Thermionic electrons

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 saturation current observed in thermionic electron emission is related to temperature by the famous Richardson-Fowler law. 

With a thermionic electron source, and a foil thickness of 100 nm, the volume of specimen excited is of the order 10-5 pm3. With a FEG source in a dedicated STEM, however, with a foil of 1 nm thickness this specimen-beam interaction volume can be as small as 10 8pm3. Very small signal levels are thus to be expected in AEM, hence the importance of employing higher brightness sources and the need to modify the specimen-detector configuration to maximize the collection angle. 

Since the technique employs low energy electrons, it is necessary to use a UHV environment. The high energy resolution in the incident electron beam is achieved by monochromatizing a thermionic electron source by means of a CHA. A second CHA is used as an energy analyser, and the basic experimental geometry is as illustrated schematically in Figure 5.47. 

Fig. 3. Eloetron-optical reproduction of a crystalline nickel plate emitting thermionic electrons in cesium vapor [according to Schenk (31)].

Fi(i. 13. Distribution of thermionic electron emission of a tungsten-monocrystal wire in cesium vapor in the [Oil] zone at A 2,000°, B 900°, C 850°K. Cesium vapor pressure corresponds to room temperature [according to Johnson and Shockley (35)]. 

Bosworth and Rideal (67,68), too found with the contact-potential method an increase of the work function by 1.38 volts when nitrogen was adsorbed on tungsten at 90°K. Since the method described in section III,Id, was used, where a second tungsten filament is heated for the emission of thermionic electrons, it is to be expected that in this case, too, N atoms were formed, which reached the cold wire near the hot one and were adsorbed. 

Work Function (WF) plays a key role in the physics and chemistry of materials. Phenomena such as the semiconductor field effect, photo- and thermionic electron emission (Allen and Gobelli, 1962), catalysis (Vayenas et al 1996), and the like are dominated by the WF. This fundamental property of electronic materials is defined as the minimum work required to extract an electron from the Fermi level Ep of a conducting phase, through the surface and place it in vacuum just outside the reach of the electrostatic forces of that phase (Trasatti and Parsons, 1986). The reference level for this transfer is thus called the vacuum reference level. Because even a clean surface is a physical discontinuity, a surface dipole t] with its associated electric field always appears at the surface of the condensed phase. Thus, the work of extracting the electron can be conceptually divided between the work required to... 

The velocity spectrum of thermionic electrons may be carried down to about 1/10 volt by means of a Hertz equipotential cathode. 

Fullerenes and their derivatives not only represent the most massive and most complex single particles in interference experiments until recently, they also mark a qualitative step towards the mesoscopic world. In many aspects they resemble rather small solids than simple quantum systems they possess collective many-particle states like plasmons and excitons, and they exhibit thermionic electron, photon and particle emission [Mitzner 1995 Hansen 1998] - which may be regarded as microscopic analogs of glow emission, blackbody radiation and thermal evaporation. Fullerenes contain about two... 

As is well known, the thermionic electron emission from the tungsten hot plate in the cesium gas has an excellent nature that the thermionic electron current from the tungsten hot plate becomes large at the lower hot plate temperature than 1000 K, although it depends on the hot plate temperature and cesium gas pressure. This is because the hot plate is covered by cesium thin layer, which reduces the work function of the hot plate to around 2.0 eV at low hot plate temperature. This nature seems to be very attractive from the view point of the development of thermionic energy converter because it enables to the operation of thermionic energy converter at the low emitter temperature. 

The thermionic electron emission current jT is described by the Dushman-Richardson relation [93,95] ... 

Mass spectrometry, which is the only technique that can be used to characterize met-cars and related metal-carbide clusters, implies that the detected clusters are ionized. This requirement opens a route to a variety of experimental procedures enabling insight to be gained into physical properties such as ionization energies, electron affinities, structure, and collective electronic properties such as thermionic electron emission and delayed atomic ion emission. 

X-rays are usually generated using thermionic electron sources. Even when driven by ultrashoit laser pulses [15] the emitted electrons have long pulse widths (> 10 ns) and are therefore not suitable for picosecond pulse x-ray generation. We have successfully utilized photoemission as a means of generating ultrashort electron bunches and subsequently picosecond duration x-ray pulses. Photoemission is known to have an extremely short response time in most materials, consequently the electron current practically follows the laser pulse intensity envelope under the appropriate conditions. 

Most commercially available X-ray spectrometers utilize a sealed X-ray tube as an excitation source, and these tubes typically employ a heated tungsten filament to induce the emission of thermionic electrons in a vacuum chamber. After acceleration by means of a high voltage X the electrons are directed towards a layer of high purity metal (e.g., Cr, Rh, W, Mo, Rh, Pd,. ..) that serves as anode. In the metal layer, a bremsstmhlung continuum is produced, onto which the characteristic lines of the anode material are superimposed. The broad band radiation is well suited for the excitation of the characteristic lines of a wide range of atomic numbers. The higher the atomic number of the anode material, the more intense the beam of radiation produced in the tube. Fig. 11.11 shows a schematic cross-section of a sealed X-ray tube. 

The BE in APS is obtained directly from the recorder plots by applying the correction for the work function of the thermionic electron soiuce. To avoid the imcertainty introduced due to this correction in BE measiuements, Fukuda et al. [34] have used a field-emission soiurce. In earlier measurements in APS, the BE was determined in a simple way by the intersection of the extrapolated projection of the bac ound and positive going low energy slope of the peak. Since the APS yield is proportional to the self-convolution of the density of the final electron states broadened by the finite lifetime of the core hole and other effects stated earlier, precise knowledge about BE can be obtained by using deconvolution techniques. Successful deconvolution techniques have been developed by Fukuda et al. [34], Dose et al. [35,36], and Schulz et al. [37]. 

Schematic diagram of Auger electron appearance potential spectrometer. Thermionic electrons passing through an aperture in the anode impinge on the sample. Signal is extracted by the potential modulation technique.

The use of the liquid scintillation counter for chemiluminescence in the out-of-coincidence mode, at phototube-amplifier gains such that single photoelectrons are detected also means that the thermionic emission of the phototube photocathode will be counted with equal efficiency. If N and N represent the thermionic electron noise of the phototubes, the signal-to-noise ratio for chemiluminescence measurements will be approximately... 

Emissivity A measure of the degree of emission of thermionic electrons from a hot cathode or filament Mu Dimensionless voltage amplification factor of a vacuum tube. 

Solid state metal borides are characteristically extremely hard, involatUe, high melting and chemically inert materials which are industrially important with uses as refractory materials and in rocket cones and turbine blades, i.e. components that must withstand extreme stress, shock and high temperatures. The borides LaBg and CeB are excellent thermionic electron emission sources, and single crystals are used as cathode materials in electron microscopes (see Box 13.8). 

Furthermore, in 2013, two novel mechanisms for molecular oxygen activation hy MNPs have been ascertained. The first lies on an indirect photothermal pathway whereby induction of extreme heat on the gold nanoparticle surface through excitation of the plasmon resonance leads to the nanoparticle fragmentation and to an increased thermionic electron emission, responsible for generation. Alternatively, it has also been proved that molecular 0) gen adsorbed onto the metal nanoparticle surface can additionally be activated through electron transfer. 

The work functions of metal surfaces show a marked dependence on the crystal planes that constitute the surface. For polycrystal metal surfaces, the experimentally determined value of the work function is an average of the work functions of all the crystal planes that are present on the surface. It is made up of the contributions from the individual crystal planes. If /a, / and tp are defined as the mean effective work functions for thermionic electron emission, positive surface ionization, and negative surface ionization, respectively, it is generally evident that /+ > /, and / =... 

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