True and measured outgassing rates of a vacuum chamber with a reversibly adsorbed phase

The modeling for the pumpdown of a vacuum chamber with a reversibly adsorbe d phase is carried out and the outgassing equation which predicts variation s of surface coverage and gas density with time is derived. When it is assu med that the wall surface of a vacuum chamber consists of oxide layer and t his layer involves weakly chemisorbed (nondissociate) H2O with desorption e nergies from 20 to 30 kcal/mol, it is shown that the calculated outgassing rate from the outgassing equation well explains the experimental result tha t the outgassing rate measured by the orifice pumping method in the unbaked stainless steel chamber is dependent on pumping speed. (C) 2001 American V acuum Society.