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Paper: astro-ph/0511548
Date: Fri, 18 Nov 2005 02:06:46 GMT (955kb)

Title: Formation of Hydrogen, Oxygen, and Hydrogen Peroxide in Electron
Irradiated Crystalline Water Ice

Authors: Weijun Zheng, David Jewitt, and Ralf I. Kaiser

Comments: ApJ, March 2006, v639 issue, 43 pages, 7 figures
\\
Water ice is abundant both astrophysically, for example in molecular clouds,
and in planetary systems. The Kuiper belt objects, many satellites of the outer
solar system, the nuclei of comets and some planetary rings are all known to be
water-rich. Processing of water ice by energetic particles and ultraviolet
photons plays an important role in astrochemistry. To explore the detailed
nature of this processing, we have conducted a systematic laboratory study of
the irradiation of crystalline water ice in an ultrahigh vacuum setup by
energetic electrons holding a linear energy transfer of 4.3 +/- 0.1 keV mm-1.
The irradiated samples were monitored during the experiment both on line and in
situ via mass spectrometry (gas phase) and Fourier transform infrared
spectroscopy (solid state). We observed the production of hydrogen and oxygen,
both molecular and atomic, and of hydrogen peroxide. The likely reaction
mechanisms responsible for these species are discussed. Additional formation
routes were derived from the sublimation profiles of molecular hydrogen (90-140
K), molecular oxygen (147 -151 K) and hydrogen peroxide (170 K). We also
present evidence on the involvement of hydroxyl radicals and possibly oxygen
atoms as building blocks to yield hydrogen peroxide at low temperatures (12 K)
and via a diffusion-controlled mechanism in the warming up phase of the
irradiated sample.

\\ ( http://arXiv.org/abs/astro-ph/0511548 , 955kb)

[ljk4-1]

Paper: astro-ph/0512596

Date: Fri, 23 Dec 2005 12:20:41 GMT (572kb)

Title: Molecular dynamics study of photodissociation of water in crystalline
and amorphous ice

Authors: Stefan Andersson, Ayman Al-Halabi, Geert-Jan Kroes, Ewine F. van
Dishoeck

Comments: 71 pages and 17 figures, consisting of paper (54 pages and 11
figures) and supporting material (17 pages and 6 figures). To be published in
J. Chem. Phys. (2006)
\\
We present results of classical dynamics calculations, performed to study the
photodissociation of water in crystalline and amorphous ice surfaces at a
surface temperature of 10 K. Dissociation in the top six monolayers is
considered. Desorption of H2O has a low probability (less than 0.5% yield per
absorbed photon) for both types of ice. The final outcome strongly depends on
the original position of the photodissociated molecule. For molecules in the
first bilayer of crystalline ice and the corresponding layers in amorphous ice,
desorption of H atoms dominates. In the second bilayer H atom desorption,
trapping of the H and OH fragments in the ice, and recombination of H and OH
are of roughly equal importance. Deeper into the ice H atom desorption becomes
less important and trapping and recombination dominate. The distribution of
distances traveled by H atoms in the ice peaks at 6 - 7 Angstroms with a tail
going to about 60 Angstroms for both types of ice. The mobility of OH radicals
is low within the ice with most probable distances traveled of 2 and 1
Angstroms for crystalline and amorphous ice, respectively. OH is however quite
mobile on top of the surface, where it has been found to travel more than 80
Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and
liquid water are found to be in very good agreement with experiments.

\\ ( http://arXiv.org/abs/astro-ph/0512596 , 572kb)