The chemisorption of nitrogen on polycrystalline tungsten ribbon: kinetic and contact potential studies

Abstract

The kinetics of chemisorption of nitrogen on a polycrystalline tungsten ribbon were studied under ultra-high vacuum conditions by a flow method using a flash-filament technique similar to that described by Becker & Hartman (1953). The change of contact potential of the tungsten ribbon during the chemisorption reaction was simultaneously measured by continuous monitoring of the characteristics of a space-charge limited diode with the ribbon used as anode. Both the ‘sticking probability’ and the apparent dipole moment of the adsorbed layer change markedly at a coverage of 2⋅5 x 10 14 atoms/cm 2 at room temperature. At higher temperatures the changes in both sticking probabilities and apparent dipole moments occur at lower coverages. The requirements of a simple two-site adsorption model to explain these results are discussed, and are compared with kinetic theories recently given by Ehrlich (1955, 1956 a ), Kisliuk (1957, 1958) and Gundry & Tompkins (1956).