Studies in the Behaviour of Molecular Beams

This thesis describes the design, construction and development of a molecular beam apparatus for the study of interactions of gases with solid surfaces in systems of catalytic interest. The apparatus has been used in an investigation of the displacement, from palladium surfaces, of the surface species resulting from the adsorption of ethylene C-14 by beams of acetylene and ethylene. The apparatus produced ultimate static vacua of about 5 x 10e-9 torr. The palladium was in the form of a film, freshly deposited in vacuo, to which was admitted ethylene C-14 at a pressure of greater than 5 x 10e-2 torr, known to provide saturation of the surface with adsorbed species. The fate of the isotopically labelled surface species was followed in vacuum desorption and in beam-displacement reactions. The detector for radioactivity was, in the final form of the apparatus, a lithium-drifted silicon solid-state detector mounted close to the target inside the beam system. The maximum flux intensity which could be directed on to the surface was 2.5 x 10e13 molecules/cm 2/sec as against a background of scattered beam material of 4.1 x 10e13 molecules/cm 2/sec. The beam was produced from a source whose pressure was 2 x 10e-3 torr and whose temperature could be varied between 1