended simple point charge water in a time interval of 1 fs to 50 ns has been performed to study the single-particle dynamics of water at supercooled temperatures. In spite of the fact that upon supercooling water progressively evolves into a more open structure locally, the single-particle dynamics is nevertheless shown to be dominated by the so-called cage effect experienced by the test particle. The slow structural relaxation of the cage at low temperatures leads to the phenomenon of slow dynamics that can only be completely studied by following the trajectories into the nanosecond range. The objective of this paper is twofold. First, we test the accuracy of various approximations used in previous analyses of spectra from incoherent quasielastic neutron-scattering experiments. Second, we explore the possibility of an alternative method of analysis of high-resolution quasielastic neutron-scattering spectra, taking into account the slow dynamics in supercooled water. The approximations tested are the decoupling of the self-intermediate scattering function into a product of rotational and translational components, the physical interpretation of the origin of the experimentally observed Debye-Waller factor, the rotational-diffusion approximation of the rotational intermediate scattering function, and the random jump diffusion approximation of the translational intermediate scattering function. Various approximations used previously for the component intermediate scattering functions are not sufficiently accurate. The reason for this is that at supercooled temperatures, due to the dominant cage effect, the conventional picture of the stochastic single-particle diffusion loses its validity. The diffusion process is then progressively controlled by the structural relaxation of the cage. @S1063-651X~97!10310-5#