Outgassing rate characteristic of a stainless-steel extreme high vacuum system

Using an extremely dry nitrogen venting system, fast return to ultrahigh vacuum condition was achieved. The apparatus is similar to that of our earlier extreme high vacuum publication. The system pumps to 1.1×1^-10 and 1.4×10^-11 Torr in 5 and 50 h, respectively, after the start of pumpdown and without bakeout. The pressure-time curve for pumpdown shows that the system pressure is determined by surface outgassing which decays algebraically as t^{- α}, where the slope α changes from 2.13 for p>10^-9 Torr, 1.24 for 10^-10<p<10^-9, and 0.82 for p<10^-10 Torr. However, α=0.5, which would be expected for bulk diffusion, was not observed. Curve fitting yields desorption energies of 19, 20, 21, 22, and 23 kcal/mol for five types of adsorption sites on the surface for water. Analysis suggests that the elimination of water with desorption energies of 20 and 21 kcal/mol is essential for rapid pumpdown to the 10^-10 Torr decade. A desorption energy of 10.4 kcal/mol for hydrogen was obtained by outgassing rate measurement. This relatively small value is suggestive of a smooth oxide layer formed on the chamber wall.