The thermal desorption of ammonia (NH 3 ) from single crystal forsterite (010) has been investigated using temperature-programmed desorption. The effect of defects on the desorption process has been probed by the use of a rough cut forsterite surface prepared from the cleaved forsterite sample. Several approaches have been used to extract the desorption energy and pre-exponential factor describing the desorption kinetics. In the sub-monolayer coverage regime, the NH 3 desorption shows a broad distribution of desorption energies, indicating the presence of different adsorption sites, which results in an apparent coverage-dependent desorption energy. This distribution is sensitive to the surface roughness with the cut forsterite surface displaying a significantly broader distribution of desorption energies compared to the cleaved forsterite surface. The cut forsterite surface exhibits sites with desorption energies up to 62.5 kJ mol −1 in comparison to a desorption energy of up to 58.0 kJ mol −1 for the cleaved surface. Multilayer desorption is independent of the nature of the forsterite surface used, with a desorption energy of (25.8±0.9) kJ mol −1 . On astrophysically relevant heating timescales, the presence of a coverage dependent desorption energy distribution results in a lengthening of the NH 3 desorption time-scale by 5.9 × 10 4 yr compared to that expected for a single desorption energy. In addition, the presence of a larger number of high-energy adsorption sites on the rougher cut forsterite surface leads to a further lengthening of ca. 7000 yr.

We present temperature programmed desorption (TPD) measurements of CO, CH 4 , O 2 and CO 2 from the forsterite(010) surface in the sub-monolayer and multilayer coverage regimes. In the case of CO, CH 4 and O 2 , multilayer growth begins prior to saturation of the monolayer peak, resulting in two clearly distinguishable desorption peaks. On the other hand a single peak for CO 2 is observed which shifts from high temperature at low coverage to low temperature at high coverages, sharpening upon multilayer formation. The leading edges are aligned for all the molecules in the multilayer coverage regime indicating zero order desorption. We have extracted multilayer desorption energies for these molecules using an Arrhenius analysis. For sub-monolayer coverages, we observe an extended desorption tail to higher temperature. Inversion analysis has been used to extract the coverage dependent desorption energies in the sub-monolayer coverage regime, from which we obtain the desorption energy distribution. We found that owing to the presence of multiple adsorption energy sites on the crystalline surface the typical desorption energies of these small molecules are significantly larger than obtained in previous measurements for several other substrates. Therefore molecules bound to crystalline silicate surfaces may remain locked in the solid state for a longer period of time before desorption into the gas phase.

Context. Formamide (NH2CHO) is one of the simplest “CHON” molecules that has been observed in different environments in space. In star-forming regions, its abundance in the gas phase is correlated to isocyanic acid (HNCO), indicating a chemical relation between the two species. Many studies have investigated the different routes for the transformation of the two species from one to the other. Aims. We carry out an experimental analysis on the interaction of atomic H at 300 K with solid NH2CHO to probe whether HNCO can form. Methods. The effects of H atom irradiation on NH2CHO have been analyzed by Fourier-transform infrared spectroscopy. Results. During irradiation, a decrease in the band intensity of the C–H, C=O, and N–H modes of NH2CHO with a simultaneous increase in the N=C=O band intensity of HNCO is observed, indicating a transformation of NH2CHO to HNCO. The corresponding destruction and formation cross-sections have been estimated from the trend of the normalized column densities as a function of the H atom fluence. The transformation follows a three-step reaction sequence driven by H atom exothermic abstractions that also induce sputtering of the products. No bands of aminomethanol were detected. Conclusions. The interaction of H atoms with NH2CHO in space can be one of the promising mechanisms to explain the chemical relation between NH2CHO and HNCO. In addition, the comparison of our results with those of other energetic processing agents suggests that H atoms play a crucial role in the destruction of NH2CHO ice in dense regions of the interstellar medium.

The VIRTIS imaging spectrometer aboard Rosetta has shown that the nucleus surface of comet 67P/Churyumov-Gerasimenko (67P/CG) is characterized by a broad absorption band at around 3.2 µm. The feature is ubiquitous across the surface and its attribution to (a) specific material(s) has been challenging. In the present paper, we report an experimental investigation showing that the interaction of hydrogen atoms with Mg-rich amorphous silicates determines the formation of hydroxyl groups. The resulting IR spectrum exhibits a broad feature around 3.2 µm similar to that of comet 67P/CG. Hapke's radiative transfer model was employed to estimate an upper limit contribution of 65% of hydroxylated silicates to the observed cometary band intensity. The presence of a hydroxylated fraction in silicates on the cometary surface would represent an evolutionary link between primitive objects of the solar system and dust in the interstellar medium (ISM), where silicate grains can be hydroxylated after having interacted with hydrogen atoms. This link is consistent with the detection of the aliphatic organics in 67P/CG that also originate in the ISM.

Context. Formamide (NH2CHO) has been proposed as a potential prebiotic precursor in the scientific discourse on the origin of life. It has been observed in different environments in space, including protostellar regions and comets. The abundance and stability of NH2CHO in the early stages of star formation can be better understood by incorporating the formation and destruction data in astrochemical models. Aims. We carried out an experimental investigation to study the destruction of pure NH2CHO ice at 12 K as a result of the interaction with Lyα (121.6 nm) photons. Methods. We studied UV photo destruction of NH2CHO using Fourier-transform infrared spectroscopy. Results. After UV processing, the intensity of NH2CHO IR bands decreased and new bands corresponding to HCN, CO, NH4+ OCN−, HNCO, and CO2 appeared in the spectrum. We then derived the destruction and cumulative product formation cross-sections. Conclusions. A comparison of destruction rates derived from the cross-section in a cold and dense molecular cloud for different energetic processing agents reveals that UV photons induce NH2CHO destruction at a level that is one order of magnitude greater than that affected by cosmic rays; however, it is three orders of magnitude lower than that of H atoms.

Refractory dust grains have an important role to play in the chemistry of star and planet-forming regions. Their surfaces interact with interstellar gas and act as a catalyst for the formation of simple and complex molecules in space. Several mechanisms have been invoked to explain how molecular hydrogen is formed in reactions on dust grain surfaces in different regions of space. In this article, we give an overview of our understanding of the laboratory experiments, conducted over the last 20 years, that deal with H2 formation on interstellar grain analogs in space simulated conditions.

We present the results of an experimental study of the interaction of Lyα photons with superhydrogenated coronene films. The effects of ultraviolet (UV) irradiation have been analyzed with infrared spectroscopy. The spectral changes provide evidence for UV photodestruction of the C–D bonds of the superhydrogenated coronene with a cross section of 8 ± 2 × 10−20 cm2. The comparison of our experimental result with the prediction from theoretical modeling suggests an extension of the region inside photodissociation regions where superhydrogenated coronene can survive and contribute to H2 formation. H2 formation through abstraction in superhydrogenated coronene dominates over direct H2 loss induced by UV photodestruction.

Context. Carbon dioxide (CO 2 ) is one of the most abundant species detected in icy grain mantles in star forming regions. Laboratory experiments have shown that CO 2 molecules are efficiently formed in the solid state under interstellar conditions. Specifically, solid CO 2 can be formed through energetic (e.g. UV photolysis, electron and ion bombardment) and non-energetic mechanisms (atomaddition reactions). Aims. Here we investigate the role of low-energy cosmic-ray bombardment in the formation of solid CO 2 in quiescent dense molecular clouds. Methods. We performed laboratory experiments to study the formation of CO 2 after ion irradiation with 200 keV H + of astrophysical relevant ice mixtures. Laboratory spectra are used to fit the profile of the CO 2 bending mode band observed at about 15.2 µm (660 cm −1 ) by the Spitzer Space Telescope in the line of sight to background sources. Results. From a qualitative point of view, good fits to observations are obtained by considering either three or four laboratory components. From a quantitative point of view, a better result is obtained with four components, i.e. when a spectrum of CO 2 formed after ion irradiation of CH 3 OH ice is added to the fitting procedure. Conclusions. Our results support the hypothesis that energetic processing of icy grain mantles is an efficient formation mechanism of CO 2 ice also in quiescent dark cloud regions, and indirectly suggest the presence of CH 3 OH in icy grain mantles in interstellar cold regions.