Monte Carlo Modeling of Astrophysically-Relevant Temperature-Programmed Desorption Experiments

Abstract

The formation of molecules in the interstellar medium is significantly driven by grain chemistry, ranging from simple (e.g. H₂) to relatively complex (e.g. CH₃OH) products. The movement of atoms and molecules on amorphous ice surfaces is not well constrained, and this is a quintessential component of surface chemistry. We show that ice structure created by utilizing an off-lattice Monte Carlo kinetics model is highly dependent on deposition parameters (i.e. angle, rate, and temperature). The model, thus far, successfully predicts the densities of deposition rate- and temperature-dependent laboratory experiments. The simulations indicate, when angle and deposition rate increase, the density decreases. On the other hand, temperature has the opposite effect and will increase the density. We can make ices with desired densities and monitor how molecules, like CO, percolate through H₂O ice pores. The strength of this model lies in the ability to replicate TPD-like experiments by monitoring molecules diffusing on and desorbing from user-defined surfaces.