A technique has been developed for determining the effect of very low energy electrons (3.0 to 8.0 eV) on molecules adsorbed on metal surfaces. The electron source is a thoria coated iridium filament operated at low filament current to minimize the thermal distribution of the energies of emitted electrons (0.0 to 0.4 eV). The system is kept at 10('-3) to 10('-4) torr by allowing the gas studied to pass through the reaction vessel while the system is being evacuated by Vac Ion pumps. Analysis is performed via computer controlled quadrupole mass spectrometry. Carbon monoxide gas has been studied on gold, nickel, palladium, molybdenum and tungsten surfaces produced by vacuum sublimation of the metals. Observations fit the following series of reactions: CO(ads) + e('-) (--->) CO('*)(ads) + e('-) CO('*)(ads) + CO(ads) (--->) C(,2)O(,2)(ads) C(,2)O(,2)(ads) (--->) CO(,2)(ads) + C(ads) C(ads) + CO(ads) (--->) C(,2)O(ads) C(,2)O(ads) + CO(ads) (--->) C(,3)O(,2)(ads) Since desorption occurs along with these reactions, the mass levels of 44 (CO(,2)), 56 (C(,2)O(,2)) and 68 (C(,3)O(,2)) were monitored as a function of electron energy. Threshold potentials (ie. the potential at which the mass spectrometer ion current begins to increase for a given mass level) were determined for the surfaces studied. Good agreement has been obtained for the threshold potentials (E(,th)) determined at the three mass levels for a given metal surface. Average values are: E(,th)(gold) = 6.0 eV; E(,th)(nickel) = 6.1 eV; E(,th)(palladium) = 4.8 eV; E(,th)(molybdenum) = 4.1 eV; E(,th)(tungsten) = 3.0 eV. A relationship between these threshold potentials and the heats of adsorption for CO adsorbed on the surfaces has been proposed. The heat of adsorption is believed to be primarily a function of d(pi)(--->)p(pi)* back bonding from the metal to the surface bound carbonyl. This stabilizes the (pi)* orbital relative to the gas phase and lowers the excitation energy of CO by the heat of adsorption.
| Identifer | oai:union.ndltd.org:pacific.edu/oai:scholarlycommons.pacific.edu:uop_etds-4084 |
| Date | 01 July 1982 |
| Creators | White, James Dean |
| Publisher | Scholarly Commons |
| Source Sets | University of the Pacific |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | University of the Pacific Theses and Dissertations |
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