Variable-Angle Electron Energy-Loss Spectroscopy of Polyatomic Molecular Systems

Citation

Walzl, Kerry Neil (1987) Variable-Angle Electron Energy-Loss Spectroscopy of Polyatomic Molecular Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pzpj-5b50. https://resolver.caltech.edu/CaltechTHESIS:10292019-111940086

Abstract

The technique of variable-angle electron energy-loss spectroscopy has been used to study the electronic spectroscopy and structure of both open- and closed-shell molecules. The experiments were performed using incident electron beam energies between 25 eV and 100 eV and at scattering angles between 0° and 90°. Energy-loss regions from 0 eV to 16 eV were examined. Spin-forbidden, dipole symmetry-forbidden/quadrupole symmetry-allowed, and super-excited transitions were investigated by this method.

The three small carbonyl compounds formaldehyde (CH₂O), acetaldehyde (C₂H₄O), and acetone (C₃H₆O) were studied in the energy-loss region from 0 eV to 16 eV. Low-lying spin-forbidden n → π* and π → π* transitions were located on the basis of the angular behavior of their relative differential cross sections. High-lying (autoionizing in the case of formaldehyde) dipole symmetry-forbidden states were also assigned on the basis of differential cross section behavior. The effect of methyl substitution on the transition energies was also noted and discussed.

Five dicarbonyl compounds (biacetyl, acetylacetone, acetonylacetone, 1,2-cyclohexanedione, 1,4-cyclohexanedione) were investigated by this spectroscopic method in order to locate their low-lying spin-forbidden transitions. The energy difference between the lowest spin-allowed and spin-forbidden n → π* excitations in the cyclic dicarbonyls was found to be much larger than in the acyclic dicarbonyls; this difference was discussed.

The spectrum of the methyl radical CH₃ was investigated by the same technique as the carbonyls except that the radical was generated by pyrolysis. Three source compounds were tried (tetramethyl tin, ethyl nitrite, di-t-butylperoxide) with temperatures ranging from ambient to 800°C. Using di-t-butylperoxide at a pyrolysis temperature of about 300°C, relative differential cross sections for the lowest allowed ²A'₁ ← ²A"₂ 3s Rydberg transition (5.73 eV) were determined at incident energies of 50 eV and 25 eV. The differential cross sections for this band do not indicate the presence of any underlying spin-forbidden transition.

Finally, preliminary investigations of the spectroscopy of pyridazine and cyclohexanone were undertaken. For pyridazine, a new low-lying spin-forbidden excitation was observed. The low-lying spin-forbidden transitions of cyclohexanone were also observed along with tentative relative differential cross sections at 30 eV and 50 eV. Rydberg bands in this molecule converging to the first ionization potential were seen and the positions tabulated.

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