Incoherent quasielastic neutron scattering from water in supercooled regime

Abstract

Measurements of the quasielastic spectra have been made with a three-axis neutron spectrometer at constant-$Q$ mode in a temperature range from 38°C down to -20°C. Two energy resolutions both high ($\Delta E=100\mathrm{}$ μeV) and low ($\Delta E=800\mathrm{}$ μeV) were used to identify and separate a sharp component from a broad one. As temperature is decreased below zero the spectrum shows an increasing sharp component standing out on top of the broad one. The broad component is attributed to rotational motions of water molecules. A preliminary analysis of the linewidths gives a $Q$-independent relaxation time which has the same magnitude as the rotational relaxation time measured by nuclear magnetic resonance. The $Q$ dependence of the sharp line is analyzed by a $Q$-dependent diffusion coefficient. A temperature-independent characteristic length $l_0=0.5\mathrm{}$ Å is obtained. We then attempt to relate this length to local geometry of protons associated with hydrogen bonding.