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The condensation of hydrogen and deuterium on to liquid-helium-cooled surfaces is being studied for high-efficiency contamination-free pumping applications in plasma physics experiments. The present experimental studies show that the sticking coefficient of hydrogen and deuterium at room temperature increases during initial coverage of a liquid-helium-cooled condensing surface from around 0·5 at about 1013 molecules cm−2 to between 0·7 and 0·95 at 1016 molecules cm−2, and then remains essentially independent of further coverage. The sticking coefficient tends to increase with increasing gas incidence rate, but is only susceptible to surface temperature at the higher end of the range 2·1-3·7°K for very low gas inflow rates. During initial coverage of the condensing surface the desorption rate of gas molecules increases with surface coverage and reaches a steady value for coverages of about 3 × 1016 molecules cm−2. This desorption rate does not decrease as rapidly with surface temperature as expected from published hydrogen and deuterium vapour-pressure curves. Experiments show that this is due to thermal-radiation desorption effects. The application of condensation pumping in practice to maintain gas densities below about 5 × 108 molecules cm−3 (about 10 ntorr) thus requires some restriction of the thermal-radiation loading at the condensing surface.

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