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

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.