Desorption of organic molecules from interstellar ices, combining experiments and computer simulations: Acetaldehyde as a case study

Abstract

Context. Explaining the presence of complex organic molecules (COMs) in interstellar environments requires a thorough understanding of the physics and chemistry occurring in the interplay between the gas phase and interstellar surfaces. Experiments and computer simulations are pivotal in building a comprehensive catalogue of processes of relevance for the build up of organic molecules in those environments. Aims. We combine experiments with tailored computer simulations to study the desorption dynamics of acetaldehyde CH3CHO – an important organic precursor in cold interstellar environments - on amorphous solid water for the first time. Our goals with this paper are twofold. Firstly, we want to contextualise the role of this molecule in the evolution of organic molecules in space. Secondly, we want to suggest a joint scheme to produce quantitative information on desorption magnitudes based on the combination of computations and experiments. This scheme can be adopted to refine measurements of other molecules. Methods. We determined desorption energies and pre-exponential factors of desorption theoretically using molecular dynamics simulations that combine semi-empirical and density functional calculations. We also performed temperature-programmed desorption experiments with acetaldehyde on top of non-porous amorphous solid water. The combination of theoretical and experimental results allows us to derive reliable quantities, which are required for understanding the desorption dynamics of interstellar COMs (iCOMs) atop interstellar ices. Results. The average theoretical and experimental desorption energies found for CH3CHO desorbing from non-porous amorphous solid water (np-ASW) surfaces are 3624 K and 3774 K, respectively. The pre-exponential factor determined theoretically is vtheo = 2.4 × 1012 s−1 while from the experiments it was possible to constrain this magnitude to 1012±1 s−1. Conclusions. The comparison of the desorption energies of CH3CHO with other COMs, such as CH3NH2 or CH3NO, shows that CH3CHO is more volatile. Therefore, we suggest that, in consideration of the average binding energy, CH3CHO should undergo preferential desorption during the ice-sublimation phase in hot cores enriching the gas-phase in this particular component. In addition, the overall low binding energy suggests a possible early return to the gas phase of pre-stellar cores due to non-thermal effects (i.e. reactive desorption or cosmic-ray-induced desorption). This could explain the prevalence of CH3CHO in the gas phase of pre-stellar cores. Dedicated laboratory and theoretical efforts are required to confirm this last point.