Electron flux

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 border between two three-dimensional atomic basins is a two-dimensional surface. Points on such dividing surfaces have the property that the gradient of the electron density is perpendicular to the normal vector of the surface, i.e. the radial part of the derivative of the electron density (the electronic flux ) is zero. 

Brand, M.D. Murphy, M.P. (1987). Control of electron flux through the respiratory chain in mitochondria and cells. Biol. Rev. 62, 141-193. 

A third class of catalysts was prepared by electron beam induced deposition of XiCl4 on a polycrystalhne Au foil. Deposition of TiCU at 300 K leads to films which comprise Ti + and Ti species as inferred from XPS measurements [90]. Depending on the experimental parameters (background pressure of TiCU, electron flux, electron energy) different composition of Ti oxidation states are observed [23]. From angular-dependent measurements it was concluded that the Ti + centers are more prominent at the surface of the titanium chloride film, while the Xp+ centers are located in the bulk [90]. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

Collman JP, Devaraj NK, Decreau RA, Yang Y, Yan Y-L, Ebina W, Eberspacher TA, Chidsey CED. 2007b. A cytochrome c oxidase model catalyzes oxygen to water reduction under rate-limiting electron flux. Science 315 1565. 

Quantitative Analysis. In its basic form, AES provides compositional information on a relatively large area ( 1 mm2 ) of surface, using a broad-focussed electron beam probe. Sufficient signal may be obtained in this way with a low incident electron flux, thus avoiding potential electron-induced modifications of the surface. 

As can be seen in the different boundary conditions, the main effects of having ribs are electronic conductivity and transport of oxygen and water, especially in the liquid phase. In terms of electronic conductivity, the diffusion media are mainly carbon, a material that is fairly conductive. However, for very hydro-phobic or porous gas-diffusion layers that have a small volume fraction of carbon, electronic conductivity can become important. Because the electrons leave the fuel cell through the ribs, hot spots can develop with large gradients in electron flux density next to the channel. " Furthermore, if the conductivity of the gas-diffusion layer becomes too small, a... 

The magnitude of the electron flux at any point J(r,z) depends upon such parameters as the atomic number and density of both the resist and substrate as well as the velocity (accelerating voltage) of the electron. The resultant undesired exposure of resist in areas not directly addressed by the electron beam is called the proximity effect and imposes certain restrictions... 

A widely used technique is the so called flood-gun. An auxiliary electron gun is tuned so that the ingoing (from the gun) and outgoing (photoemitted) electron fluxes compensate. In order to achieve this equilibrium, it is necessary to record spectra during a large interval of time and to search for steady state conditions. Therefore, in view of the short time needed to build one monolayer of contamination on the surface of the sample, the use of this technique is made difficult in the case of UPS. One should mention, that very recently a similar compensation electron gun has been successfully used for electronic vibrational spectroscopy, which is even more surface-sensitive than... 

From Eqs. (1) and (3) we may write the total contribution of the observed electron flux as... 

A widely used method described by Shirley (1972) is similar, but it employs instead a summation over the observed electron flux (z)fc, which includes contributions from electrons already scattered. Our own iterative method is based on the assumption of uniform scattering of a fixed fraction of the electrons that would be observed in the complete absence of scat-... 

For thermionic emission from a uniform surface, the maximum electron flux j across the uniform surface at a temperature T in the absence of an applied field is given by... 

In the classical description of nonequilibrium systems, fluxes are driven by forces [73,76,77]. Equation (8) shows that the flux of electrons (7 ) is related to the (photo)electrochemical force (VEFn) by a proportionality factor (np ). Equation (8) and the related equation for holes can be employed as a simple and powerful description of solar photoconversion systems. However, it is useful to go beyond this analysis and break V > into its component quasithermodynamic constituents, V(7 an Vp, because this helps reveal the fundamental differences between the photoconversion mechanisms of the various types of solar cells. Equation (6) can be separated into two independent electron fluxes, each driven by one of the two generalized forces, Vf7 and Vp. Equations (9a) and (9b) are expressed in the form Flux = Proportionality factor X Force ... 

The oxygen reactivity of flavohydroquinone bound to apoflavoprotein dehydrogenases can vary considerably from fast (flavodoxins), moderate (xanthine oxidase) to nil (succinate dehydrogenase) Most, but not all, flavoprotein dehydrogenases contain one or more types of metal prosthetic groups, e.g. xanthine oxidase contains also Fe and Mo. Since these metal ions are involved in electron flux, their possible participation in the reaction with O2 cannot be excluded. Much evidence, however, indicates that the flavin is involved in the one-electron reduction of Oj, as shown in Equation (5). 

Photomultipliers are vacuum tube photocells with a sealed-in set of dynodes. Each successive dynode is kept at a potential difference of 100V o that photoelectrons emitted from the cathode surface are accelerated M each step. The secondary electrons ejected from the last dynode are Collected by the anode and are multiplied so that a 10° — 107 — fold arnpli-t tfion of electron flux is achieved. This allows simple devices such as l croammeters to measure weak light intensities. Background thermal mission can be minimised by cooling the photomultiplier. The schematic... 

One concludes that af-jj is the electron flux which is induced by the flux of atomic species j, provided that. A", = 0 (no force acting on the electrons). Also, from Eqn. (4.16) it follows that in a homogeneous solid, if an external electric field is applied (i.e, X = Zj e0 E and Xs = e0-E), then (Zj-a f) represents the effective (drift) charge of species j in the field E. 

Boundary condition l). In the absence of an external electrical circuit, current cannot flow, that is, X Zjji = 0. Inserting ionic and electronic fluxes (Eqns. (4.99) and (4.100)) into this condition, one obtains... 

The different boundary conditions given in Eqns. (6.23) thus have to be supplemented according to the magnitude of / M (rM = 0 or =b 0) in order to evaluate the growth of the spinel. This has been done for the different coupling conditions of the ionic and electronic fluxes in the product according to Figure 6-6 [T. Pfeiffer, H. Schmalzried (1989) H. Schmalzried, T. Pfeiffer (1986)]. 

Equations (8.48)-(8.50) define three independent transport coefficients for the two building units (A,h), namely L, and Lhh, in terms of the 21 independent transport coefficients of the SE set. They are sufficient to describe the transport in A O. The cross coefficient LAh expresses the coupling between the ionic and electronic fluxes. If ATh ) = 0, the electronic flux is due only to the cross effect and given by... 

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