Adsorption isotherms of He and H₂ at liquid He temperatures

The adsorption isotherms of He and H₂ at 4.2 K on stainless steel and Pyrex glass and of He on stainless steel at eight different temperatures between 1.9 and 4.2 K for pressures between 10⁻¹² and 10⁻⁵ Torr have been measured. The measured data have been compared to different theoretical models describing adsorption isotherms. The Dubinin–Radushkevich–Kaganer (DRK) equation describes the measured He isotherms well for pressures down to about 10⁻⁹ Torr. The adsorption isotherms of He on stainless steel appear to follow the DRK equation as long as the energy to compress the gas from the measured equilibrium pressure to the saturated vapor pressure at the temperature of the measurement is smaller than the isosteric heat of adsorption. At pressures lower than about 10⁻⁹ Torr, the isotherms deviate from the DRK equation. The measurements show that the empirical constants in the DRK equation for He on stainless steel have similar values to He on Cu plated stainless steel while for He on the Pyrex glass surface the values are different. The isosteric heat of adsorption for He on stainless steel has been calculated from the measured isotherms and it decreases almost linearly from 8.5 meV/atom at a surface coverage of 1.8×10¹⁴ He atoms/cm² down to 2.5 meV/atom at a surface coverage of 1.1×10¹⁵ He atoms/cm². In the region where the DRK equation is not valid, the He isotherms can be described by a simple power law P=βS^α, where P is the equilibrium pressure, S the surface coverage, α=1.63, and β varies with temperature.