Low-temperature, amorphous water ice films grown by vapor deposition under high-vacuum are exposed to microwave-frequency discharge-activated oxygen in order to investigate its effect on the ice surface. Adsorption of methane is used to probe alterations to microscale structures and surface morphology. Films are interrogated throughout the experiment by grazing-angle Fourier-transform infrared reflection-absorption spectroscopy, and after the experiment by temperature-programmed desorption mass spectrometry. Multilayer Fresnel thin-film optics simulations aid in the interpretation of absorbance spectra. Using these techniques, structural alterations are observed over a range of spatial and time scales. At first, spectral absorbance features arising from incompletely coordinated water molecules disappear. The density of high-energy methane adsorption sites is reduced, lowering the equilibrium amount of adsorbed methane. At longer exposure times, this is manifested in a narrowing of the width of the primary methane desorption peak, indicating a narrower range of methane adsorption energies on the ice surface. Together these observations indicate restructuring of micropores resulting in an increase in the structural homogeneity of the film. Enhancement of small, higher-temperature methane desorption features associated with methane encapsulation during thermal annealing indicates alterations to larger pore structures by the same restructuring process. Attribution of these effects to various energetic species in active oxygen is discussed. Based on their abundance, O(3P) and O2(a1Δg) are the most likely candidates; other trace atomic and molecular species may also contribute.
Bibliographical noteFunding Information:
This material is based upon work supported by the National Aeronautics and Space Administration under Grant No. 05-SRT05-48, issued through the Office of Geospace Sciences, Sun-Earth System Division. This research was additionally supported by an award from Research Corporation, Grant No. CC6673. Dr. Boulter is a Cottrell Scholar of Research Corporation. Additional support came from the University of Wisconsin-Eau Claire Office of Research and Sponsored Programs, The Kell Container Corporation Scholarship for Faculty/Student Collaborative Research, and start-up funding from UWEC. Thanks go to J. White for preliminary O-atom concentration measurements and to M. Hooper for early work developing the experimental vacuum chamber.