Scattering electron-specimen interactions

Electron-Specimen Interactions 29.2.3.3.1. Elastic and Inelastic Scattering... 

It is a fundamental principle of quantum mechanics that electrons bound in an atom can have only discrete energy values. Thus, when an electron strikes an atom its electrons can absorb energy from the incident electron in specific, discrete amounts. As a result the scattered incident electron can lose energy only in specific amounts. In EELS an incident electron beam of energy Eq bombards an atom or collection of atoms. After the interaction the energy loss E of the scattered electron beam is measured. Since the electronic energy states of different elements, and of a single element in different chemical environments, are unique, the emitted beam will contain information about the composition and chemistry of the specimen. 

By considering only elastic scattering events, the interaction of the specimen with the electron beam can be described through a complex transmission function (object wave-function) 0(f) which represents the ratio between the outgoing and the incoming electron wave-functions f = (x, y) is a two-dimensional vector lying on a plane perpendicular to the optic axis z which is parallel, and in the same direction, to the electron beam. In the standard phase object approximation ... 

When a solid is bombarded with high energy electrons the interaction produces secondary electrons (elastic), back-scattered electrons (inelastic), low loss electrons. Auger electrons, photo electrons, electron diffraction, characteristic x-rays, x-ray continuum, light, hole electron pairs and specimen current. These interactions are used to identify the specimen and elements of the specimen and can also be used to physically characterize particulate systems. 

Electrons which have lost energy by interaction with the specimen (inelastically scattered electrons) also exit the specimen. The amounts of energy lost by the electrons can be measured and form the basis for several types of electron energy-loss spectroscopies (EELS) which allow high-resolution chemical analysis to be performed. 

Backscattered (high-energy) electrons are elastically scattered beam electrons. The electrcms are deflected back out of the specimen interaction volume. 

The interaction of the primary electrons with the specimen results in the emission of secondary electrons (SE), i.e., inelastically scattered electrons that escape from the sample and reach the respective detector if emitted per definition from a depth less than about 50 nm equivalent of solid phase (Figure 21.2). The resolutirm of the SE is determined by the quality of the focus of the primary beam and the area of the sample surface from which they are emitted. 

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