AVS 45th International Symposium
    Surface Science Division Friday Sessions
       Session SS2-FrM

Paper SS2-FrM3
A Study of Amorphous Solid Water (ASW) Morphology using N@sub 2@ Gas Adsorption and Thermal Desorption

Friday, November 6, 1998, 9:00 am, Room 309

Session: Water and Ice Interfaces
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
Correspondent: Click to Email
We have probed the morphology of amorphous solid water (ASW) thin films grown below 140 K using N@sub 2@ gas adsorption and temperature programmed desorption (TPD). Surprisingly, we find that ASW films grown with increasing angles of incidence from a directed vapor source show 20-100 fold increase in the uptake of N@sub 2@ gas at 26 K. The integrated N@sub 2@ TPD signals obtained from subsequent thermal desorption experiments reflect significant increases in the available surface area and porosity of ASW films with increasing incident angle. In comparison, ASW films grown by ambient H@sub 2@O backfilling of the experimental apparatus show integrated N@sub 2@ signals most comparable with ASW films grown at oblique angles of incidence. This observation has important implications for those studying ASW in laboratory settings, since the angle of incidence as an experimental control variable has not been widely appreciated in the preparation of ASW thin films. Further, we have investigated the affects of growth temperature, film thickness, and annealing on ASW morphology. The available ASW film surface area is observed to decrease with increasing growth temperature until 80 K where it becomes relatively constant thereafter. For a given incident angle, a roughly linear increase with increasing film thickness is observed for ASW films grown at 22 K. ASW films annealed above 120 K show a significant collapse of the micropore structure consistent with previously published accounts. These results have important implications for understanding the chemical and physical properties of ASW found in astrophysical media such as comets, planetary satellites, and interstellar grains. @FootnoteText@ Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.