Ion-Neutralization Spectroscopy of Solids and Solid Surfaces

A new method of spectroscopy of solids is presented. By means of it, information concerning the state densities in solids is extracted from measured kinetic-energy distributions of electrons ejected from the solid by slow noble-gas ions. The basic electronic process involved is the radiationless, two-electron, Augertype transition which neutralizes the ion to its ground state at the atomically clean solid surface and simultaneously excites another electron in the solid. The method starts with measured energy distributions and, making a minimum of assumptions, works its way back to what may be called the transition density function. This function, which specifies the relative probability that an electron at a given band energy will be involved in the neutralization process, depends on state densities in initial and final states of the process, transition probability, final-state interaction, and other factors. The information obtained is of the same general kind as that obtained by soft-x-ray spectroscopy and photoelectron spectroscopy. When the surface is atomically clean the transition density involves bulk-state densities and transition probabilities which depend on the surface. For surfaces with monolayers involving foreign atoms, the transition density is modified by the state-density and wave-function changes resulting from the presence of the two-dimensional surface crystal. The method, feasibility, and characteristics of the ion-neutralization spectroscopy are discussed using experimental data for copper and nickel as illustrative examples. Experimental apparatus and operating conditions are discussed only briefly.