Adsorption of CO on amorphous water-ice surfaces

Abstract

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 i = 0.01 eV (116 K) and surface temperature T 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 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 T s , such that at T s = 90 K, τ = 300 ns. The pre-exponential factor τ ν 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.

Spatial averaging for small molecule diffusion in condensed phase environments

“…Favorable positions on the surface identified here are in good agreement with positions obtained by unbiased sampling. 6 The barriers for H 2 migration from unbiased sampling ͑where available͒ and from spatial averaging simulations are low ͑Յ1 kcal/ mol͒ and agree with each other, and are in quite good agreement with previous estimates from experiment. 40 A wider exploration of the available phase space with spatial averaging and subsequent unbiasing of the barriers leads to the same conclusion.…”

“…6 The energetic barriers between neighboring positions in this system are low since the energy difference between different positions on the surface originate only from Van-derWaals ͑VdW͒ interactions and electrostatics, and the VdW interactions are very similar for neighboring po-sitions on the surface. The desorption energy in contrast is significantly higher.…”

“…Such structural properties of favorable sites are in good agreement with favorable positions found from MD simulations in earlier studies. 6 …”

Molecular dynamics simulations of D2O ice photodesorption

“…[40][41][42][43] Starting from the normal hexagonal ice (I h ) crystalline ice configuration (containing 8 bilayers (BLs) (16 MLs) with 60 (30) molecules in each ML), the amorphous ice surface was set up at 10, 20, 60, or 90 K using the "fast quenching" method" [46][47][48] Further details can be found in our previous studies. [41][42][43] Since the resulting amorphous ice surface has a more irregular bonding structure than the crystalline ice surface, 41,47 assigning molecules to MLs is not straightforward.…”

Dynamics of Co in Amorphous Water-Ice Environments

“…Second, because CO forms in the gas phase and freezes out on the grain mantles only under certain conditions (Pontoppidan 2006), the ice mantles probably have a layered structure (Allamandola et al 1999;Öberg et al 2011) where the interface between the layers may be especially interesting for astrochemistry. Also, given its importance in the chemical evolution of molecular clouds, the H 2 O-CO system has been extensively studied both experimentally (Bar-Nun et al 1985;Devlin 1992;Allouche et al 1998;Manca et al 2000;Collings et al 2003b;Ayotte et al 2001) and theoretically (Al-Halabi et al 2004a, 2004bManca et al 2001), so there is ample reference material.…”