Electron emission

The energy required by an electron to escape from the surface of a crystalline solid is called the work function (9) of the material. It is a characteristic parameter for its electron emission behavior. The work function of metals is in the range of 2 to 6eV and correlates mainly with the electronegativity of the element (Table 1.23). 

The temperature coefficient of the work function of polycrystalline tungsten (dcp/r/T) is in the range of 60-110peV K [1.38]. 

Thermionic emission. The number of electrons which escape from the metal surface increases rapidly with temperature (thermionic emission). In general, the higher the temperature and the lower the work function, the higher is the electron emissivity. The current density can be calculated by the Richardson-Dushman equation (in the absence of an external electrical field), according to i — AT exp(—rp/kT), where A is the Richardson constant (A cm K ), T is the temperature (K), and p is the work function (eV). For pure tungsten A — 60.2 (A cm K ) [1.91]. The thermionic current (A cm ) can then be calculated as i — 60.2r exp(—52230/T) [1.37]. 

FIGURE 1.21. Variation of emission current density with tungsten for pure and thoriated tungsten electrodes [1.93] according to Ref. 1.92. 

Tungsten is the most important metal for thermoemission applications, not only because of its high emissivity but because of its high thermal and chemical stability (extremely low vapor pressure at service temperature high hot strength and rigidity excellent corrosion resistance against metal and oxide vapors). 

Campagna, Siegmann [4], and I = 4.2eV in the field-induced ferromagnetic state at 

Busch etal. [5]. Metastable disordered films produced at 4.2 K have only I = 3.85 0.1 OeV in the antiferromagnetic state. Possible origins for this behavior have been discussed in the paper [4]. On other polycrystalline films (produced at room temperature) the photoelectric threshold energy and the work function had. the same low value of 2.8 0.3 eV due to photoemission from impurity states at the Fermi level Ep extending down to 1.4 eV below Ep, Eastman et al. [6]. 

The energy distribution curves of the photoelectron emission from cleaved single crystals at 300 K for photon energies hv = 6.5 to 9.7 eV (see Fig. 115) reveal peaks attributed to4f levels at 1.6 eV below Ep with a peak location and peak width independent of exciting photon energy, and to p states ca. 3 eV below Ep. The peak at 5.5 eV below Ep, observable only for hv 9 eV, cannot be explained it may result from scattered electrons [1,3]. Earlier studies at hv = 6.5 eV on single crystals [2] and at hv = 6.5 to 10.2 eV on (ordered) polycrystalline films [6] showed that the 4f levels lie above the p valence band but that emission from 4f is very weak. Studies with 40 eV synchrotron radiation photons reveal the intense 4f peak at 1.8 eV below Ep and a broad p band peak around 3 eV below Ep. But there also is an unidentified broad peak between 8 and 11 eV (not observable for 61 eV photons) and a weak broad peak at 13 eV below Ep (not studied for 61 eV), which was tentatively attributed to the outermost s band of selenium, Sato etal. [7]. 

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J. H. de Boer, Electron Emission and Adsorption Phenomena, Macmillan, New York, 1935. 

While field ion microscopy has provided an effective means to visualize surface atoms and adsorbates, field emission is the preferred technique for measurement of the energetic properties of the surface. The effect of an applied field on the rate of electron emission was described by Fowler and Nordheim [65] and is shown schematically in Fig. Vlll 5. In the absence of a field, a barrier corresponding to the thermionic work function, prevents electrons from escaping from the Fermi level. An applied field, reduces this barrier to 4> - F, where the potential V decreases linearly with distance according to V = xF. Quantum-mechanical tunneling is now possible through this finite barrier, and the solufion for an electron in a finite potential box gives... 

Measuring the electron emission intensity from a particular atom as a function of V provides the work function for that atom its change in the presence of an adsorbate can also be measured. For example, the work function for the (100) plane of tungsten decreases from 4.71 to 4.21 V on adsorption of nitrogen. For more details, see Refs. 66 and 67 and Chapter XVII. Information about the surface tensions of various crystal planes can also be obtained by observing the development of facets in field ion microscopy [68]. 

OSEE Optically stimulated exoelectron emission [143] Light falling on a surface in a potential held produces electron emission Presence and nature of adsorbates... 

NakatsujI H, Kuwano R, Merita H and Nakal H 1993 Dipped adcluster model and SAC-CI method applied to harpooning, chemical luminescence and electron emission in halogen chemisorption on alkali metal surface J. Mol. Catal. 82 211-28... 

The section on Spectroscopy has been expanded to include ultraviolet-visible spectroscopy, fluorescence, Raman spectroscopy, and mass spectroscopy. Retained sections have been thoroughly revised in particular, the tables on electronic emission and atomic absorption spectroscopy, nuclear magnetic resonance, and infrared spectroscopy. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon ICP, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. 

To obtain an accurate value of Tig for the ground electronic state is virtually impossible by vibrational spectroscopy because of the problems of a rapidly decreasing population with increasing v. In fact, most determinations are made from electronic emission spectra from one, or more, excited electronic states to the ground state. 

Figure 8.21 The competitive processes of X-ray fluorescence (XRF) and Auger electron emission...

If we require similar information regarding the ground state potential energy surface in a polyatomic molecule the electronic emission specttum may again provide valuable information SVLF spectroscopy is a particularly powerful technique for providing it. 

Edx is based on the emission of x-rays with energies characteristic of the atom from which they originate in Heu of secondary electron emission. Thus, this technique can be used to provide elemental information about the sample. In the sem, this process is stimulated by the incident primary beam of electrons. As will be discussed below, this process is also the basis of essentially the same technique but performed in an electron spectrometer. When carried out this way, the technique is known as electron microprobe analysis (ema). 

Fig. 14. Schematic of the Auger electron emission process induced by creation of a K level electron hole.

An alternative mechanism of excess energy release when electron relaxation occurs is through x-ray fluorescence. In fact, x-ray fluorescence favorably competes with Auger electron emission for atoms with large atomic numbers. Figure 16 shows a plot of the relative yields of these two processes as a function of atomic number for atoms with initial K level holes. The cross-over point between the two processes generally occurs at an atomic number of 30. Thus, aes has much greater sensitivity to low Z elements than x-ray fluorescence. 

Fig. 16. Relative probabiUties of Auger electron emission and x-ray fluorescence for initial iClevel electron hole as a function of atomic number (19).

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

The overall function of the electron gun is to produce a source of electrons emanating from as small a spot as possible. The lenses act to demagnify this spot and focus it onto a sample. The gun itself produces electron emission from a small area and then demagnifies it initially before presenting it to the lens stack. The actual emission area might be a few pm in diameter and will be focused eventually into a spot as small as 1 or 2 nm on the specimen. 

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... 

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