Electronic Transitions of Molecules by Electron Impact and Multiphoton Ionization Spectroscopy

Citation

Rianda, Ronald (1982) Electronic Transitions of Molecules by Electron Impact and Multiphoton Ionization Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/b7ed-jg16. https://resolver.caltech.edu/CaltechTHESIS:11192009-134206825

Abstract

The experimental work discussed in this thesis is concerned primarily with the detection of electric dipole forbidden transitions of molecules in the gas phase. The thesis is divided into two parts. The first part describes measurements made using the technique of low-energy, variable-angle, electron impact spectroscopy. The second part describes investigations performed using resonance enhanced multiphoton ionization (REMPI) spectroscopy.

The low-energy variable-angle electron impact technique has been used to study the electronic spectroscopy of molecules. Both dipole allowed and dipole forbidden transitions have been investigated. Transitions having excitation energies from 0 eV to 20 eV have been studied using incident electron beam energies ranging from 25 eV to 75 eV and scattering angles from 0° to 80°. Molecules studied included nitric oxide (NO), uranium hexafluoride (UF₆), tungsten hexafluoride (WF₆), nitrogen dioxide (NO₂), hydrogen cyanide (HCN), acetonitrile (CH₃CN), propionitrile (C₂H₅CN), butyronitrile (C₃H₇CN), and malononitrile (CH₂(CN)₂).

Weak structure was observed in the spectrum of nitric oxide between 5.22 eV and 5.60 eV. These bands have been assigned as vibronic bands belonging to the X²π → a⁴π transition. Additional structure extending from 5.7 eV to about 7 eV was assigned to the X²π → b⁴Σ^- transition. Several higher lying transitions were observed which have been tentatively assigned as doublet → quartet in nature.

In order to elucidate the electronic structure of uranium hexafluoride the electron impact spectra at UF₆ and WF₆ were determined. Eleven features were observed in UF₆ with intensity maxima at 3.26, 4.2, 4.7, 5.8, 7.0, 7.86, 9.26, 11.01, 11.75, 12.5 and 13.2 eV. Features were observed in the spectrum at 7.25, 7.9, 8.5, 9.85, 11.75, 12.6 and 13.5 eV. Comparison of the spectra indicate that the primary contribution to transition intensity in UF₆ above 5.8 eV and in WF₆ results form charge transfer excitations from fluorine p orbitals to metal d orbitals. Tentative assignments based on previous theoretical studies are made.

A previously unreported doublet → quartet transition was observed at 4.49 eV in the electron impact spectrum of NO₂, in excellent agreement with theoretical calculations. Doublet → doublet transitions were observed with maxima at 2.95, 5.81, 7.48, 8.64, 9.69, 10.52, 10.68, 10.94 and 11.20 eV in agreement with previous studies.

The series of C Ξ N containing molecules, Aydrogen cyanide, acetonitrile, malononitrile, propionitrile and butyronitrile, have also been studied using the electron impact technique. Results for hydrogen cyanide are in excellent agreement with previous work. Previously undetected singlet → triplet transitions of acetonitrile, propionitrile and butyronitrile are reported. In addition the first study of the electronic spectrum of malononitrile is reported.

Two appendices to Part One are included. The first of these reports the results of generalized valence bond and configuration interaction studies of the low lying states of ammonia. The second appendix discusses an electron impact study of the electronically excited states of 1,3,5-cycloheptatriene.

Part Two of this thesis describes the theory of multiphoton ionization and reports results obtained using this technique. The application of REMPI spectroscopy to the detection of spin forbidden transitions is examined. It is shown in a study of the X¹Σ⁺_g → a³A₂ state of CS₂ to offer potential for the detection of spin forbidden transitions at high resolution and with great sensitivity.

Finally a preliminary study of the two-photon resonance enhanced multiphoton ionization of p-xylene is reported. Several elements of the X¹Ag → ¹B_2u transition observed previously in benzene and p-difluorobenzene are reported.

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