Demian Marchione

Laboratory studies of electron and ion irradiation of solid acetonitrile (CH3CN)

The structure and bonding of solid acetonitrile (CH3CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250–400 eV electrons are quantified using transmission infrared spectroscopy, reflection–absorption infrared spectroscopy and temperature-programmed desorption, with the assistance of basic computational chemistry and nuclear materials calculations. The thermal desorption profiles are found to depend strongly on the balance between CH3CN–surface and CH3CN–CH3CN interactions, passing from a sub-monolayer regime (binding energy: 35–50 kJ mol−1) to a multilayer regime (binding energy: 38.2±1.0 kJ mol−1) via a fractional order desorption regime characteristic of islanding as the coverage increases. Calculations using the SRIM code reveal that the effects of the ion irradiation are dominated by electronic stopping of incident protons, and the subsequent generation of secondary electrons. Therefore, ion irradiation and electron irradiation experiments can be quantitatively compared. During ion irradiation of thicker CH3CN films, a cross section for secondary electron-promoted chemical destruction of CH3CN of 4 (±1)×10−18 cm2 was measured, while electron-promoted desorption was not detected. A significantly higher cross section for electron-promoted desorption of 0.82–3.2×10−15 cm2 was measured during electron irradiation of thinner CH3CN films, while no chemical products were detected. The differences between the experimental results can be rationalized by recognizing that chemical reaction is a bulk effect in the CH3CN film, whereas desorption is a surface sensitive process. In thicker films, electron-promoted desorption is expected to occur a rate that is independent of the film thickness; i.e. show zeroth-order kinetics with respect to the surface concentration.

Laboratory investigations of irradiated acetonitrile-containing ices on an interstellar dust analog

Reflection-absorption infrared spectroscopy is used to study the impact of low-energy electron irradiation of acetonitrile-containing ices, under conditions close to those in the dense star-forming regions in the interstellar medium. Both the incident electron energy and the surface coverage were varied. The experiments reveal that solid acetonitrile is desorbed from its ultrathin solid films with a cross section of the order of 10−17 cm2. Evidence is presented for a significantly larger desorption cross section for acetonitrile molecules at the water–ice interface, similar to that previously observed for the benzene–water system.

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.

Electrons, excitons and hydrogen bonding: electron-promoted desorption from molecular ice surfaces

Desorption of benzene (C6H6) from thick methanol (CH3OH) and diethyl ether (CH3CH2OCH2CH3) ices during irradiation with 250 eV electrons is reported and compared with our previous work on C6H6 desorption from water (H2O) ice systems. C6H6 electron-promoted desorption (EPD) is seen to be sensitive to the chemical nature of the substrate reflecting both the importance of the excitations localised around the O-atom versus those involving the C-atom; and the role of hydrogen bonding interactions in transporting non-dissociative electronic excitation to the substrate/C6H6 interfaces during the electron irradiation.