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Electronic spectroscopy of various molecular systems by low-energy, variable-angle, electron impact


Frueholz, Robert Paul (1978) Electronic spectroscopy of various molecular systems by low-energy, variable-angle, electron impact. Dissertation (Ph.D.), California Institute of Technology.


The technique of low-energy, variable-angle, electron impact spectroscopy has been used to study the electronic spectroscopy of several groups of molecules. In each of the molecules, spin-forbidden and spin-allowed transitions have been investigated as well as excitations lying above the first ionization potential. Studies were performed at incident electron beam energies between 25 eV and 75 eV,at scattering angles ranging from 0° to 80°. The excitation energy region studied was from 0 eV to energies greater than 15 eV. Molecules investigated were selected because of their inherent spectroscopic interest; elucidation of their spectra would aid in explaining photochemical processes in which they are involved; and in some cases the molecules were of an energy-related nature. Ketene (H_2CCO) is a molecule of particular photochemical interest. Transitions were observed in its spectra at energies of 3. 7 eV, 5.3 eV and 5.86 eV. They are assigned, respectively, to an n → π^*, X^1A_1 → ^1A^2 transition, a π → π^*, X^1A_1 → ^3A_1 transition and an n → 3s, singlet → singlet, Rydberg transition. Both 1, 3, 5-cycloheptatriene and 1,3,5, 7-cyclooctatetraene are also of photochemical interest. They have both been found to be efficient dye laser quenchers and their low-lying triplet states are of importance in these quenching mechanisms. Three singlet → triplet transitions were located in 1, 3, 5, 7- cyclooctatetraene's spectrum with maxima at 3.05 eV, 4. 05 eV, and 4.84 eV. The spin-allowed transitions are also discussed. In the spectrum of 1,3, 5-cycloheptatriene two triplet states were located at 3.05 eV and 3.95 eV. Electron-impact spectra of benzene and eleven fluorine substituted derivatives were obtained. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, m-difluorobenzene, and 1, 3, 5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.7 eV. Three singlet → singlet transitions analogous to the 4.90 eV, 6.20 eV, and 6.95 eV benzene excitations are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition which is most clearly observed in the hexafluorobenzene spectrum at 5.32 eV. The electronic spectroscopy of conjugated cis-1, 3-dienyl systems has been investigated by obtaining the spectra of 1, 3-cyclopentadiene, 1, 3-cyclohexadiene, and 1, 3-cycloheptadiene. In each molecule a single low-lying triplet state was detected with maximum intensity at 3.10 eV, 2.94 eV, and 2.99 eV, respectively. The singlet-state spectra were also investigated and discussed in terms of interacting ethylenic units. The spectrum of 1, 3-cyclopentadiene was compared to those of the heterocycles, furan (C_4H_4O), thiophene (C_4H_4S) and pyrrole (C_4H_4NH). 1, 3-Cyclopentadiene's spectrum is best discussed in terms of a cis-1, 3-dienyl system. The electron-impact spectra of cyclohexene and norbornene were obtained along with those of the non-conjugated dienes, 1,4-cyclohexadiene and norbornadiene. In cyclohexene and norbornene a single singlet → triplet excitation was observed at 4,24 eV and 4.10 eV, respectively. Transitions corresponding to it π → π ^* singlet excited states and π → 3s Rydberg states were also observed. The spectra of norbornadiene and 1,4-cyclohexadiene are discussed in terms of ethylenic units interacting via through-bond and through-space effects. The lowest feature in the norbornadiene spectrum appears to be due to the superposition of two singlet → triplet transitions with approximate locations at 3.4 eV and 3.9 eV. In the spectrum of 1, 4-cyclohexadiene, the lowest feature is also believed to result from two singlet → triplet transitions; however, the transitions overlap so heavily that individual locations could not be determined. Finally, in two appendices the electronic spectroscopy of nitric oxide and UF_6 and WF_6 are discussed. Weak structure in the NO spectrum occurring between 5.22 eV and 5.60 eV has been assigned as vibronic bands belonging to the X^2∏ → a^4∏ transition. Structure with an apparent Franck-Condon envelope extending from 5.70 eV to approximately 7 eV with intensity maxima at 6.29 eV was assigned to the X^2∏ → b^4Σ^- transition. The UF_6 and WF_6 spectra were discussed in terms of recent calculations and by drawing parallels between the two spectra.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Kuppermann, Aron
Thesis Committee:
  • Unknown, Unknown
Defense Date:16 May 1978
Record Number:CaltechTHESIS:11042009-104248477
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5348
Deposited By: Tony Diaz
Deposited On:17 Nov 2009 23:42
Last Modified:26 Dec 2012 03:18

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