12 research outputs found
Laboratory astrochemistry of dust and ice
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