Sources electron

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

Instmmentation for tern is somewhat similar to that for sem however, because of the need to keep the sample surface as clean as possible throughout the analysis to avoid imaging surface contamination as opposed to the sample surface itself, ultrahigh vacuum conditions (ca 10 -10 Pa) are needed in the sample area of the microscope. Electron sources in tern are similar to those used in sem, although primary electron beam energies needed for effective tern are higher, typically on the order of ca 100 keV. 

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. 

Radiation Sources. Ordinarily, electron beams are produced from soHds in vacuo by thermal or field-assisted processes. Plasmas also serve as electron sources, but are more uniquely used as ion sources. Whereas ions can be produced by sputtering and field assisted processes in the absence of plasmas, most ion sources involve plasmas (75). 

As femtomolar detection of analytes become more routine, the goal is to achieve attomolar (10 molar) analyte detection, corresponding to the detection of thousands of molecules. Detection sensitivity is enhanced if the noise ia the analytical system can be reduced. System noise consists of two types, extrinsic and intrinsic. Intrinsic aoise, which represents a fundamental limitation linked to the probabiHty of finding the analyte species within the excitation and observation regions of the iastmment, cannot be eliminated. However, extrinsic aoise, which stems from light scatteriag and/or transient electronic sources, can be alleviated. 

The electronic environment of an a-substituent on pyridine (319) approaches that of a substituent on the corresponding imino compound (320) and is intermediate between those of substituents on benzene and substituents attached to carbonyl groups (321, 322) (cf. discussion in Chapter 2.02). Substituents attached to certain positions in azole rings show similar properties to those of a- and y-substituents on pyridine. However, the azoles also possess one heteroatom which behaves as an electron source and which tends to oppose the effect of other heteroatom(s). 

Azolinones and azole iV-oxides possess systems which can act either as an electron source or as an electron sink, depending on the requirements of the reaction. 

Ring opening by intramolecular attack by an electron source located on a /3-carbon is represented by the Wharton reaction (Scheme 62) (8UCS(P1)2363) and the Eschenmoser fragmentation (Scheme 63) (81S276). 

Remaining terms to right of the bracket relate to properties to be measured within the matrix. The factor/is adjusted from 0 to 1 in relationship to the purpose of testing. A low value of/is indicated for scarce elements such as precious metals in electronic-source scrap. Moisture content has a high/value. The factor g is adjusted from 0 to... 

There are three major types of electron sources thermionic tungsten, LaBg, and hot and cold field emission. In the first case, a tungsten filament is heated to allow... 

It is easiest to discuss the electron optics of a TEM instrument by addressing the instrument from top to bottom. Refer again to the schematic in F ure la. At the top of the TEM column is an electron source or gun. An electrostatic lens is used to accelerate electrons emitted by the filament to a h potential (typically 100-1,000 kV) and to focus the electrons to a cross-over just above the anode (the diameter of the cross-over image can be from 0.5 to 30 Mm, depending on the type of gun ). The electrons at the cross-over image of the filament are delivered to the specimen by the next set of lenses on the column, the condensers. 

There are three types of instruments that provide STEM imaging and analysis to various degrees the TEM/STEM, in which a TEM instrument is modified to operate in STEM mode the SEM/STEM, which is a SEM instrument with STEM imaging capabilities and dedicated STEM instruments that are built expressly for STEM operation. The STEM modes of TEM/STEM and SEM/STEM instruments provide useful information to supplement the main TEM and SEM modes, but only the dedicated STEM with a field emission electron source can provide the highest resolution and elemental sensitivity. 

Future developments of this instrumentation include field emission electron sources at 200-300 kV that will allow better elemental detectability and better spatial resolution. Multiple X-ray detectors having large collection angles will also improve elemental detectability in X-ray microanalysis. The higher accelerating... 

Scanning Auger Electron Spectroscopy (SAM) and SIMS (in microprobe or microscope modes). SAM is the most widespread technique, but generally is considered to be of lesser sensitivity than SIMS, at least for spatial resolutions (defined by primary beam diameter d) of approximately 0.1 im. However, with a field emission electron source, SAM can achieve sensitivities tanging from 0.3% at. to 3% at. for Pranging from 1000 A to 300 A, respectively, which is competitive with the best ion microprobes. Even with competitive sensitivity, though, SAM can be very problematic for insulators and electron-sensitive materials. 

The cross-section for electron impact ionization has already been mentioned in Sect. 2.2.2.2 in connection with electron sources, and a variety of experimental and theoretical cross-sections have been shown in Fig. 2.18 for the particular case of the K-shell of nickel. The expression for the cross-section derived by Casnati et al. [2.128] gives reasonably good agreement with experiment the earlier expression of Gry-zinski [2.131] is also useful. 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

Simple electroless techniques have been used for the formation of CdTe layers following an anodic or a cathodic route of deposition. For instance, spontaneous cathodic formation of CdTe was observed on Ti or glass electrodes short circuited with a corroding A1 contact (electron source) in a solution of Cd " " and HTe02 ions [96]. After thermal treatment and subsequent growth of an a-Pb02 layer on them, the as-obtained CdTe thin films were found to exhibit n-type behavior in alkaline polysulfide PEC cells. 

A strain of Rhodopseudomonas palustris, which was isolated by enrichment with taurine, could use this as electron source, and as a source of sulfur and nitrogen during photo-autotrophic growth with CO2. Taurine was metabolized to sulfoacetaldehyde and acetyl phosphate by a pathway, which has already been noted (Novak et al. 2004). 

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