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Photoelectron spectroscopic studies of some polyatomic molecules Sandhu, Jagjit S.


The study of the kinetic energies of photoelectrons ejected from gaseous atoms and molecules by 584 Å (21.21 eV) photons is relatively new, and has been named photoelectron spectroscopy. Photoelectrons are ejected with energies of 21.21 - I₁ eV, 21.21 - I₂ eV, etc., where I₁ I₂, etc., are successive ionization potentials of the molecule. Retarding fields, a 180° magnetic analyzer, a 127° electrostatic analyzer and a 180° hemi-spherical analyzer have been used to measure the electron energies. In this work only the first method was used. A spherical photoelectron spectrometer was used to make the measurements. This analyzer utilizes a retarding potential between the spherical inner grid and the spherical collector, with a second grid to repel positive ions. The 584 Å photoelectron spectra from the triatomic molecules C0₂, COS, CS₂ S0₂, H₂0 and H₂S, and polyatomic molecules C₂H₂, CF₄, SiF₄, CH₃OH, C₆H₅C1, PH₃, C₂H₄, propylene, isobutylene, cis-butene-2, trans-butene-2, trimethyl-ethylene, tetramethyl-ethylene, vinyl chloride, 1:1 dichloro-ethylene, cis-dichloro-ethylene trans-dichloro-ethylene, trichloro-ethylene and tetrachloro-ethylene are described and are shown to give all the ionization potentials less than 21.21 eV in each case. The results obtained are compared with results from other sources, and agreements and differences explained. The results are interpreted in terms of the electronic structures of these molecules as given by the molecular orbital theory which grew out of the pioneering work of Mulliken. In some cases new electronic structures are tentatively assigned. The effect of substituents on ionization potentials is also considered. Not only may the binding energies of the various molecular orbitals be determined, but also the relative electron transition probabilities to the molecular ionic states at the 584 Å incident radiation may be assessed directly from the photoelectron-stopping curve. In favourable cases the bonding character of different orbitals may be inferred from the shape of the photo-electron-stopping curve. Both adiabatic and vertical ionization potentials may be determined from photoelectron energy measurements. Vertical ionization potentials were determined for a series of methyl substituted ethylenes and chloro-ethylenes. The experimental values were compared with the theoretical values obtained by employing the equivalent orbital method of G. G. Hall.

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