Physical understanding of gas desorption mechanisms

Abstract

A new set of equations is introduced for the analysis of the evacuation process. The equations are based on the surface processes of adsorption and desorption of gas molecules taking place on the inhomogeneous metal surface but not on the diffusion process. Because the equations are nonlinear, it is difficult to get the analytic solution, and numerical calculations, as well as analytic considerations were made. The results tell us that (i) the outgassing phenomena can only be understood under nonequilibrium conditions, (ii) the evacuation time dependence of the outgassing rate depends on the distribution function of the adsorbing site, i.e., the adsorption site density as a function of the activation energy of desorption E, and (iii) a pressure buildup phenomenon, which is accompanied by a sudden stop of evacuation, can be easily understood. Finally, a new measurement method is proposed for the investigation of surface properties. In this method, a chamber made of a test material is evacuated for a long time and then a measurement follows. In the measurement, one adjusts the pumping speed so as to keep the vacuum pressure in the chamber constant. The amount of outgas is determined as a function of time by knowing the pumping speed. By this method, one can get quite new information concerning the surface properties.