EFFECT OF BAKE-OUT ON THE ADSORPTION-KINETICS OF GASES IN A VACUUM CHAMBER

Adsorption and desorption of gases in a vacuum chamber can be characte rized by sticking probability, s, mean sojourn time, tau, and amount, sigma, of adsorption on the surface in the chamber. The values of s an d tau can be obtained through the analysis of pressure changes in the chamber under nonequilibrium conditions. These conditions were realize d by means of a pressure pulse generated by pulsed-laser-beam irradiat ion of the stainless steel surface in the chamber. After laser beam ir radiation, ion currents of a quadrupole mass spectrometer were measure d for m/q = 2, 18, and 28. The time constant of pressure decrement for water vapor desorbed was much smaller than the value calculated. The difference can be explained by a small s and a large tau on the chambe r surface. For a new stainless steel chamber, s of water vapor was con stant, at about 9 x 10(-4), when bake-out temperature was lower than 1 50-degrees-C, and increased markedly after bake-out at 250-degrees-C.