High resolution photoelectron spectra of a number of polyatomic molecules have been obtained using Zero-Kinetic-Energy Pulsed-Field-Ionisation (ZEKE-PFI) spectroscopy. The molecules studied were trifluoromethyl iodide (CF<sub>3</sub>I), all three geometric isomers of the aminobenzotrifluorides (CF<sub>3</sub>C<sub>6</sub>H<sub>4</sub>NH<sub>2</sub>) and 4-fluorobiphenyl (C<sub>6</sub>H<sub>5</sub>V<sub>6</sub>H<sub>4</sub>F). The ground ionic state (<sup>2</sup>E<sub>3/2</sub>) of CF<sub>3</sub>I has been studied by two separate excitation schemes. The first involved a direct two-photon (one-colour) process from the neutral ground state. The spectrum observed was found to be very similar to that of the conventional photoelectron spectrum with progressions being observed in all three totally symmetric normal modes reflecting the direct Franck-Condon overlap between the two states. The spectrum is very different to that seen for CH<sub>3</sub>I where long vibrational progressions were observed due to intermediate bond stretching. The lack of such effects in CF<sub>3</sub>I is ascribed to the weakness of the one-photon absorption at the wavelength required for two-photon ionisation. A second excitation scheme involved a two colour (2 + 1') process via the [<sup>2</sup>E<sub>3/2</sub>]6p Rydberg states. The ZEKE-PFI spectra obtained allowed the assignment of the majority of the vibrational structure of the intermediate. This structure, assigned to normal mode v<sub>3</sub> (the C-I stretch), has a vibrational frequency far lower (144 cm<sup>-1</sup>) than either the neutral (286 cm<sup>-1</sup>) or ionic (205 cm<sup>-1</sup>) ground states. This is ascribed to interactions between the 6p Rydberg states and an ion-pair state. The aminobenzotrifluorides have been studied in a two-colour (1 + 1') scheme via the S<sub>1</sub> valence state. The high resolution of ZEKE-PFI spectroscopy allows the torsional structure of the ion to be resolved for all three isomers. This structure could be simulated using a one-dimensional rigid rotor model. Trends in barriers to internal rotation were in line with previous studies on other toluenes but some noticeable differences were found 2-ABTF shows substantial activity in both the CF<sub>3</sub> and NH<sub>2</sub> torsions indicating a substantial interaction between the two substitutions. The electron withdrawing nature of the CF<sub>3</sub> makes the formation of an intramolecular hydrogen bond possible. For all three isomers considerable activity was observed in torsional transitions which are forbidden by symmetry selection rules.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:654393 |
| Date | January 2000 |
| Creators | Macleod, Neil Angus |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11836 |
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