Influence of oxygen and water vapor on the ion induced desorption from stainless steel

Abstract

Ion induced neutral gas desorption, residual gas composition, and pumpdown times were measured after 1 h exposures of a baked stainless steel tube (SS 304 L) to the following atmospheres (atmospheric pressure): (a) pure N2, (b) normal air, (c) pure 80% N2, 20% O2 mixture, (d) pure N2 with the system open during the exposure. It is shown that exposure to oxygen-containing atmospheres increases the subsequent ion induced desorption of CO, CH4, and CO2 while the H2 desorption is reduced. Exposure to water-containing atmospheres, on the other hand, stimulates the ion induced desorption of all the four commonly desorbing species (CO, H2, CH4, and CO2). The effects of exposure to ordinary air on the ion induced desorption and residual gas composition is attributed mainly to its water vapor content. It is shown that if filaments (ion gauges, etc.) are on during low pressure oxygen exposures, the ratio between the ion induced desorption of CO and CO2 will be affected, probably due to the formation of excited oxygen molecules at the filaments. Backfilling the vacuum system with pure N2 before opening to air is shown to give a considerable decrease in surface contamination, which is seen from the ion induced desorption response as well as the reduced evacuation time.