Hydrogen and Nitrogen Desorption Phenomena Associated with a Stainless Steel 304 Low Energy Electron Diffraction (LEED) and Molecular Beam Assembly

Abstract

The rates of hydrogen desorption from the walls of a standard LEED stainless steel vacuum chamber isolated from the ion pump have been measured using an oxygen-free copper orifice of known conductance, which acts as the final focussing slit in a molecular beam assembly, and found unacceptably high for the maintenance of a clean metal single crystal surface. The causes are discussed and the rates of desorption are presented after the stainless steel apparatus had been subjected to vacuum degassing at 850–950 °C in a vacuum furnace at pressures ≤10−5 Torr. Desorption rates are decreased ∼130 times. Rates of desorption of nitrogen from Varian post-acceleration type two and three grid LEED optics screens (P-4 CR 422 phosphor) have been measured. Possible displacement reactions with residual hydrogen, small quantities of which are adsorbed during nitrogen desorption, are discussed. It is concluded that the two processes are independent of each other. Contamination of the single crystal samples with the nitrogen desorbed for the LEED screen is also discussed. Brief reference is made to the adsorption properties of stainless steel 304 with special reference to hydrogen and nitrogen.