The influence of surface oxygen on uptake and release of deuterium by tantalum

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Abstract
Here we examine the influence of surface oxygen on both the absorption of deuterium (D) from gas into solution in tantalum (Ta) and the release from solution back to the gas. The D uptake rate was proportional to gas pressure with a sticking coefficient of 0.0085±0.0025 for clean Ta surfaces. Exposure to 10 Langmuirs (L) of O2, giving about one monolayer of chemisorbed oxygen, decreased the D uptake rate by about two orders of magnitude. D release was studied using the D(3He,p)α nuclear reaction to measure the concentration of D in the Ta versus time during release at constant temperature. D release was surface-limited and obeyed second-order kinetics for both clean and oxygen-covered Ta surfaces. The second-order kinetics show that D recombines from sites which are not saturated and therefore must have a smaller binding energy than the low-coverage chemisorption sites. Exposure to 106 L of O2 resulted in an additional energy barrier to D recombination of 0.25±0.04 eV/D which greatly reduces the D release rate. This activation barrier should also reduce D dissociation and uptake rates, and this is observed. The measured uptake and release rates are consistent with detailed balance.

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