Resonant mechanisms for negative ionization of secondary emitted atoms from sputtered metals

A model is presented to describe negative ionization of low energy, secondary atomic particles ejected from sputtered metal surfaces. Focus is made on the diatomic systems formed, in the collision cascade generated by the primary ion beam, between secondary emitted atoms and their nearest-neighbor substrate atoms that provide the initial impulse for ejection. Two different resonant ionization mechanisms are investigated such that a conduction electron may tunnel into the affinity orbital of the ejected atom either by direct hopping or after an intermediate transition via the affinity orbital of the substrate atom. A numerical method is outlined to calculate the negative ionization probability of secondary emitted atoms. A good agreement is found with van Der Heide's measurements of the Cu− population sputtered from a clean Cu-surface, at emission energies below 100 eV.