Electron-beam interactions

The incoming electron beam interacts with the sample to produce a number of signals that are subsequently detectable and useful for analysis. They are X-ray emission, which can be detected either by Energy Dispersive Spectroscopy, EDS, or by Wavelength Dispersive Spectroscopy, WDS visible or UV emission, which is known as Cathodoluminescence, CL and Auger Electron Emission, which is the basis of Auger Electron Spectroscopy discussed in Chapter 5. Finally, the incoming... 

Figure 1 Signals generated when the focussed electron beam interacts with a thin specimen in a scanning transmission electron microscope (STEM).

Fig. 8 Schematic of electron beam interaction with a sample and the electron beam interaction volumes for electron-specimen interactions.

Although x-ray microanalysis in the STEM is the most developed form of analytical electron microscopy, many other types of information can be obtained when an electron beam interacts with a thin specimen. Figure 2 shows the various signals generated as electrons traverse a thin specimen. The following information about heterogeneous catalysts can be obtained from these signals ... 

Much of the available experimental information on intermetallic compounds comes from high-energy electron diffraction (HEED) measurements. In electron diffraction, the electron beams interact with the electrostatic potential in the crystal. The electron structure factor is therefore directly dependent on this... 

In an electron microscope, a stream of electrons is formed by the electron source and is accelerated toward the specimen using a positive electric potential. The stream is focused into a thin, monochromatic beam by using metal apertures and magnetic lenses. The electron beam interacts with the specimen and the effects of these interactions are detected and transformed into an image. 

The electron microprobe is similar to the scanning electron microscope however, its primary function is to detect characteristic X-rays produced by the electron beam interaction with the specimen. The X-ray emissions can be used to determine the elemental composition of the specimen quantitatively and the location of a particular element within the morphology or topological structure of the specimen. 

Some crystallites will dissociate in the beam while others tend to agglomerate (14). The mass of the crystallite, support-metal interaction, chemical environment, oxidation state of the metal, etc., all have an influence on how the crystallite and electron beam interact. In order to formulate a correlation of these variables with crystallite reactivity with the beam, the crystallite site chemistry is required. This is difficult if not impossible to do because the site chemistry is altered during microscopic examination. With parallel EELS detection the time may be sufficiently reduced that useful chemical information can be obtained and correlations of the type previously described can be made. 

In a LWA, the electron and laser pulses are inherently synchronized, so the time jitter sources associated with photocathode accelerators are not an issue. The ultimate time resolution should depend only upon the cross correlation between the laser and electron pulses and the physics of the electron beam interaction with the... 

With direct observation, the sample must be kept cold in the electron microscope, and care is required to prevent sample damage in the beam and to prevent microscope contamination. In addition, these frozen samples are often difficult to image because of charging effects that distort the image. The benefit of this extra care in sample handling, however, is that electron beam interactions with the sample produce characteristic X-ray signals that allow identification of components of the emulsion being observed. This technique has been refined to the point where, in special cases, chemical compositional differences at the emulsion interface can be identified, as well as the composition of the dispersed and continuous phases 109, 110),... 

Figure 4.9 Monte Carlo electron trajectory simulation of an electron beam interaction with iron E = 20 keV. (Reproduced with kind permission of Springer Science and Business Media from J.I. Goldstein et al, Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed., Plenum Press, New York. 1992 Springer Science.)...

In a transmission electron microscope, a highly coherent electron beam passes through a thin sample. The electron beam interacts with the sample and is transferred to the specimen s exit plane. The electron wave at the exit plane is magnified in order to form an image or alternatively a diffraction pattern of the sample. 

The primary electrons of the electron beam interact with the specimen producing a large number of detectable signals. 

Krasheninnikoy S.I. (1980), Electron Beam Interaction with Chemically Active Plasma, Ph.D. Dissertation, Kurchatov Institute of Atomic Energy, Moscow. 

The electron microprobe or WDS can provide accurate chemical analysis or a chemical profile across the interface. The wavelength of the X-rays emitted when the electron beam interacts with the sample is measured. Wavelength dispersive spectroscopy is more accurate than XEDS, but is a serial acquisition, so it is slower. Table 10.10 compares WDS and XEDS. 

Robin, 1978, J. Physique Lett. 39, L265-269. Draper, C.W., FJ.A. Den Breeder, D.C. Jacobson, E.N. Kaufmann, M.L. McDonnald and J.M. Vandenberg, 1982, in Laser and Electron Beam Interactions with Solids, eds. B.R. Appleton and G.K. Celler (Elsevier, Amsterdam). 

For quantitative X-ray analysis, electron probe microanalyzers (EPMA, EPA, or EMMA) are able to determine the elemental concentration by X-ray emission from the microvolume of paint samples where a static electron beam interacts. However, due to inhomogeneity of paint samples nonquantitative... 

Upon entering the sample, the electron beam interacts with the sample, and both electron and photon signals are generated (Figure 2). 

Usually a traveling wave tube means a forward wave tube. In the vacuum tube, the electron beam and forward waves interact with each other. There is a vacuum tube in which the electron beam interacts with backward waves. This type of tube is termed the backward wave tube. The backward wave tube is inherently highly regenerative (built-in positive feedback) therefore, it is usually an oscillator. Such an oscillator is termed a backward wave oscillator (BWO). The BWO has a traveling wave structure inside, but usually it is not called a traveling wave tube. Details of a BWO are presented in Chap. 6.4.3. The oscillation frequency of a BWO is dominated by the electron speed, which is determined by the anode voltage. Therefore, a BWO is a microwave frequency voltage controlled oscillator (VCO). 

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