On the secondary ion emission probability in the presence of Fermi sea excitations due to dynamical screening; application to C 60

Abstract

The Fermi sea excitation state plays a large role in surface ionization. When it leaves a target, an atom creates a time-varying localized potential which causes important Fermi sea excitations. A similar dynamical screening effect appears in photoemission when a deep hole suddenly appears in a metal. This last phenomenon has already received a theoretical treatment for the bulk (Nozières and de Dominicis, Friedel) and more recently for aggregates, in particular C 60. Here, we show that the dynamical screening effects result in a local excitation of the ejected atom, in a positive ionization process, its population is less than the adiabatic population. In all our calculations we only study π electrons. From a general point of view we also discuss how the model (where the parameter variations are sudden) could be adapted to any continuous variation by introducing a successive-step process. We show that a connection is sometimes possible between the ionization probability studied here and the valence excitation spectrum (photoemission spectrum satellite structure). A correspondence rule is then obtained between two a priori different fields. We also study the ionization probability in the ejection of a diatomic molecule. In agreement with experiment it is larger than for a simple particle.