Physisorption characteristics of hydrogen on a cold metal surface were investigated in the range of a monolayer for the purpose of establishing a cryopumping technique in extremely high vacuum. Electron stimulated desorption (ESD) technique was applied to measure the adsorption density of hydrogen molecules. Hydrogen was adsorbed on a copper substrate at 3 - 4.2 K of temperature and was irradiated with a pulsed electron beam of a few hundreds nsec pulse width and 45 - 170 eV electron energy. ESD ions, H+, H2+, and H3+, were mass-separated by a time-of-flight measurement with the acceleration bias of 100 V. In submonolayer range, the signal intensity of ESD H+ was a useful measure for the amount of physisorbed H2 and the monolayer completion of H2, which was accompanied by two dimensional phase condensation, was detected by the rapid rise of H+ intensity followed by its saturation.

An adsorption isotherm was obtained as the ESD signal intensities of H+ as a function of the hydrogen pressure, which was gradually increased from 10-9 to 10-4 Pa so as to keep adsorption equilibrium. The observable lower end of the isotherm was 1x10-8 Pa of the equilibrium pressure and 0.001 of H2 coverage, which was limited by back ground signal of H+ originated from water or hydrocarbons adsorbed on the metal substrate. Though the validity of the assumption that the H+ desorption yields is proportional to the H2 physisorption density should be further examined, the H+ yield curve between 1x10-8 and 2x10-6 Pa represents the adsorption isotherm between 1 and 0.001 of H2 coverage.

Two transient methods were applied to measure the mean residence time, τ, of physisorbed H2. A time development of ESD H+ intensity was monitored after either (1) a stepwise H2 pressure rise to a certain fixed value or (2) YAG laser cleaning of the substrate under a constant H2 pressure. In either case, the time constant of the increase of ESD H+ intensity is the direct measure of the mean residence time of H2. Typical value of τ obtained by the method (1) at nearly monolayer and at 4.2 K was 62±3 sec, which corresponds to 1.19 kJ/mol of the adsorption energy on assumption of τ0 = 10-13 sec.

The adsorpion isotherms and the close examinations of the dependency of τ on the temperature and the coverage can reveal the fundamental properties of the hydrogen physisorption in low pressure and low coverage range.