Dynamics of Co in Amorphous Water-Ice Environments

Abstract

The long-timescale behavior of adsorbed carbon monoxide on the surface of amorphous water ice is studied under dense cloud conditions by means of off-lattice, on-the-fly, kinetic Monte Carlo simulations. It is found that the CO mobility is strongly influenced by the morphology of the ice substrate. Nanopores on the surface provide strong binding sites, which can effectively immobilize the adsorbates at low coverage. As the coverage increases, these strong binding sites are gradually occupied leaving a number of admolecules with the ability to diffuse over the surface. Binding energies and the energy barrier for diffusion are extracted for various coverages. Additionally, the mobility of CO is determined from isothermal desorption experiments. Reasonable agreement on the diffusivity of CO is found with the simulations. Analysis of the 2152 cm −1 polar CO band supports the computational findings that the pores in the water ice provide the strongest binding sites and dominate diffusion at low temperatures.

Grain Surface Models and Data for Astrochemistry

“…Once a significant number of H atoms become trapped, the subsequent H atoms recombine with the trapped H atoms. Therefore, the effective diffusion rate becomes This picture is also observed at an atomistic level through adaptive kinetic Monte Carlo simulations of CO diffusion on an ASW surface (Karssemeijer et al 2014b). Here, for a single CO molecule on an ASW surface, the diffusion is limited by diffusion out of the deep binding sites (E CO diff = 84-114 meV), whereas when the deep sites are filled with CO molecules, the diffusion rate is increased (E CO diff = 48-79 meV).…”

“…This method is limited to stable species with a desorption temperature below that of ASW. Surface diffusion rates for CO molecules have been studied this way (Mispelaer et al 2013;Karssemeijer et al 2014b;Lauck et al 2015).…”

CO Diffusion in Layered Water Ices

“…A larger pore type is called a mesopore that is formed when ices are deposited at an angle where shadowing effects of the columnar growth result in large voids up to tens of nm wide (Smith et al 2011). Karssemeijer et al (2014) describes the properties of another type of pore, a nanopore, which is a small (6Å), strong-binding well on the surface of a water matrix. The different sizes and characteristics of the pores provide several environments that affect diffusion of molecules within the ice mantle, and by studying these processes, information obtained can be correlated to the interstellar diffusion process.…”

“…These experiments were performed under ultra high vacuum conditions with thin (10-32 ML) ices at temperatures ranging from 23 to 27 K. Other recent experiments measured CO diffusion by monitoring the desorption of CO from a layered sample, both under high vacuum conditions. Mispelaer et al (2013) report a diffusion barrier of 10±15 meV for CO diffusing out of an ice film consisting of a pure layer of amorphous water on top of a layer of pure CO. Karssemeijer et al (2014) report a value of 26±15 meV for the diffusion of CO out of a mixture of CO:H 2 O layered underneath pure water. Both of these experiments consisted of layers with thicknesses >450 ML, and were performed at temperatures ranging from 32 to 50 K.…”

“…With a successful model, the components of the exponential and preexponetial terms can be broken down to extract the barrier and diffusion coefficient more accurately. Fick's 2nd Law of Diffusion has been used recently (Karssemeijer et al 2014;Mispelaer et al 2013) to describe CO diffusion in water at temperatures 35 K and above. In a collaborative effort with the Cuppen group at Radboud Univeristy, this method was applied to the experiments presented here.…”