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The detection of many complex organic molecules (COMs) in interstellar space has sparked the study of their origins. While the formation of COMs detected in hot cores is attributed to photochemistry on warming grain surfaces followed by recombination of radicals and desorption, the formation routes in colder regions are still a debated issue with a number of theories such as cosmic ray bombardment on interstellar ice mantles or non-diffusive surface chemistry. Here, we present another method with reactions involving metastable atomic oxygen in the O(1D) state, which is initially produced by photodissociation of oxygen-containing species in interstellar ices. As a first example, we study the reactions of metastable oxygen atoms and methane in ices to form both formaldehyde and methanol. The reaction is studied incorporating two different surface processes: diffusive and non-diffusive chemistry. The formation of methanol and formaldehyde via metastable oxygen atoms is compared with well-known formation routes of both to understand the O(1D) contributions at different temperatures.

Temperature programmed desorption has been used to probe the distribution of binding energies of carbon monoxide (CO) to molecular solid thin films of astrophysical relevance. Measurements are reported for solid water (both compact amorphous solid water and crystalline water), ammonia, and methanol surfaces. Binding energy distributions and optimized pre-exponential factors based on the inversion method are tabulated. These are compared to existing data on these systems and astrophysical conclusions drawn.

In this study, we employed broadband X-rays ($6-2000$ eV) to irradiate the frozen acetone CH$_3$COCH$_3$, at the temperature of 12 K, with different photon fluences up to $2.7\times 10^{18}$ photons cm$^{-2}$. Here, we consider acetone as a representative complex organic molecule (COM) present on interstellar ice grains. The experiments were conduced at the Brazilian synchrotron facility (LNLS/CNPEN) employing infrared spectroscopy (FTIR) to monitor chemical changes induced by radiation in the ice sample. We determined the effective destruction cross-section of the acetone molecule and the effective formation cross-section for daughter species. Chemical equilibrium, obtained for fluence $2\times 10^{18}$ photons cm$^{-2}$, and molecular abundances at this stage were determined, which also includes the estimates for the abundance of unknown molecules, produced but not detected, in the ice. Timescales for ices, at hypothetical snow line distances, to reach chemical equilibrium around several compact and main-sequence X-ray sources are given. We estimate timescales of 18 days, 3.6 and 1.8 months, $1.4\times 10^9-6\times 10^{11}$ years, 600 and $1.2\times 10^7$ years, and $10^7$ years, for the Sun at 5 AU, for O/B stars at 5 AU, for white dwarfs at 1 LY, for the Crab pulsar at 2.25 LY, for Vela pulsar at 2.25 LY, and for Sagittarius A* at 3 LY, respectively. This study improves our current understanding about radiation effects on the chemistry of frozen material, in particular, focusing for the first time, the effects of X-rays produced by compact objects in their eventual surrounding ices., Comment: Accepted in MNRAS. This is a preprint. 14 pages, 7 figures

We have undertaken a detailed investigation of the adsorption, desorption and thermal processing of the astrobiologically significant isomers glycolaldehyde, acetic acid and methyl formate. Here, we present the results of laboratory infrared and temperature programmed desorption (TPD) studies of the three isomers from model interstellar ices adsorbed on a carbonaceous dust grain analogue surface. Laboratory infrared data show that the isomers can be clearly distinguished on the basis of their infrared spectra, which has implications for observations of interstellar ice spectra. Laboratory TPD data also show that the three isomers can be distinguished on the basis of their thermal desorption behaviour. In particular, TPD data show that the isomers cannot be treated the same way in astrophysical models of desorption. The desorption of glycolaldehyde and acetic acid from water-dominated ices is very similar, with desorption being mainly dictated by water ice. However, methyl formate also desorbs from the surface of the ice, as a pure desorption feature, and therefore desorbs at a lower temperature than the other two isomers. This is more clearly indicated by models of the desorption on astrophysical time-scales corresponding to the heating rate of 25 and 5 M⊙ stars. For a 25 M⊙ star, our model shows that a proportion of the methyl formate can be found in the gas phase at earlier times compared to glycolaldehyde and acetic acid. This has implications for the observation and detection of these molecules, and potentially explains why methyl formate has been observed in a wider range of astrophysical environments than the other two isomers.

Solid state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices toward young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of the present work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals - one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.

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N2 is believed to lock considerable part of nitrogen elemental budget and, therefore, to be one of the most abundant ice constituent in cold dark clouds. This laboratory-based research utilizes high energetic processing of N2 containing interstellar ice analogues using 200 keV H+ and He+ ions that mimics cosmic ray processing of the interstellar icy grains. It aims to investigate the formation of (iso)cyanates and cyanides in the ice mantles at the conditions typical for cold dark clouds and prestellar cores. Investigation of cosmic ray processing as a chemical trigger mechanism is explained by the high stability of N2 molecules that are chemically inert in most of the atom- and radical-addition reactions and cannot be efficiently dissociated by cosmic ray induced UV-field. Two sets of experiments are performed to closer address solid-state chemistry occurring in two distinct layers of the ice formed at different stages of dark cloud evolution, i.e. `H2O-rich' and `CO-rich' ice layers. Formation of HNCO and OCN- is discussed in all of the performed experiments. Corresponding kinetic curves for HNCO and OCN- are obtained. Furthermore, a feature around 2092 cm-1 assigned to the contributions of 13CO, CN-, and HCN is analysed. The kinetic curves for the combined HCN/CN- abundance are derived. In turn, normalized formation yields are evaluated by interpolation of the obtained results to the low irradiation doses relevant to dark cloud stage. The obtained values can be used to interpret future observations towards cold dark clouds using James Webb Space Telescope.

The formation of C=C and C≡C bonds from the processing of pure c-C6H12 (cyclo- hexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by highly-charged, and energetic ions (219 MeV 16 O 7+ and 632 MeV 58 Ni 24+ ) is studied. The experiments simulate the physical chemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ices analogs. The measurements were performed inside a high vacuum chamber at the heavy-ion acceler- ator GANIL (Grand Accelerateur National d'Ions Lourds) i n Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometry at different ion fluences. Dis- sociation cross section of cyclohexane and its half-life in astrophysical environments were determined. A comparison between spectra of bombarded ices and young stellar sources in- dicates that the initial composition of grains in theses env ironments should contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several species containing double or triple bounds were identified in the radiochemical produc ts, such as hexene, cyclohexene, benzene, OCN − , CO, CO2, as well as several aliphatic and aromatic alkenes and alkynes. The results suggest an alternative scenario for the product ion of unsaturated hydrocarbons and possibly aromatic rings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment.

Infrared emission features assigned to gas phase polycyclic aromatic hydrocarbons (PAHs) are observed in space along many lines of sight. In regions where interstellar ices are present, these emissions are largely quenched. It is here that PAHs form agglomerates covered by ice or freeze out on to dust grains, together with volatile species such as H2O, CO, CO2 and NH3. Upon exposure to the Lyα-dominated interstellar radiation field, PAHs are expected to participate in photo-induced chemical reactions, explicitly also involving the surrounding ice matrix. In this paper, a systematic laboratory-based study is presented for the solid-state behaviour of the PAHs pyrene and benzo[ghi]perylene upon Lyα irradiation in ammonia and mixed NH3:H2O astronomical ice analogues. The results are compared to recently published work focusing on a pure water ice environment. It is found that the ice matrix acts as an ‘electronic solid-state switch’ in which the relative amount of water and ammonia determines whether positively or negatively charged PAHs form. In pure water ice, cations are generated through direct ionization, whereas in pure ammonia ice, anions form through electron donation from ammonia-related photoproducts. The solid-state process controlling this latter channel involves electron transfer, rather than acid–base type proton transfer. In the mixed ice, the resulting products depend on the mixing ratio. The astronomical consequences of these laboratory findings are discussed.

The surface formation of CO 2 at low temperatures through the reaction CO + OH and direct dissociation of the resulting HO-CO complex is shown by hydrogenation of a CO:O 2 ice mixture. Such a binary ice is not fully representative for an interstellar ice, but the hydrogenation of O 2 ice produces OH radicals, which allows the investigation of the interstellar relevant CO + OH solid state reaction under fully controlled laboratory conditions. Similar recent astrophysical ice studies have focused on the investigation of isolated surface reaction schemes, starting from the hydrogenation of pure ices, like solid CO or O 2 . For such ices, no CO 2 formation is observed upon H-atom exposure. The hydrogenation of binary ice mixtures presented here allows to investigate for the first time the influence of the presence of other species in the ice on the pure ice reaction shemes. Mixtures of CO:O 2 are deposited on a substrate in an ultra high vacuum setup at low temperatures (15 and 20 K) and subsequently hydrogenated. The ice is monitored by means of Reflection Absorption InfraRed Spectroscopy (RAIRS). Results show that solid CO 2 is formed in all studied CO:O 2 mixtures under our laboratory conditions. Within the experimental uncertainties no dependency on ice temperature or composition is observed. The laboratory results show a correlation between the formation of CO 2 and H 2 O, which is consistent with the astronomical observation of solid CO 2 in water-rich environments. The results also show that the contemporary presence of CO and O 2 molecules in the ice influences the final product yields of the separate CO + H (H 2 CO, CH 3 OH) and O 2 + H (H 2 O 2 and H 2 O) channels, even though the formation rates are not significantly affected.

We consider a model of diffuse cloud dust in which carbon is deposited slowly on the surfaces of silicate cores. The solid carbon is also processed by the interstellar radiation so that its optical properties change with time. The computed interstellar extinction curves (ISECs) for this model evolve in time, lie close to the observed mean ISEC for substantial periods but generate at later times significant dispersions in the far-ultraviolet (for ISECs normalized in the visual). The requirement on the carbon budget is well within that available, while essentially all the available silicon must be in the silicate cores. The variety of observed ISECs may therefore be interpreted (at least, in part) as due to evolutionary changes in a time-dependent model of interstellar dust.

The importance of tunnelling in the formation of CO2 from O( 3 P) + CO at very low temperatures is studied with harmonic quantum transition state theory. To optimize the quantum transition state, we have used both fitted potential energy surfaces and direct dynamics, where forces and energies are calculated by a quantum chemistry code on the fly as they are needed. Whilst tunnelling does increase the classical reaction rate for the O( 3 P) + CO addition reaction at very low temperatures, the onset of tunnelling is at too low temperatures for the reaction to significantly contribute to the formation of interstellar CO2. Instead, it is more likely that an oxygen atom impinging on an interstellar ice would form a loosely bound complex with adsorbed CO, which may subsequently react with a hydrogen atom to ultimately yield CO2.

We use laboratory data on the conversion of solid methane into large hydrocarbons by particle radiation to estimate the fraction of interstellar carbon converted by this process into organics. We find that the maximum fraction of carbon that can be converted into large hydrocarbons during the life of a dense core within an interstellar cloud is in the range of 1 - 5 percent. The implication of this result is that, in order for enough carbon to be converted into organics, frequent passage of material into and out of dense cores is required. If so, then interstellar clouds must exist for at least 10 million years. However, these conclusions should be regarded as preliminary until confirmed by further laboratory studies of the particle irradiation of complex ice mixtures.

Solid CO2 is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO2 at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO2 can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently.

We present 20–110 µm absorbance spectra of H2O ice, deposited on amorphous carbon and silicate substrates, obtained over the 10–140 K temperature range. The measurements have been carried out in a manner that simulates the deposition, warming and cooling of H2O ice mantles on interstellar and circumstellar grains. For H2O ice films deposited on these substrates we find (i) similar 44-µm-band peak wavelength temperature dependences, (ii) no bandshape differences in the respective spectra, and (iii) a structural phase transition occurring between 120 and 130 K. In comparison with published data obtained using a polyethylene substrate, the 52-µm feature (the longitudinal optical mode) observed in our spectra is less prominent. This suggests the presence of material-dependent substrate effects that can alter the appearance of the H2O far-infrared spectrum. The crystallization temperature of H2O ice films deposited on our amorphous silicate substrate is significantly different from that reported by Moore et al. (1994), who found crystallization temperatures down to

The fundamental processes of H2 formation via H + H H2 on the surfaces of icy mantles of interstellar dust have been investigated consistently within a single model based on a classical molecular dynamics (MD) computational simulation. As a model surface for icy mantles of dust grains, an amorphous water ice slab was generated at 10 and 70 K under periodic boundary conditions. The first and second incident H atoms were then ‘thrown’ on to the model surface. Two MD procedures were employed: (i) the H2)O molecules were treated as rigid (hard ice model); (ii) the intramolecular vibrational modes of H2O were included (soft ice model). The amorphous water ice slabs produced by our MD simulations are found to be good models for the surfaces of icy mantles of dust grains. For the various fundamental processes of H2 formation on the dust surface, the following results emerge. (1) For the sticking of an H atom on to the surface, a sticking probability that depends on the temperature of the incident H atom is obtained. (2) For the diffusion of an H atom on the surface, it is found that the first incident H atom diffuses via the thermal hopping mechanism at the first stage, and then it is always trapped in one of the stable sites on the amorphous ice. The migration length and time have been calculated for the mobility of the incident H atom before it is trapped. The time-scales of thermal diffusion and desorption of H atoms after trapping have also been estimated. (3) For the reaction of two H atoms on the surface, the following three reaction patterns are observed: (i) H2 is produced via the Langmuir–Hinshelwood mechanism; (ii) H2 is produced via the Eley–Rideal mechanism; (iii) the almost elastic collision of two H atoms occurs without H2 being formed. The effective reactive cross-section is estimated at about 40 A2. The reaction probabilities are found to be near unity. (4) For the ejection of H2 from the ice surface, the product H2 is subsequently ejected after the reaction process, using part of the excess energy derived from the H2 formation. The average lifetime of ejection is about 400–600 fs. Most of the ejected H2 molecules are found to be in vibrationally and rotationally excited states.

Temperatures of grain models using interstellar radiation field in solar neighborhood and optical data for graphite and ice