Collision Theories of Cathode Sputtering of Metals at Low Ion Energies

The periodicity of the threshold energies, as seen in a plot vs atomic number, is more pronounced for threshold energies determined from the part of the yield curve at the lowest ion energies, which obeys a quadratic law, than for "cut-in" energies determined with a linear law from the adjacent somewhat higher ion energy range. The relationship of the "empirical sputtering relation" of Wehner [Phys. Abstr. 62, 719 (1959)] with the threshold formula, formerly derived by the author, is clarified. It is shown that in the case that the rebounding particle is believed to be a displaced target atom, the assumption of a very large "effective" mass of the struck atom must also be made to explain the rebound at angles as observed in ejection patterns of single-crystal planes. Arguments are presented that the author's model is consistent with the observed higher yields of metals with completely filled $d$ shells, such as Cu, Ag, and Au. The energy losses connected with the impulses used up in Debye waves, as assumed in the author's theory, are shown to be in good agreement with Silsbee's calculations of the energy losses in similar collisions.