Low-temperature studies of adsorption of gases on metals

Abstract

A molecular beam method has been used to study the kinetics of oxygen adsorption at sample temperatures T s ⩾ 4.2 K and gas temperatures in the effusion source 78 ⩽ T g ⩽ 550 K on the (100) and (112) tungsten faces and at T g = 200 K on the (110) face of tungsten. Adsorption properties of tungsten faces manifest themselves in the kinetics of dissociative oxygen adsorption within the first layer. The second and third monolayer filling for the three investigated tungsten faces shows practically the same kinetics during oxygen adsorption into the low-energy states with binding energies of ∼ 0.13 and ∼ 0.08 eV and desorption temperatures T s ≃ 45 K and T s ≃ 27 K, respectively. The oxygen condensation with a binding energy of ∼ 0.07 eV and sublimation temperature T s ≃ 25 K extends to the fourth monolayer. From the experimental data on the T s influence upon the initial sticking coefficient S 0 for the three tungsten faces and also for the niobum (110) face investigated earlier at temperatures T s ⩾ 78 K and at T g = 200 K conclusions have been drawn concerning the mechanisms of the elementary processes during dissociative adsorption. Using the experimental data on the T g influence upon the initial sticking coefficient in the second layer the critical translational energy for capture of incident oxygen molecules has been determined ( E c ⩽ 0.06 eV).