According to Zalm84 ¹⁾ there ary two mechanisms for electron ejection upon ion bombardment:

• Potential Emission: If the ion neutralization energy exceeds twice the electron work function of the material, electron emission is possible. The secondary electron emission yield resulting from this mechanism is energy dependent and applies for rather low ion incident energies up to a few 100 eV.
• Kinetic Emission: At higher energies, electron emission by direct ionization of metal atoms by the ion projectile and/or by energetic recoils becomes important. This mechanism becomes relevant for ion incident energies beyond 1 keV.

Given the relevant energy range, only the first mechanism seems relevant for magnetron sputtering. The secondary electron emission coefficient related to potential emission can be calculated as:

• = Ionization energy of the ion (= 15.8 eV for Ar⁺)
• = Electron work function of the metal
• = Fermi energy of the electrons within the metal

In the following, representative numerical values are given. Most of the values are taken from the following sources:

• Table of fermi energies from http://hyperphysics.phy-astr.gsu.edu/hbase/tables/fermi.html#c1 from the Hyperphysics project (University of Georgia)
• Wikipedia article about work function