Thermal desorption of C6H6 from surfaces of astrophysical relevance.

The thermal desorption of C(6)H(6) from two astrophysically relevant surfaces has been studied using temperature programmed desorption. Desorption from an amorphous SiO(2) substrate was used as a mimic for bare interstellar grains, while multilayer films of amorphous solid water (ASW) were used to study the adsorption of C(6)H(6) on grains surrounded by H(2)O dominated icy mantles. Kinetic parameters were obtained through a combination of kinetic modeling, leading edge analysis, and by considering a distribution of binding sites on the substrate. The latter is shown to have a significant impact on the desorption of small exposures of C(6)H(6) from the amorphous SiO(2) substrate. In the case of adsorption on ASW, dewetting behavior and fractional order desorption at low coverage strongly suggest the formation of islands of C(6)H(6) on the H(2)O surface. The astrophysical implications of these observations are briefly outlined.

Laboratory surface astrochemistry experiments.

Although several research groups have studied the formation of H2 on interstellar dust grains using surface science techniques, few have explored the formation of more complex molecules. A small number of these reactions produce molecules that remain on the surface of interstellar dust grains and, over time, lead to the formation of icy mantles. The most abundant of these species within the ice is H2O and is of particular interest as the observed molecular abundance cannot be accounted for using gas-phase chemistry alone. This article provides a brief introduction to the astronomical implications and motivations behind this research and the requirement for a new dual atomic beam ultrahigh vacuum (UHV) system. Further details of the apparatus design, characterisation, and calibration of the system are provided along with preliminary data from atomic O and O2 beam dosing on bare silica substrate and subsequent temperature programmed desorption measurements. The results obtained in this ongoing research may enable more chemically accurate surface formation mechanisms to be deduced for this and other species before simulating the kinetic data under interstellar conditions.

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.

Meteorite nanoparticles as models for interstellar grains: synthesis and preliminary characterisation.

Dust particles and their interaction with gases play important roles in star formation and in solar nebulae. Appropriate model dust grains are needed for the laboratory simulation of gas-grain interactions. Nanoparticles formed from carbonaceous meteorites may be particularly suitable, as these particles are formed from materials that were formed originally from interstellar/nebula dust. Extending our previous studies with grounded meteorite powders, we demonstrate here the production of nanoparticles formed from meteorites using the laser desorption/controlled condensation method developed in our laboratory. The product nanoparticle aggregates have porous, web-like morphologies similar to interstellar dust grains, indicating that they can present large specific surface areas for gas/grain interactions. In this paper, we present polarisation modulation reflection-absorption infrared spectra (PM-RAIRS) of supported thin films and compare these spectra with the known silicate bands in the spectra of interstellar dust recorded during the ISO mission. We also report an ultrahigh vacuum (UHV) temperature programmed desorption (TPD) study of the adsorption of CO on the supported nanoparticle films. The latter allow us to estimate the CO binding energy on the meteorite nanoparticles as 13.5 +/- 3.0 kJ mol(-1), cf. a value of 9.8 +/- 0.2 kJ mol(-1) for CO binding to a water ice substrate. Such thermochemical data can be useful for computational modelling of gas-grain interactions under the diverse conditions in interstellar clouds and solar nebulae.