University of Sussex
Stubbing, James William.pdf (10.37 MB)

The determination of molecular parameters of ices relevant to astrochemistry

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posted on 2023-06-09, 21:09 authored by James StubbingJames Stubbing
Dust grains in the interstellar medium (ISM) provide a surface onto which molecules can accrete and form icy mantles. This thesis describes laboratory investigations of interstellar ice analogues. In particular, the design, installation and testing of a novel, newly developed, ultraviolet/visible (UV/vis) reflection spectrometer is presented. This apparatus allows simultaneous measurement of the ice thickness and refractive index at cryogenic temperatures (˜ 25 K) for the first time. These data are essential for modelling the spectra and radiative transfer of energy in the ISM. The key feature of the apparatus is an ultra-high vacuum (UHV) compatible variable angle lens assembly. A Python program has been written to analyse the spectra in order to determine the wavelength dependent complex refractive index of ices. The advantages that the apparatus has over previous methods of measuring the refractive index and ice thickness are discussed. Prior to the measurement of the refractive index, temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) were used to characterise the ices. Several molecules were studied in this thesis, and the carbonaceous dust grain analogue surface used was highly oriented pyrolytic graphite. Benzene was used as a test system due to its well defined UV/vis spectrum and the fact that is has been detected in space. Toluene offers a good comparison to benzene, to examine the effect of a small molecular modification on surface behaviour. Finally, methyl formate is an example of an interstellar complex organic molecule (COM, molecules containing > 6 atoms), and has been shown in previous work to interact with water in a mixed ice. Previously, it has been assumed that the refractive index of a mixed ice can be taken to be the weighted average of the ice constituents. This work challenges that assumption.


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University of Sussex

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