Attainment of Ultrahigh Vacua, Reduction in Surface Desorption, and the Adsorption of Hydrogen by Evaporated Molybdenum

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Abstract
Deposition of molybdenum by vaporization from simple hairpin filaments has been found to reduce pressures in both unbaked and moderately baked stainless steel vacuum systems to 4×10−10 mm Hg. A deposit of 300 mg from a single filament, 0.050 in. in diameter and 6 in. long, has maintained an unbaked 85‐liter volume at pressures below 10−9 mm Hg for over 40 hr with the aid of a small well‐trapped diffusion pump. For a substrate area of 8×103 cm2, the initial pumping speed of a molybdenum deposit was found to be as high as 105 liters/sec for hydrogen and 8×104 liters/sec for deuterium. The sticking probability for either hydrogen or deuterium on this deposit is estimated at 0.3. A similar system, baked at 200°C for several days while pumped by an oil‐free ion pump, attained 2×10−10 mm Hg with molybdenum evaporation. When the ion pump was valved off, the pressure in this 75‐liter system remained at 2×10−10 mm Hg for two weeks with no external pumping.

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