Work Function and Thermal Desorption in the System Thorium-on-Rhenium

Abstract

This investigation consists of two different but related parts. 1. The thermionic properties of rhenium and rhenium with an adsorbed layer of thorium atoms, were investigated. The Richardson constants for rhenium were found to be φ=4.85 eV, A=66 A per cm2‐deg2. The work function decreases with increasing thorium coverage σ to a minimum of 3.15 eV at σ=4.2×1014 atoms per sq cm then rises to a constant value of 3.3 eV at σ=8×1014 atoms per sq cm. A comparison with the system tungsten—thorium shows that: (a) φmin occurs at the same surface density in both cases, (b) φconst is attained at the same surface density in both cases, and this density is that of the atoms in a (100) plane of thoriummetal, and (c) the values of φconst are equal and different from that of bulk thorium by only 0.1 eV. On the basis of these regularities a model is proposed to explain the variation of φ with σ. 2. The thermal desorption of the adsorbed atoms was studied at temperatures between 2203° and 2468°K. At low coverage the desorption rate per atom η0, was found to be constant with coverage, and to obey the relation η0=CT exp (‐ε/kT) with C=4×1010 sec−1 deg−1 and ε=8.30 eV. The desorption rate increased for coverages between 0.05 and 0.5 monolayer and this increase agreed well with the predictions of a theory based on interactions among the adsorbed atoms.