Thermionic Emission of Electrons

Here we shall prove the two formulse which we gave in the text for the thermionic emission of electrons (the Richardson effect) firstly on the basis of classical statistics and secondly on the basis of the 

Eermi-Dirac statistics. Eor this we require to calculate tEe number of electrons striking one square centimetre of tbe boundary in tbe metal per second, suck that tbe kinetic energy of tbeir motion normal to tbe boundary is sufficient to carry tbe electron over tbe energy barrier of beigbt wbicb represents tbe boundary. We bave therefore to determine, on tbe basis of tbe distribution law, tbe number of electrons for wbicb, e.g., 

We begin with classical statistics. On tbia basis tbe number of electrons whose velocity lies between v and is given by 

To find tbe number of electrons falling on unit area of tbe boundary per second we bave first to divide tbe above number by F, to get tbe density of tbe electrons, and then multiply by Vg, as in unit time there impinge on tbe boundary all tbe molecules with tbe component Vg, vrbicb were contained in tbe layer of breadth Vg in front of tbe boundary 3, p. 5). We thus obtain tbe emission current by evaluating tbe integral 

Tbe calculation takes a similar course in tbe case of tbe Fermi distribution. Here we start from tbe distribution function (p. 216) 

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For example, the //V characteristics of devices based on the aluminum chelate complex Alq3, where Ag-Mg or ln-Mg are used as the cathode, can be described by thermionic emission of electrons over the barrier height at the electron injection contact/Alq3 [78]. 

Oxygen and tungsten reacted in a totally different manner. A minute pressure of oxygen formed an adsorbed film over the whole surface of the filament. The presence of this film was shown in three different ways it was found to diminish the thermionic emission of electrons from the heated wire at 1,500° K. it was found to inhibit the dissociation of hydrogen at the surface of the wire and it was shown by quantitative studies that the rate of reaction of the filament covered by adsorbed oxygen, with further supplies of the gas, was so great as to require the presence of a monatomic film of adsorbed oxygen, already combined with the surface of the filament for, p. 1150). 

Impurities in the interior of the filament can sometimes diffuse to the surface, and form there a surface film apparently just like those formed by combination with a gas. Thus thorium (i, p. 2280) diffuses to the surface between 2,000 and 2,500° K., forming a film which enormously increases the thermionic emission of electrons. This film grows till it is one thorium atom thick, and then ceases to grow. For further information as to the effect of layers of foreign atoms on electron emission, see Chap. VIII, 4. 

Various FID modifications have been developed to protect the flame from draft, obtain an optimal air flow pattern, curtail thermionic emission of electrons from the detector jet tip, etc. An interesting development is the use of a Tiorizontal flame in a high-eflBciency FID designed to minimize combination reactions (i7). 

We find that laser induced thermionic emission of electrons is a very well suited method for our purposes. The idea consists simply of irradiating the molecules by a tightly focused laser beam. When many photons are absorbed by a single molecule, the latter is heated such that it emits an electron. The resulting positive ion is then extracted and detected. 

Such methods have been available for many years. For example, the change in thermionic emission of electrons after adsorption of a... 

In the case of the use of an uncooled cathode, with a current density of about 0.1A cm 2, an additional thermionic emission of electrons takes place. This results in another avalanche effect and since the output impedance of the supply limits the voltage, a discharge with low voltage and high current density commences. 

A. Types of Element. Rectifiers are devices that allow the passage of current in only one direction. Thermionic emission of electrons, for example, is a one-way process and was the basis for vacuum-tube diodes. The cathode must be held negative with respect to the collector in order for electrons to flow. This explains the use of the word valve to describe what has been known in the United States as a tube. ... 

The so-called dark noise introduced by a discrete dynode SEM when operated under appropriate conditions is largely Johnson noise, the result of random thermionic emission of electrons from the dynode surfaces, and is minimized by appropriate choice of materials. When SEMs are used appropriately the dark noise is generally low (< lO A, negligible in applications of interest here) even when operated to provide Gsem = 10 . (The maximum output current from such devices can be as high as 10 A before saturation occurs.) In addition, some dark current can arise from leakage through the ceramic supports for the dynodes, particularly when they become... 

This value of may be significantly lower than that found from equation 33 for the thermionic emission of electrons. 

Second, the target is heated so that the positrons trapped in the surface state are thermally activated to form thermal positronium in the ground state[21j. Fig. 6 shows the thermal activation of Ps from a Cu(lll) surface. The process is analogous to thermionic emission of electrons or to thermal desorption of hydrogen, and can be described by the usual statistical mechanics arguments[28]. 

Flame charging occurs when particles are formed in or pass through a flame. At the high temperature of the flame, direct ionization of gas molecules creates high concentrations of positive and negative ions and thermionic emissions of electrons... 

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