Relaxational dynamics of supercooled water in porous glass

We have made a high-resolution quasielastic incoherent neutron scattering (QENS) study of the translational dynamics of supercooled water contained in micropores of Vycor glass at different hydration levels. QENS spectra from the confined H2O are analyzed in terms of the cu-relaxation dynamics predicted by mode-coupling theory of supercooled liquids and by a recent computer molecular-dynamics simulation of extended simple point charge model water. We verify that the stretched exponential relaxation description of the long-time test-particle dynamics is consistent with the measured QENS spectral line shape. We are thus able to determine the wave-number dependence of magnitudes of the structural relaxation rate 1/tau and the stretch exponent beta as functions of temperature and coverage. A power-law dependence of the average relaxation time on the magnitude of the scattering vector Q is observed. In the Q range studied, the exponent starts out with nearly -2.0, at room temperature, indicating a continuous diffusion, and gradually becomes less negative as the temperature is decreased to below the freezing temperature. [S1063-651X(99)13302-6].