The diffusion of molecules in interstellar ice is a fundamental phenomenon to take into account while studying the formation of complex molecules in this ice. This work presents a theoretical study on the diffusion of H2O, NH3, CO2, CO, and H2CO in the bulk of a low density amorphous (LDA) ice, while taking into account the physical conditions prevailing in space, i.e. temperatures below 150 K and extremely low pressure. This study was undertaken by means of molecular dynamics simulations. For CO2 for which no experimental data were available we conducted our own experiments. From our calculations we show that, at low temperatures, the diffusion of molecules in the bulk of a LDA ice is driven by the self-diffusion of water molecules in the ice. With this study we demonstrate that molecular dynamics allows the calculation of diffusion coefficients for small molecules in LDA ice that are convincingly comparable to experimentally measured diffusion coefficients. We also provide diffusion coefficients for a series of molecules of astrochemical interest.

Ammonia and carbon dioxide play an important role in both atmospheric and interstellar ice chemistries. This work presents a theoretical and experimental study of the kinetics of the low-temperature NH3 and CO2 solid-state reaction in ice films, the product of which is ammonium carbamate (NH4(+)NH2COO(-)). It is a first-order reaction with respect to CO2, with a temperature-dependent rate constant fitted to the Arrhenius law in the temperature range 70 K to 90 K, with an activation energy of 5.1 ± 1.6 kJ mol(-1) and a pre-exponential factor of 0.09-0.08(+1.1) s(-1). This work helps to determine the rate of removal of CO2 and NH3, via their conversion into ammonium carbamate, from atmospheric and interstellar ices. We also measure first-order desorption energies of 69.0 ± 0.2 kJ mol(-1) and 76.1 ± 0.1 kJ mol(-1), assuming a pre-exponential factor of 10(13) s(-1), for ammonium carbamate and carbamic acid, respectively.