A study of some atmospherically important molecules and reactions using photoelectron spectroscopy

Abstract

The main objective of the work reported in this thesis was to investigate molecules of atmospheric importance and their reactions. This has been achieved using spectroscopic methods, notably photoelectron spectroscopy, u.v.-visible spectroscopy and infrared spectroscopy, as well as kinetics simulations, global modelling and quantum chemical calculations.

Photoelectron spectroscopy (PES) was used to investigate the pyrolysis behaviour of two hydrofluorocarbon (HFC) fire suppression agents, pentafluoroethane (CF3CHF2) and 2-H heptafluoropropane (CF3CHFCF3) to determine their thermal decomposition pathways. The thermal decomposition of flowing pentafluoroethane (CF3CHF2) diluted in argon was studied over the temperature range 600-1600 ?C. At lower temperatures, (< 1000 ?C) there is evidence of production of the thermal decomposition products C2F4 and HF and at higher temperatures (over 1000 ?C) the decomposition products CF2 and CF3H are observed. The adiabatic and vertical ionisation energy (AIE and VIE) have been measured for the first band of CF3CHF2 from the recorded photoelectron spectrum as (12.71 +- 0.05) eV and (13.76 +- 0.02) eV respectively. The main pathways for decomposition have been established over the temperature range investigated.

Pyrolysis of 2-H heptafluoropropane (CF3CHFCF3), at low pressure, diluted in argon, has been studied over the temperature range 600-2000 ?C. Comparison of the results obtained has been made with results of recent electronic structure calculations of possible decomposition pathways. The most favoured reaction thermodynamically, to produce CF3CF=CF2 + HF, is found to be the main decomposition reaction at lower temperatures, 600-900 ?C. At higher temperatures, 900-1200 ?C, decomposition gave C2F4 + CF3H and it was found that C3F6 decomposes to C2F4 + CF2, and C2F4 decomposes to CF2 at temperatures above 1400 ?C.

Three ozone-alkene reactions (O3 + ethene, O3 + 2-methylpropene (2MP) and O3 + 2, 3 dimethyl-2-butene (DMB)) were separately investigated, each at low pressure, using a flow-tube interfaced to a photoelectron spectrometer. Photoelectron spectra, recorded as a function of reaction time, have been used to estimate partial pressures of the reagents and products, using photoionisation cross-sections for selected photoelectron bands of the reagents and products, (which have been measured separately) for each reaction. The yields of all the main products have been determined for each reaction. For each reaction, oxygen was observed as a product for the first time and for the O3 + ethene reaction acetaldehyde was measured as a product for the first time. Kinetics simulations were performed using reaction schemes which were developed for these reactions in order to determine the main reactions for production of the observed products.

A feasibility study was carried out on the first PE bands of four selected non-linear triatomic molecules, SF2, HO2, HOCl and Cl2O in order to optimise the experimental production conditions for a threshold PES study using synchrotron radiation. Reliable methods were developed to record threshold PE spectra of SF2, HOCl and Cl2O consistently for upwards of 4 hours. The work on HO2 is still ongoing and the method developed requires further optimisation.

Absorption spectroscopy was used to determine the photolysis rate coefficient, in the atmosphere, of the atmospherically important molecule monochlorodimethyl sulphide (CH3SCH2Cl) from u.v. photoabsorption cross-section measurements in the gas-phase.

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Identifiers

ORCID for John M. Dyke: orcid.org/0000-0002-9808-303X

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