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Absolute photoabsorption and photoionization studies of large polyatomic molecules by high energy electron impact and mass spectromentry

University of British Columbia

Electronic excitation spectra and absolute differential oscillator strengths (cross sections) have been measured from the visible up to the soft X-ray energy regions (2-200 eV) for the valence-shell photoabsorption of a series of normal alkanes (C₅H₁₂, C₆H₁₄, C₇H₁₆, and C₈H₁₈), single carbon Freons (CFCl₃, CF₂CI₂, CF₃CI, and CF₄), nitrogen dioxide (NO₂) and phosphorus halides (PF₃, PC1₃, and PF₅) at 1 eV resolution using dipole (e,e) spectroscopy. The absolute differential oscillator strength data for PF₃, PC1₃, and PF₅have also been extended through the P 2p,2s and CI 2p,2s inner shells up to 300 eV. In addition, the discrete structures in the valence region and in the vicinity of the P 2p and CI 2p (PC1₃) inner shells of these molecules have been studied at higher resolution (0.05-0.1 eV fwhm). Comparisons have been made to previously reported optical measurements in those limited energy regions where such data exist. The accuracy of the absolute differential oscillator strength scales have been critically evaluated by comparison of the static electric-dipole polarizabilities of the normal alkanes, single carbon Freons, NO2, PCI3, and PF5 derived from the present data using the S(-2) sum rule with those determined from refractivity and dielectric constant measurements in the literature. The S(-2) sum rule has also been used with the differential oscillator strength data to determine the experimental dipole polarizability of PF₃ for the first time. The feasibility of using atomic and molecular mixture rules as well as group additivity concepts for predicting differential oscillator strengths for the valenceshell photoabsorption of long-chained alkane molecules has been investigated over a wide energy range from 20 to 200 eV. The predictions are discussed with reference to the experimental measurements for the normal alkanes obtained in the present work (CnH₂n +₂, n = 5-8) as well as for the smaller alkanes (n = 1-4) previously reported in my B.Sc. undergraduate thesis project. Dipole (e,e+ion) coincidence spectroscopy has been used to obtain the photoion branching ratios and absolute photoionization efficiencies of the valence shells of C₃H₈, n-C₄H₁₀, and NO₂, and the valence and inner (P 2p,2s; CI 2p,2s) shells of PF₃, PCI₃, and PF₅ . These data are used together with the absolute photoabsorption differential oscillator strengths to determine the absolute partial differential oscillator strengths for the molecular and dissociative photoionization channels of these molecules. A consideration of the photoabsorption and photoionization measurements of PF3 together with thermodynamic data and results from previously published photoelectron branching ratios and photoelectron-photoion coincidence (PEPICO) studies provides quantitative information on the valence-shell dipole-induced breakdown pathways of PF₃ in the photon energy region below 100 eV. Some qualitative deductions have also been made concerning the dipole-induced breakdown pathways of C₃H₈, n-C₄H₁₀, PCI₃ PF₅ and NO₂ under UV and vacuum UV radiation.

Thesis/Dissertation

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2009-04-16

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http://hdl.handle.net/2429/7272

Chemistry

University of British Columbia

UBCV

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