Secondary ion emission of alloys in relation with their electronic structure

Abstract

Abstract When an atom crosses a metal-vacuum surface, the perturbation acting upon outer electronic shells produces excited states the energy of which exceeds, in some cases, the ionization energy of the atom; then, one deals with auto-ionizing states. We propose a secondary positive ion emission process from metals and alloys based upon the formation of auto-ionizing states. In such a process the ion yield K+ of an element is proportional to the occupation probability of auto-ionizing states. This probability is closely connected to the density of states around the element inside the metal or alloy, in a small bandwidth energy near the Fermi level: the greater the fraction of valence electrons of the atom in this bandwidth, the higher the occupation probability will be. This connection explains enhancement observed experimentally in the ion emission of transition elements (from Ti to Cu) in dilute alloys. This effect is particularly noticeable when d electrons of transition elements are distributed in a virtual bound state which crosses the Fermi level as in the case of chromium in nickel matrix or transition elements in aluminium or copper matrix.