Beam-Induced Indeterminacies In The Ivem

Abstract

Intermediate-voltage electron microscopes accelerate electrons to energies U between 200 keV and 600 keV to take advantage of a U-3/8 improvement in resolving power which enables sub-0.2nm atomic structure projections to be routinely imaged. This exciting prospect is, however, purchased at the price of uncertainties in the integrity of the images obtained, because the investigating probe unavoidably perturbs the structure which is being imaged. Such perturbations have been relegated historically to the anecdotal realm of “radiation effects” but are really a perverse manifestation of the Uncertainty Principle. That electron microscopists are able to resolve the structure of solids at all is a tribute to the strength and mutliplicity of atomic bonds in solids and the large “recoiless fraction” (in the parlance of nuclear spectroscopies like the Mossbauer effect) for which the whole solid, and not an individual atom, recoils from the impact of the investigating probe.Atomic structure is sensitive to the impact of incident IVEM electrons because they lose energy (at a rate of order 1 GeV/m) in traversing a solid, a small portion of which is available as kinetic energy to restructure atomic positions. About 98% of the energy loss goes into electronic excitations which can in certain materials (notably organic materials, halides, silicates and almost anything explosive) destabilize atomic positions within the specimen interior with unnervingly high efficiency by radiolytic processes. Analogous radiolytic loss or restructuring of surface atoms, whose bonding constraints are fewer than for atoms in the interior, has been coined “desorption induced by electronic transitions” (DIET). Of the remaining 2% of the energy loss, which is transferred instead to atomic nuclei, most goes into the generation of heat (the eventual fate of most non-radiative electronic transitions as well), but about 1 part in 105 (or 1 part in 107 overall) is available to generate atomic displacements ballistically, by direct knockon: at the surface for almost all solids examined by IVEM (leading to sputtering) and in the interior for many medium atomic-weight solids.