On the Z₁ dependence of ion-induced kinetic electron emission

The electron emission from clean gold, silver, palladium, copper, and titanium surfaces was investigated for bombardment with up to 23 different ion types from Li+ to Sn+ with a kinetic energy of 20 keV. The electron yield depends on the electronic structures of both projectile ion and metal atom. The dependence on the atomic number of the projectile, Z 1, exhibits maxima and minima and is the same for gold, silver and palladium. A different dependence is observed for each of copper and titanium. The Z 1 dependence is not correlated with the electronic stopping power, nor is it sensitive to the band structure of the metal. Rather, an enhanced electron yield is observed when the outermost bound electron levels of the target atoms are matched by electron levels in the projectile. This supports the conjecture that electron promotion is an important mechanism for electron emission at low projectile energies.