Scanning Microscopy


The energy and angular distributions of electrons ejected by fast charged particles in ionizing collisions provide detailed information regarding the effects of atomic, molecular, and condensed-phase structure on the energy loss process. Analysis of the wide range of available data has lead to several general conclusions. For ionization of atomic and molecular targets by protons having energies above a few hundred keV, the cross sections for electron production have been found to scale as the number of loosely bound target electrons. The more subtle features of the ejected electron energy spectra are, however, dependent on the electronic structure of the target, especially for emission of low-energy electrons. Although ab initio theoretical techniques are currently limited to simple systems, cross sections for electron production in collisions of bare charged particles with atomic targets can be reliably calculated using Born theory. For more complex targets, models have been developed that provide singly-differential electron-emission cross sections for a wide range of ion energies. These models rely on experimental data to determine parameters that are difficult or infeasible to obtain by ab initio theory. Although great strides have been made in understanding ionization processes involving bare ions and atomic and molecular targets, understanding the collision process for structured ions, i.e., ions that carry bound electrons, as well as collision processes in solid targets, presents a greater challenge. With structured ions, the screening of the ion's nuclear charge by its bound electrons results in an effective interaction potential that depends on the collisional energy loss. In addition, this screened potential has been found to vary with energy loss in a functionally different manner for different light ions. For solid targets, differential ionization cross sections for ion impact are fragmentary, and theoretical results exhibit only qualitative agreement.

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