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

Paper SS3-MoM6
The Molecular Volcano: Release of Trapped Volatile Gases by Amorphous Solid Water

Monday, October 25, 1999, 10:00 am, Room 612

Session: Water-Surface Interactions
Presenter: K.P. Stevenson, Pacific Northwest National Laboratory
Authors: K.P. Stevenson, Pacific Northwest National Laboratory
Z. Dohnálek, Pacific Northwest National Laboratory
G.A. Kimmel, Pacific Northwest National Laboratory
R.S. Smith, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
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The trapping and release of volatile gases by amorphous solid water (ASW) has important astrophysical implications, predominately for the adsorption of gases on icy planetary surfaces and the outgassing behavior of cometary bodies. A previous study involving ASW and CCl@sub 4@ introduced the concept of the molecular volcano,@footnote 1@ wherein the abrupt release of trapped CCl@sub 4@ was driven by the amorphous to crystalline ice phase transition. The mechanism for the release of gas was postulated to be the formation of connected pathways in the ASW film during the nucleation and growth of the crystalline ice. We have investigated the mechanisms for trapping and release of volatile gases by ASW using temperature programmed desorption (TPD) from N@sub 2@, O@sub 2@, and Ar adsorbed underneath thin ASW films. The TPD of physisorbed N@sub 2@ monolayers from the ASW surfaces was used to probe the crystalline ice surface fractions and surface area changes during crystallization of the ASW films. The isothermal desorption and TPD of N@sub 2@, O@sub 2@, and Ar deposited beneath thin ASW overlayers results in the desorption of the trapped gas at the onset of crystallization in the ASW. During crystallization, the trapped gas escapes in concert with a cracking of the ASW film as observed by a significant change in film surface area. For thicker ASW overlayers (~600 BL), the gas release is delayed until after the ASW film has crystallized. A comparison with the isothermal desorption of pure ASW films, reveals that the amorphous to crystalline ice phase transition kinetics are independent of the trapped gas in the ASW films. @FootnoteText@ *Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830. @footnote 1@ R. S. Smith, C. Huang, E. K. L. Wong, and B. D. Kay, PRL, 79, (1997) 909.