Sequential adsorption of D2 and C2H4 on single-crystal planes of tungsten

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Abstract
The sequential adsorption of deuterium and ethylene has been studied on the (110), (100), (112), and (111) planes of tungsten at [inverted lazy s] 135°K using thermal desorption mass spectroscopy. Adsorption of D2 followed by C2H4 on both the (110) and (112) planes, respectively, leads to 0.24 and 0.37 monolayers of ethylene adsorbed with no displacement of deuterium for an ethylene exposure which would just be sufficient to saturate a bare surface. Since the resulting spectra are apparently superpositions of ethylene decomposition and deuterium desorption, it appears that ethylene and deuterium in part occupy separate sites. On both the (100) and (111) planes adsorption of ethylene occurs through displacement of 1.3 molecules of deuterium for each molecule of ethylene adsorbed. This displacement is more extensive on the (111) plane: θ C2H4=0.86 and θ D2=0.36 on the (111) plane vs θC2H4=0.52 and θD2=0.67 on the (100) plane. Displacement on the (100) plane occurs through adsorption of ethylene on sites separate from deuterium with the ethylene perturbing deuterium sufficiently to lead to desorption. Arguments are given for this interaction being relatively short range. On the (111) plane hydrogen displacement is a thermal process with both D2 displaced by C2H4 and H2 by D2. The D2 is able to displace an amount of H2 corresponding to all the β1 and β2 states and [inverted lazy s] 3/4 of the β3 state. The extent of this displacement indicates that there is extensive interchange between the β states on W(111). Previous data for the sequential adsorption of D2 and C2H4 on polycrystalline tungsten were interpreted as a composite of contributions from individual planes with specifically the (100) plane contributing a high C2H4:D2 ratio and the (110) plane a low C2H4:D2 ratio. The single‐crystal results are shown to be in agreement with this. If we assume the polycrystalline surface was partially carbon contaminated, both the spectra and the coverages resulting from sequential adsorption of D2 and C2H4 and the ethylene decomposition spectrum, can be reconstructed using a reasonable linear combination of the results from these four planes.

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