AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA4
Physicochemical Properties of Water and Ammonia Ice Thin Films

Tuesday, October 3, 2000, 3:00 pm, Room 210

Session: Water/Surface Interactions
Presenter: D.J. Safarik, University of Texas at Austin
Authors: D.J. Safarik, University of Texas at Austin
R.J. Meyer, University of Texas at Austin
C.T. Reeves, University of Texas at Austin
C.B. Mullins, University of Texas at Austin
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We have studied the physical and chemical properties of water and ammonia ice films utilizing surface science techniques. Ultrathin films of amorphous solid water (ASW) and crystalline ammonia ice were grown via supersonic molecular beam on an Ir(111) substrate and characterized with temperature programmed desorption (TPD) and low energy electron diffraction (LEED). Overlayers of ASW dramatically alter the desorption of ammonia, resulting in a TPD spectrum that is distinctly different than free ammonia desorption yet unlike the desorption of carbon tetrachloride through water overlayers (the "molecular volcano") as reported by Smith et al.@footnote 1@ Whereas uncovered ammonia multilayers desorb with zero order kinetics at approximately 100 K, covered films desorb in a sequence of one to three distinct and rapid bursts. The number of ammonia releases, the temperature at which they occur, and their intensity varies with the thickness of both the ASW and ammonia ice films. These phenomena cannot be completely explained by a diffusional model. The overlying water film desorbs with zero order kinetics at approximately 160 K, apparently unaffected by the ammonia discharge. However, TPD, isothermal desorption, and LEED experiments indicate that the ammonia underlayer accelerates the kinetics of ASW crystallization and reduces the volatility of the remaining solid water. @FootnoteText@ @footnote 1@ R. S. Smith, C. Huang, E. K. L. Wong, and B. D. Kay. The Molecular Volcano: Abrupt CCl4 Desorption Driven by the Crystallization of Amorphous Solid Water. Phys. Rev. Lett. 79, 909-912 (1997).