Laboratory astrochemistry of dust and ice
Taj, Skandar Mahmmood
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Thin film growth and desorption behaviour of simple molecules on interstellar dust grain analogue surfaces has been investigated using a range of surface science techniques including temperature programmed desorption (TPD), reflection-absorption infrared (RAIR) and reflection-adsorption UV-Visible spectroscopy. The systems investigated use amorphous silica (aSiO2) as a mimic for bare interstellar dust grains and thin adsorbed films of ammonia (NH3), benzene (C6H6), carbon monoxide (CO), compact and porous amorphous solid water (c-ASW and p-ASW) crystalline solid water (CSW), methanol (CH3OH) and methyl formate (HCOOOCH3, MF). The optical properties for benzene (C6H6) were investigated using a newly designed and constructed UV/Visible spectrometer. Preliminary measurements of C6H6 on a highly-orientated pyrolytic graphite (HOPG) surface give the refractive index (n) as 1.43 ± 0.07 for a film of thickness (d) 261±5 nm. MF on aSiO2 was investigated using TPD, RAIRS and ab initio calculations. The TPD of MF is consistent with wetting of the aSiO2 surface. The binding energy of the monolayer was found to be 29.8±0.1 kJ mol-1 and that of the multilayer is 26.4±5.5 kJ mol-1 . This indicates that MF coupling to the aSiO2 surface is weak and only slightly stronger than the MF interaction with itself. Below 95 K, MF is in an amorphous phase and above 95 K, it is crystalline. A combination of measurements of spontaneous dipole orientation and RAIR spectra with computational chemistry supports the idea that the basis motif of the lattice in crystalline cis-MF is a ring dimer structure. A simple method was developed to synthesise the vibrational line profile of CO on a heterogeneous surface. The procedure developed allows the conversion of a distribution of binding energies, Edes, into a continuous distribution of vibrational frequencies, which can in turn be compared with experimental RAIRS data. The interaction of CO with a range of astrophysically surfaces including CH3OH, CSW, c-ASW, amorphous silica and NH3 on the aSiO2 substrate was investigated using TPD. Extended Inversion Analysis was used to determine the pre-exponential factor, distribution of Edes and the entropy of activation (∆ǂS) for desorption of CO from each surface.