Vieira, Paulo Manuel Assis Loureiro Limao; (2003) Electronic excitation and dissociation of global warming and ozone depleting molecules. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The research described in this thesis covers a wide range of experimental techniques used to characterise the excitation and dissociation processes of molecules relevant to atmospheric studies, with a particular emphasis on a newly discovered greenhouse gas, trifluoromethyl sulphur pentafluoride, SF5CF3. Since the excitation, dissociation and ionisation processes, together with their subsequent chemical reactivity are particularly important in the upper region of the Earth's atmosphere (e.g., in the stratosphere and ionosphere), the research aimed to provide accurate VUV absorption cross sections for a selected set of atmospheric trace gases. The spectroscopy of such molecules has been investigated by high resolution photo-absorption and, in the particular case of SF5CF3, electron energy loss spectroscopy (EELS) was also used. High resolution photo-absorption spectra were obtained in a new apparatus assembled at the Institute for Storage Ring Facilities, University of Aarhus, Denmark, where synchrotron radiation with photon energies from 3 eV up to 11 eV has been used with a FWHM resolution better than 0.1 nm. Electronic excitation into both valence and Rydberg states has been studied and assigned for dimethylsulphide and acetaldehyde, trifluoromethyl sulphur pentafluoride, dichlorodifluoromethane, acetonitrile and acrylonitrile. The present results are compared, wherever possible, with previous data. A photo-electron experiment carried out at the University of Liege to investigate trifluoromethyl sulphur pentafluoride is also described. The ionisation energies obtained were then used to assign the Rydberg series observed in the VUV studies. At stratospheric altitudes, local chemistry and physics are strongly influenced by the formation of ices and dust surfaces and therefore in this thesis we also present, for the first time, results on the reactivity of gaseous SF5CF3 on ice surfaces.

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