Adsorption of CO on amorphous water-ice surfaces

A&A 422, 777-791 (2004)
DOI: 10.1051/0004-6361:20035939

Adsorption of CO on amorphous water-ice surfaces

A. Al-Halabi^{1, 2}, H. J. Fraser³, G. J. Kroes¹ and E. F. van Dishoeck²

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)

Abstract
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 $E_{\rm i}$ = 0.01 eV (116 K) and surface temperature $T_{\rm s}$ = 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 $\tau$ 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 $T_{\rm s}$ , such that at $T_{\rm s}$ = 90 K, $\tau$ = 300 ns. The pre-exponential factor $\tau_{\nu}$ used in the Frenkel model is found to be 0.95 $\pm$ 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