Abstract
Field emission currents of electrons were measured for different planes of tungsten after the interaction of CH 4, C 2H 6, C 3H 8, and C 2H 4 at 78°K and 298°K and used to obtain work function and pre-exponential values from Fowler-Nordheim plots. A critical analysis of these data yields a conclusive result on surface potentials only if emitting areas and pre-exponentials are comparable. As demonstrated by the field emission patterns, different surface planes of W chemisorb hydrocarbons in a selective manner. Methane at 298 °K increases the average work function \\ ̄ gF as well as Φ (011) without appreciable change of the pre-exponential. A negative surface potential is formed by chemisorption. Methane at 78 °K forms multilayers which facilitate resonance tunneling. With ethane and less pronounced with propane, remarkable changes of the pre-exponential are observed due to surface corrosion and/or whisker formation. Ethylene preferentially decreases the emission in areas between the (011), (112), and (001) surfaces, with an increase of the work function in all cases. The differences in the bahavior of the hydrocarbons studied are related to different chemisorption states.
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