Ion Energy Distributions in Field-Ion Microscopy

Abstract

Two currently proposed theories, resonance tunneling and surface-plasmon creation, are critically compared for the interpretation of the energy distributions of field-emitted ions measured by Jason. The properties of the spacings and intensities of the peaks of the observed oscillatory energy distribution curves are explicable in terms of an ion-surface-plasmon inelastic scattering mechanism. Some of the most important features are incompatible with the resonance tunneling effect: (i) The spacings are relatively insensitive to the chemical nature and to the pressure of the imaging gas as well as to the tip crystal face used as ion source; (ii) different metal tips, i.e., W, Pt, and Mo, produce the same peak spacings; (iii) the peak intensities strongly depend on field strength. A detailed theoretical study of a model ion-plasmon interaction Hamiltonian shows that, as a result of the strong dependence of peak spacings and intensities on field strength, field-ion emission could provide a new experimental method for investigating selectively surface collective excitations of metals. The ion-plasmon scattering effects also have important consequences for the energy distributions of ions in field desorption or evaporation and place a limitation on the mass resolution of the atom-probe microscope.