Surface Residence Time Measurements of Ba Atoms by Molecular Beam Relaxation Spectrometry

Abstract

Molecular beam relaxation spectrometry has been applied to measure the residence time of Ba atoms on such refractory metal surfaces as W and W-26Re, which are commonly used as substrates for dispenser cathodes. It is found that there are at least two chemisorption states on both metal surfaces, which yield different residence times at the same temperature. The chemisorption energies of Ba on W-26Re surface are found to be 2.4 and 0.4 eV respectively. The dependency of the residence time (signal phase shift) on the Ba impinging flux suggests the existence of a third adsorption state. In the dispenser cathode operation temperature range, 900–1200°C, the role of the highest energy chemisorption sites is crucial in determining the surface coverage of the mono-atomic order layer, the electric dipole of which reduces the work function, thus enhancing electron emission of dispenser cathodes.