Trapping and desorption of complex organic molecules in water at 20 K

Binding energies of ethanol and ethylamine on interstellar water ices: synergy between theory and experiments

Experimental and computational chemistry are two disciplines used to conduct research in astrochemistry, providing essential reference data for both astronomical observations and modeling. These approaches not only mutually support each other, but also serve as complementary tools to overcome their respective limitations. Leveraging on such synergy, we characterized the binding energies (BEs) of ethanol (CH3CH2OH) and ethylamine (CH3CH2NH2), two interstellar complex organic molecules (iCOMs), on crystalline and amorphous water ices through density functional theory (DFT) calculations and temperature-programmed desorption (TPD) experiments. Experimentally, CH3CH2OH and CH3CH2NH2 behave similarly, in which desorption temperatures are higher on the water ices than on a bare gold surface. Computed cohesive energies of pure ethanol and ethylamine bulk structures allow describing of the BEs of the pure species deposited on the gold surface, as extracted from the TPD curve analyses. The BEs of submonolayer coverages of CH3CH2OH and CH3CH2NH2 on the water ices cannot be directly extracted from TPD due to their co-desorption with water, but they are computed through DFT calculations, and found to be greater than the cohesive energy of water. The behaviour of CH3CH2OH and CH3CH2NH2 is different when depositing adsorbate multilayers on the amorphous ice, in that, according to their computed cohesive energies, ethylamine layers present weaker interactions compared to ethanol and water. Finally, from the computed BEs of ethanol, ethylamine and water, we can infer that the snow-lines of these three species in protoplanetary disks will be situated at different distances from the central star. It appears that a fraction of ethanol and ethylamine is already frozen on the grains in the water snow-lines, causing their incorporation in water-rich planetesimals.

Using Surface Science Techniques to Investigate the Interaction of Acetonitrile with Dust Grain Analogue Surfaces : Behaviour of acetonitrile and water on a graphitic surface

Surface science methodologies, such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), are ideally suited to studying the interaction of molecules with model astrophysical surfaces. Here we describe the use of RAIRS and TPD to investigate the adsorption, interactions and thermal processing of acetonitrile and water containing model ices grown under astrophysical conditions on a graphitic dust grain analogue surface. Experiments show that acetonitrile physisorbs on the graphitic surface at all exposures. At the lowest coverages, repulsions between the molecules lead to a decreasing desorption energy with increasing coverage. Analysis of TPD data gives monolayer desorption energies ranging from 28.8‐39.2 kJ mol−1 and an average multilayer desorption energy of 43.8 kJ mol−1. When acetonitrile is adsorbed in the presence of water ice, the desorption energy of monolayer acetonitrile shows evidence of desorption with a wide range of energies. An estimate of the desorption energy of acetonitrile from crystalline ice (CI) shows that it is increased to ~37 kJ mol−1 at the lowest exposures of acetonitrile. Amorphous water ice also traps acetonitrile on the graphite surface past its natural desorption temperature, leading to volcano and co-desorption. RAIRS data show that the C≡N vibration shifts, indicative of an interaction between the acetonitrile and the water ice surface.

Mechanisms of methyl formate production during electron-induced processing of methanol-carbon monoxide ices

The formation of methyl formate (CH3OCHO) upon electron irradiation of mixed ices of carbon monoxide (CO) and methanol (CH3OH) has been monitored by post-irradiation thermal desorption spectrometry (TDS). The energy dependence of the product yields obtained with electron energies between 3 and 18 eV was studied. These energies are characteristic of secondary electrons that are released in vast numbers under the effect of ionizing radiation. Our results reveal that the reactions leading to methyl formate are initiated by a number of different electron-molecule interactions that produce CH3O˙ radicals. Dissociative electron attachment (DEA) to CH3OH around 5.5 eV and neutral dissociation (ND) above 7 eV release CH3O˙ radicals that can add to CO to initiate a reaction sequence leading to formation of methyl formate. Around 10 eV, DEA to CO yields an oxygen radical anion that reacts with CH3OH to also produce CH3O˙ radicals. Alternatively, CH3OH can also release H˙ radicals upon both DEA and ND. These can also add to CO to form HCO˙ radicals as an intermediate to formaldehyde (H2CO), which was also investigated to unravel the reaction mechanisms leading to formation of methyl formate. The recombination of HCO˙ and CH3O˙ as minority radical species is considered as an alternative but less probable pathway to the formation of methyl formate. To the best of our knowledge, this is the first study showing considerable contributions of DEA to the formation of methyl formate in CH3OH containing ices. Thus, our study has important implications for current astrochemical models.

Desorption and crystallisation of binary 2-propanol and water ices adsorbed on graphite

Strong interactions between 2-propanol and water ice cause marked changes in the crystallisation kinetics and desorption of water.

Infrared study on the thermal evolution of solid state formamide

Formamide synthesized in interstellar ice analogues after energetic processing remains trapped in the refractory residue simultaneously produced.