Temperature-programmed desorption and reflection-absorption infrared spectroscopy have been used to explore the interaction of oxygen (O 2 ), nitrogen (N 2 ), carbon monoxide (CO) and water (H 2 O) with an amorphous silica film as a demonstration of the detailed characterization of the silicate surfaces that might be present in the interstellar medium. The simple diatomic adsorbates are found to wet the silica surface and exhibit first-order desorption kinetics in the regime up to monolayer coverage. Beyond that, they exhibit zero-order kinetics as might be expected for sublimation of bulk solids. Water, in contrast, does not wet the silica surface and exhibits zero-order desorption kinetics at all coverages consistent with the formation of an islanded structure. Kinetic parameters for use in astrophysical modelling were obtained by inversion of the experimental data at sub-monolayer coverages and by comparison with models in the multilayer regime. Spectroscopic studies in the sub-monolayer regime show that the C-O stretching mode is at around 2137 cm −1 (5.43 μm), a position consistent with a linear surface-CO interaction, and is inhomogenously broadened as resulting from the heterogeneity of the surface. These studies also reveal, for the first time, direct evidence for the thermal activation of diffusion, and hence de-wetting, of H 2 O on the silica surface. Astrophysical implications of these findings could account for a part of the missing oxygen budget in dense interstellar clouds, and suggest that studies of the sub-monolayer adsorption of these simple molecules might be a useful probe of surface chemistry on more complex silicate materials.

A low-temperature flow tube and ultra-high vacuum apparatus were used to explore the uptake and heterogeneous chemistry of acetylene (C2H2) on cosmic dust analogues over the temperature range encountered T below 600 km. The uptake coefficient, , was measured at 181 K to be (1.6 ± 0.4) × 10 -4 , (1.9 ± 0.4) × 10 -4 and (1.5 ± 0.4) × 10 -4 for the uptake of C2H2 on Mg2SiO4, MgFeSiO4 and Fe2SiO4, respectively, indicating that  is independent of Mg or Fe active sites. The uptake of C2H2 was also measured on SiO2 and SiC as analogues for meteoric smoke particles T but was found to be below the detection limit ( < 6 × 10 -8 and < 4 × 10 -7 , respectively). The rate of cyclo-trimerization of C2H2 to C6H6 was found to be 2.6 × 10 -5 exp(-741/T) s -1 , with an uncertainty ranging from ± 27 % at 115 K to ± 49 % at 181 K. A chemical ablation model was used to show that the bulk of cosmic dust particles (radius 0.02 -10  T mass loss through sputtering), thereby providing a significant surface for heterogeneous 2 chemistry. A 1D model of dust sedimentation shows that the production of C6H6 via uptake of C2H2 on cosmic dust, followed by cyclo-trimerization and desorption, is probably competitive with gas-phase production of C6H6 between 80 and 120 km.

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Studtite is known to exist at the back-end of the nuclear fuel cycle as an intermediate phase formed in the reprocessing of spent nuclear fuel. In the thermal decomposition of studtite, an amorphous phase is obtained at calcination temperatures between 200 and 500 °C. This amorphous compound, referred to elsewhere in the literature as U2O7, has been characterised by analytical spectroscopic methods. The local structure of the amorphous compound has been found to contain uranyl bonding by X-ray absorption near edge (XANES), Fourier transform infrared and Raman spectroscopy. Changes in bond distances in the uranyl group are discussed with respect to studtite calcination temperature. The reaction of the amorphous compound with water to form metaschoepite is also discussed and compared with the structure of schoepite and metaschoepite by X-ray diffraction. A novel schematic reaction mechanism for the thermal decomposition of studtite is proposed.