Isotopic Mixing in Nitrogen Chemisorbed on W

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Abstract
The chemisorption of an equimolar mixture of 14N2 and 15N2 on polycrystalline tungsten has been studied. The flash-filament mass-spectrometer technique was used at sample temperatures of 77°K and above, in an ultrahigh-vacuum system. The molecular identity of the low-temperature γ state was confirmed since the γ—N2 desorption spectrum showed no formation of 14N15N. On the other hand, the strongly held β-nitrogen was completely isotopically mixed on desorption. A careful study of β-nitrogen, using 15N2, has revealed two overlapping binding states, β1 and β2, associated with surface heterogeneity in the W substrate. The β1 state shows first-order desorption kinetics, and the more tightly bound β2 state appears to desorb with higher-order kinetics. Under saturation conditions, the coverage of β-nitrogen is essentially constant from 300°—1100°K.

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