Thermal and non-thermal processes of simple molecules on model interstellar ices

Thin film growth and desorption behaviour of simple molecules have been studied by means of surface science techniques, such as mass spectrometry and reflectionabsorption infrared spectroscopy (RAIRS), in order to understand the physiochemical processes and intermolecular interactions in model interstellar ices. The systems of interest comprise a silica surface, representing the bare grains in the interstellar medium, and films of water (H2O), methanol (CH3OH), diethyl ether ((CH3CH2)2O) and benzene (C6H6). While H2O and CH3OH are key components of the icy mantles, (CH3CH2)2O and C6H6 are found in lower abundances being two products, among many, of the rich chemistry occurring in these environments. Temperature programmed desorption and IR signatures of pure solid H2O, CH3OH, and (CH3CH2)2O adsorbed on amorphous silica were compared as a function of surface coverage and temperature. H2O and (CH3 CH2)2O display opposite behaviours, consistent with two-dimensional island formation and wetting of the amorphous silica surface respectively. CH3OH, being intermediate between the two species, exhibited aspects of both behaviours. Temperature programmed RAIRS has revealed evidence for thermal activation of di↵usion of H2O over the amorphous silica surface between 40 K and 60 K, and of CH3OH between 20 K and 40 K, while no conclusive evidence was found for such with (CH3CH2)2O. Experiments have been performed to study the thermal desorption and the IR features of C6H6 on CH3OH and (CH3CH2)2O solids in comparison to those on a solid H2O substrate at 110 K. The results give a clear picture of the C6H6 film growth from low to high coverages. Ab initio quantum chemical calculations highlight the key interactions between the two species for each system, C6H6/H2O, C6H6/CH3OH and C6H6/(CH3CH2)2O, in support of the interpretation of the data. Building on this basis, 250 eV electron irradiation of C6H6 on thick ices of H2O, or CH3OH, or (CH3CH2)2O was investigated to demonstrate the crucial role of hydrogen-bonding in propagating electronic excitation to the solid-vacuum interface where C6H6 desorption can occur. Competitive electron-induced chemistry in the form of molecular hydrogen (H2) formation was also observed. The electron beam used in the these experiments is inelastically scattered by the molecules in the solid ices forming a similar flux of electrons to that associated to cosmic rays. Conclusions related to the impact of these observations on the early phase of icy interstellar grain chemistry are discussed

Non-covalent Interaction of Benzene with Methanol and Diethyl Ether Solid Surfaces

We have investigated the interactions involved at the interface of binary, layered ices (benzene on methanol and on diethyl ether) by means of laboratory experiments and ab initio calculations on model clusters.</p

Electrons, Excitons and Hydrogen Bonding: Electron-promoted Desorption from Molecular Ice Surfaces

Desorption of benzene from methanol and diethyl ether ices during irradiation with 250 eV electrons is reported and compared with our previous work on benzene/water ices to highlight the role of hydrogen bonding in excitation transport.</p

Efficient Electron-promoted Desorption of Benzene from Water Ice Surfaces

We study the desorption of benzene from solid water surfaces during irradiation of ultrathin solid films with low energy electrons.</p

Peeling the astronomical onion

Water ice is the most abundant solid in the Universe. Understanding the formation, structure and multiplicity of physicochemical roles for water ice in the cold, dense interstellar environments in which it is predominantly observed is a crucial quest for astrochemistry as these are regions active in star and planet formation. Intuitively, we would expect the mobility of water molecules deposited or synthesised on dust grain surfaces at temperatures below 50 K to be very limited. This work delves into the thermally-activated mobility of H2O molecules on model interstellar grain surfaces. The energy required to initiate this process is studied by reflection-absorption infrared spectroscopy of small quantities of water on amorphous silica and highly oriented pyrolytic graphite surfaces as the surface is annealed. Strongly non-Arrhenius behaviour is observed with an activation energy of 2 kJ mol-1 on the silica surface below 25 K and 0 kJ mol-1 on both surfaces between 25 and 100 K. The astrophysical implication of these results is that on timescales shorter than that estimated for the formation of a complete monolayer of water ice on a grain, aggregation of water ice will result in a non-uniform coating of water, hence leaving bare grain surface exposed. Other molecules can thus be formed or adsorbed on this bare surface

Molecular hydrogen production from amorphous solid water during low energy electron irradiation.

This work investigates the production of molecular hydrogen isotopologues (H2, HD, and D2) during low energy electron irradiation of layered and isotopically labelled thin films of amorphous solid water (ASW) in ultrahigh vacuum. Experimentally, the production of these molecules with both irradiation time and incident electron energy in the range 400 to 500 eV is reported as a function of the depth of a buried D2O layer in an H2O film. H2 is produced consistently in all measurements, reflecting the H2O component of the film, though it does exhibit a modest reduction in intensity at the time corresponding to product escape from the buried D2O layer. In contrast, HD and D2 production exhibit peaks at times corresponding to product escape from the buried D2O layer in the composite film. These features broaden the deeper the HD or D2 is formed due to diffusion. A simple random-walk model is presented that can qualitatively explain the appearance profile of these peaks as a function of the incident electron penetration

Photo-conductivity measurements of water-rich ices at cryogenic temperatures

Solid water (H_2O) ice is found in high abundances in a large variety of astrophys. environments: e.g. icy mantles coating interstellar dust grains, in icy bodies like comets and satellites like Europa where exposure to energetic photons, ions and electrons is significant. In these conditions, solid H O is known to dissoc. into H and OH fragments under VUV irradn. while promoting ionization of org. impurities such as polycyclic arom. hydrocarbons (PAHs). This was explained in terms of the relatively high electron affinity of the H_2O mols. and of the OH fragments which, in synergy with the thermodn. stability assocd. with the solvation of ions in solid H_2O, enhances the ionization of pos. charged PAHs. If such ions are indeed produced, these will change the dielec. properties of the ice and can move inside the H_2O matrix when a bias is applied, depending on the ice temp. Our work reports on the investigation of photocurrents in millimeter-sized, water-rich ices contg. org. impurities (e.g. toluene) and aims at understanding how these measurements correlate with the desorption of volatiles during vacuum UV (VUV, e.g. 121.6 nm) and electron (2 keV) irradn

The chemical bond in gold(I) complexes with N-heterocyclic Carbenes

In this contribution we report a comparative analysis of the chemical bond between an N-heterocyclic carbene and different Au(I) metal fragments of general formula [(NHC)AuL]+ or [(NHC)AuL], where NHC is imidazol-2-ylidene and L is chosen from some ligands frequently used both in coordination and in organometallic chemistry. The focus is on the nature of the Au(I)-C (of NHC) bond in terms of Dewar-Chatt-Duncanson components and its modulation by the ancillary ligand L. In the case of L = Cl (metal fragment AuCl), we present a comparative analysis of the binding mode with 1,3-dimethylimidazol-2-ylidene and 13-diphenylimidazol-2-ylidene, where the hydrogens bonded at the nitrogens of NHC have been substituted with methyl and phenyl groups. We applied a model-free definition and a theoretical analysis of the electron-charge displacements making up the donation and back-donation components of the Dewar-Chatt-Duncanson model. We thus show that the nature of the NHC-gold bond is strongly dependent on the electronic structure of the ancillary ligand L. The results clearly confirm that the NHC is not a purely -donor for our systems, but has a \u3c0-back-donation component that amounts to up to half of the -donation (as found in NHC-AuCl) or is entirely negligible (as found in [NHC-AuCO] +). \ua9 2014 American Chemical Society