The self-diffusivity of amorphous solid water near 150 K

Molecular beam techniques are used to create nanoscale thin films composed of different isotopes of amorphous solid water (ASW). The metastable ASW composites are then heated above the glass transition temperature, T-g, and the extent of isotopic intermixing is determined using temperature programmed desorption. The observed self-diffusion in the 150-160 K range is roughly a millionfold greater than that expected for crystalline ice. The magnitude and temperature dependence of the self-diffusivity are consistent with an amorphous solid that melts into a deeply supercooled liquid prior to crystallization. The overall temperature dependence for the diffusivity of liquid water, supercooled liquid water (238-273 K), and of ASW (150-160 K) is well described by the Vogel-Fulcher-Tamman equation. These results suggest that ASW above its T-g is a deeply supercooled metastable extension of normal liquid water prior to crystallizing near 160 K. Additionally, rapid H/D isotopic exchange occurs in concert with the translational diffusive motion. (C) 2000 Elsevier Science B.V. All rights reserved.