AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS3-MoM

Paper SS3-MoM10
The Effect of Porosity on the Crystallization of Amorphous Solid Water

Monday, October 25, 1999, 11:20 am, Room 612

Session: Water-Surface Interactions
Presenter: Z. Dohnálek, Pacific Northwest National Laboratory
Authors: Z. Dohnálek, Pacific Northwest National Laboratory
G.A. Kimmel, Pacific Northwest National Laboratory
K.P. Stevenson, Pacific Northwest National Laboratory
R.S. Smith, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
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The annealing and crystallization of amorphous solid water (ASW) is important to understand the properties of ice in extraterrestrial environments. Here we show a dramatic effect of the initial porosity of the ASW films on their crystallization kinetics. The porosity of the films was varied by changing the incident angle of the H@sub 2@O molecules during the deposition.@footnote 1@ The crystallization kinetics were followed using the temperature programmed desorption (TPD) of N@sub 2@(g) monolayers from the ASW film surface. Despite the fact that the films densify upon annealing, before any crystallization takes place, a strong dependence of the crystallization kinetics on the initial porosity exists. The crystallization of dense ASW films (grown at zero angle of incidence) proceeds via classical bulk nucleation and 3-dimensional growth. In the case of porous films (grown at high angles of incidence) the crystallization half time is reduced by more than an order of magnitude and the apparent activation energy decreases to a value that corresponds to the growth of the crystalline phase in the absence of nucleation. We propose that the lowering of the activation energy is due to seeding of the porous films that is likely to occur during the densification. This conclusion is supported by detailed Monte Carlo simulations of the effect of seeding on the crystallization kinetics. @FootnoteText@ * Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830. @footnote 1@ K. P. Stevenson, G. A. Kimmel, Z. Dohnálek, R. S. Smith, and B. D. Kay, Science 283 (1999) 1505.