Issue |
A&A
Volume 422, Number 3, August II 2004
|
|
---|---|---|
Page(s) | 777 - 791 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20035939 | |
Published online | 16 July 2004 |
Adsorption of CO on amorphous water-ice surfaces
1
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
2
Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: ewine@strw.leidenuniv.nl
3
Raymond and Beverly Sackler Laboratory for Astrophysics at Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
Received:
23
December
2003
Accepted:
4
March
2004
We present the results of classical trajectory
calculations of the adsorption of thermal CO on the surface of
compact amorphous water ice, with a view to understanding the
processes governing the growth and destruction of icy mantles on
dust grains in the interstellar medium and interpreting solid CO
infrared spectra. The calculations are performed at normal
incidence, for Ei = 0.01 eV (116 K) and surface temperature
Ts = 90 K. The calculations predict high adsorption
probabilities (~1), with the adsorbed CO molecules having
potential energies ranging from -0.15 to -0.04 eV with an
average energy of -0.094 eV. In all the adsorbing trajectories,
CO sits on top of the surface. No case of CO diffusion inside the
ice or into a surface valley with restricted access was seen.
Geometry minimizations suggest that the maximum potential energy
of adsorbed CO (-0.155 eV) occurs when CO interacts with a
“dangling OH” group, associated with the 2152 cm-1 band
seen in laboratory solid-state CO spectra. We show that relatively
few “dangling OH” groups are present on the amorphous ice
surface, potentially explaining the absence of this feature in
astronomical spectra. CO also interacts with “bonded OH” groups,
which we associate with the 2139 cm-1 infrared feature of
solid CO. Our results for CO adsorption on amorphous ice are
compared with those previously obtained for CO adsorption to
crystalline ice. The implications of the spectroscopic assignments
are discussed in terms of the solid-CO infrared spectra observed
in interstellar regions. Using the Frenkel model, the lifetime
τ for which CO may remain adsorbed at the surface is
calculated. At temperatures relevant to the interstellar medium,
i.e. 10 K, it is longer than the age of the universe, but
decreases dramatically with increasing Ts, such that at Ts =
90 K, τ = 300 ns. The pre-exponential factor
used in the Frenkel model is found to be 0.95 ± 0.02 ps. These
data are compared to recent experimental results. The
astrophysical implications of these calculations are discussed,
with particular reference to the CO binding sites identified on
amorphous ice surfaces, their adsorption energies, probabilities
and lifetimes.
Key words: astrochemistry / line: identification / molecular data / molecular processes / ISM: molecules / ISM: lines and bands
© ESO, 2004
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