Contribution of the Landau-Zener-Stueckelberg model to the understanding of positive secondary-ion emission.

Abstract

In sputtering experiments, the enhancement of the emission of positive metal ions in the presence of chemically active elements can result from bond-breaking processes occurring at the surface of the sample. Although the Landau-Zener-Stueckelberg model describes the dissociation of chemical bonds of ionic character, its application to metallic emission processes cannot be made without a preliminary study that takes into account the specificities of a bond involving a metallic cation. In this paper, the possible breaking of an isolated diatomic bond involving a Cu atom is studied. It is shown that the ground state does not generally participate in the dissociation of such a bond into its ionic components, and that the first excited states have to be taken into account. In fact, the states involved in the process are comprised between two limits determined by the electron affinity of the partner of the metal atom. These states can have very different probabilities to dissociate under an ionic form, according to their quantum numbers and to the energy. In an emission process a complete computation of the final ionization probability cannot be done as long as the probabilities for the diatomic system to reach its excited states remain unknown. It is not impossible that they could be obtained with the help of ``local thermal equilibrium'' theory. Also, the high values of the electron affinities of some molecules lead us to believe that more complex bond breakings could be responsible for the observed ``matrix effects.''