Structure and dynamics of amorphous water ice

Abstract

Further insight into the structure and dynamics of amorphous water ice, at low temperatures, was obtained by trapping in it Ar, Ne, ${\mathrm{H}}_2$, and ${\mathrm{D}}_2$. Ballistic water-vapor deposition results in the growth of smooth, ∼1×0.2 μ${\mathrm{m}}^2$, ice needles. The amorphous ice seems to exist in at least two separate forms, at T<85 K and at 85<T<136.8 K, and transforms irreversibly from one form to the other through a series of temperature-dependent metastable states. The channels formed by the water hexagons in the ice are wide enough to allow the free penetration of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ into the ice matrix even in the relatively compact cubic ice, resulting in ${\mathrm{H}}_2$- (${\mathrm{D}}_2$-) to-ice ratios (by number) as high as 0.63. The larger Ar atoms can penetrate only into the wider channels of amorphous ice, and Ne is an intermediate case. Dynamic percolation behavior explains the emergence of Ar and Ne (but not ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$) from the ice, upon warming, in small and big gas jets. The big jets, each containing ∼5×${10}^{10}$ atoms, break and propel the ice needles. Dynamic percolation also explains the collapse of the ice matrix under bombardment by Ar, at a pressure exceeding 2.6 dyn ${\mathrm{cm}}^{\mathrm{-}2}$, and the burial of huge amounts of gas inside the collapsed matrix, up to an Ar-to-ice ratio of 3.3 (by number). The experimental results could be relevant to comets, icy satellites, and icy grain mantles in dense interstellar clouds.