The thermal dissociation of hydrocarbon molecules on a hot tungsten emitter results in the appearance of dark (334) planes. Between the elapsed time
to needed for the appearance and the pressure
P of methane, the following relation was found: log
to=-μ log
p+α where μ ranges from 0. 82
8 to 4. 7
4 depending upon tip temperatures; and the change of μ with the tempera-ture is not monotonous.
This dependence of μ on the temperature can be explained by considering that (1) the mean residence time of methane molecules on the tungsten surface has to be long enough for the dissociation of these molecules, by mutual bonding of each molecule: (2) μ is the mean number of molecules making the mutual bonding; (3) vibrational energy levels of methane are involved, in a resonance-like manner, with its dissociation.
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