Abstract
An ultrahigh vacuum (UHV) chamber is generally supposed to have a constant base background outgassing rate q0 over very long periods of time. In experiments with ionized or molecular hydrogen, this level might be increased by absorption of a small fraction of hydrogen. In this work, absorption of molecular hydrogen is monitored in a small austenitic stainless steel UHV chamber during 3 h exposures at initial pressures from 0.01 to 1 mbar at room temperature. An absorbed dose of ∼1013–1014 atom H cm−2 was recorded. After gas removal, desorption period in the closed vacuum system lasted for 20 h, wherein most of the absorbed hydrogen was released. The initial desorption rate was 10–100 times higher than the previous q0. Calculations based on well accepted models for hydrogen–metal interactions confirm that the observed absorption–desorption processes are related mainly to the native oxide layer. Its reported thickness is between 1 and 3 nm and our results express its high solubility, which at 1 nm ranges from Ksox(294 K) = 2.0 × 1022 to 3.8 × 1022 atom H cm−3 bar−0.5. The recombination rate coefficient ranges from KLox(294 K) = 8.9 × 10−19 to 6.45 × 10−18 cm4 s−1. The bulk metal was not involved noticeably in 24 h cycles as the oxide layer–bulk metal interactions proceed substantially slower.
Published Version
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