Vaporization of impurities in copper, including those introduced by ion bombardment∗

Abstract

A mass spectrometric analysis has been made of particles sputtered and/or evaporated from the (221) plane of a high-purity single crystal copper surface in high vacuum. The results are relevant to such processes as heterogeneous catalysis and nucleation, as well as to the technique of cleaning solid surfaces by ion bombardment and annealing. (1) Copper oxide layers may be completely removed from a copper surface (or at least reduced to a very small fraction of a monolayer) by repeated cycles of inert gas ion bombardment and annealing. (2) During ion bombardment, particles of the bombarding beam and the ambient gas are physically trapped by the surface. These may be released from the copper by heating it to 200°C. The activation energy for this release is 0.3 ± 0.1 eV (measured only for argon). (3) Sodium and potassium bulk impurities diffuse readily to the surface at a temperature of 600°C with activation energies of 2.8 ± 0.1 and 2.3 ± 0.1 eV, respectively. (4) These same impurities also desorb from copper at 600°C with an activation energy (for potassium) of 1.2 ± 0.1 eV. (5) The surface concentration of sodium and potassium, while always less than a monolayer, was never found to be zero; it was less after annealing, however, than after ion bombardment.