Dissociation of Hydrogen on Tantalum Using a Modulated Molecular Beam Technique

Abstract

A molecular beam–mass spectrometric apparatus has been developed to study the kinetics of gas–solid reactions at low pressures. The system has been used in an investigation of the dissociation of hydrogen on tantalum. A modulated beam of molecular hydrogen, formed by effusion from a multichannel source, impinged upon the tantalum target. The reflected H₂ and H atoms emitted after surface dissociation were monitored by a mass spectrometer in the vacuum system. The beams were modulated and the products measured by a phase‐sensitive detector to improve the signal‐to‐noise characteristics of the detection system. The degree of dissociation was studied as a function of the surface temperature, beam temperature, and beam intensity. For a fixed target temperature, dissociation increased with beam temperature with an apparent activation energy of 1.4 kcal/mole. The rate of dissociation varied linearly with beam intensity. Mixed hydrogen–deuterium beams were employed to investigate the isotope exchange reaction, which, within the sensitivity of the detection system, did not take place. The results are consistent with a model in which the surface concentration of hydrogen is controlled by competition between atom evaporation and surface diffusion. The presence of the latter loss mechanism reduces the surface concentration to the point where second‐order recombination is negligible. The atom‐evaporation and surface‐diffusion processes are satisfactorily described by the two‐dimensional ideal‐gas model. Collisions occur between migrating adatoms and scattering centers on the surface. The mean free path for this interaction was 0.02 cm for all temperatures investigated.