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.

26The results given in this paper for the soluble model are in complete agreement with the asymptotic results obtained previously for the f and g equations of the BBGKY hierarchy.New resonances have been observed in the absorption spectrum of neutral cadmium vapor at wavelengths around 30 k These are believed to be at present the shortest-wavelength metal-vapor absorption features recorded optically. The 2. 5-GeV electron synchrotron at Bonn University was used as a background source of continuous radiation. Hartree-Fock calculations enabled identification of the resonances as caused by excitation from the 3d shell, although somewhat larger differences between computed and observed wavelengths were found than were expected. Following our previous work on the absorption spectra of metal vapors' and the work recently published by Ederer, Lucatorto, and Madden on the Liz absorption spectrum, we report here absorption features caused by excitation of a 3d innershell electron in Cd r. vapor, the structure being more than 400 V above the first ionization potential of Cd z. Our results were obtained in a collaborative experiment between the European Space Research Institute and the Synchrotron Group of the Physikalisches Institut at the University of Bonn.In some earlier work ' using the Baloffet, Rome, nd, and Vodar (BRV) sources to provide a background radiation continuum, spectra of alkalimetal vapors were recorded down to 180 A. At wavelengths below about 100 A, the BRV continuum became too weak to record absorption spectra. The furnace design used in Ref. 2 allowed a metalvapor pressure of several Torr to be achieved in the central region of the furnace while maintaining a pressure of about 10 Torr at the ends, butdid not meet the much more stringent requirements (vacuum better than 10 ' Torr) for operation on a synchrotron. Ederer, Lucatorto, and Madden have used a heat-pipe furnace system together with 1000-Athick aluminum windows to contain Li vapor at pressures of up to 2 Torr. %ith this apparatus, they were able to study the Liz absorption spectrum between 170 and 210 A using the background radiation continuum of the National Bureau of Standards 180-MeV synchrotron.They were not, however, able to observe structures much below 170 A, because of the I, » cutoff in the aluminum filters.Vfe have further developed the furnace described in Ref. 2, essentially by increasing the efficiency of the vapor trapping system. Details of our arrangement will be published separately. Spectra were recorded up to temperatures corresponding to a Cd vapor pressure of 3 Torr at the center of the furnace, the over-all length of the furnace being about 1.5 m. However, owing to the steep pressure gradients in the trapping system, the pressure-path product is probably much smaller than these figures would indicate. Our spectra were recorded using a 2-m grazing incidence spectrograph equipped with a 1200-line/mm Bausch and Lomb platinized replica grating, the entrance slit being set for a 2-deg grazing angle. The slit width was 6. 3 p, ...

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