Dynamics of ice layers deposited on MgO(001): Quasielastic neutron scattering experiments and molecular dynamics simulations

The dynamical behavior of a thin film of ice Ih deposited on MgO(001) surfa ce has been investigated both experimentally and theoretically. Incoherent neutron quasielastic scattering experiments, using uniform MgO powders, sho w that a quasiliquid water layer of monolayer thickness exists at T=265 K. The translational mobility of this layer, with a diffusion coefficient D-t= 1.5x10(-5) cm(2) s(-1), is close to that of liquid water. At T=270 K, the t hickness of the quasiliquid layer increases to about two layers, showing no appreciable change in the D-t value but an increase of the rotational mobi lity from 6x10(9) s(-1) to 1.2x10(10) s(-1). Classical molecular dynamics s imulations are performed to determine the translational and orientational o rder parameters and diffusion coefficients of the supported ice film as a f unction of temperature within 190 and 270 K, and to compare the results wit h those obtained for bulk ice. It is shown that the whole supported ice fil m is much more disordered than bulk ice, with melting temperature around 23 5 K for the TIP4P potential used, while the melting temperatures of the out ermost layer are nearly the same (around 220 K) for the supported film and bulk ice. Comparison of the values of the translational and orientational d iffusion coefficients obtained in simulation and experiments displays a goo d agreement. Although the calculated value of the surface melting temperatu re is underestimated by 15% with respect to the experimental result, the pr esent study indicates clearly the influence of the support on the melting p rocess. (C) 2001 American Institute of Physics.