Thermoionic emission

2 Production of Gas-phase Alkali Metal ions 4,2.1 Thermoionic Emission 

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The low level comparator will reject pulses of low intensity which are generated by thermoionic emission in the dynodes. It is obvious that an electron emitted by dynode 5 for instance will produce a much smaller avalanche at the anode than an electron emitted by the photocathode but of course thermoionic emission from the photocathode itself would still appear as a genuine pulse, and for this reason PM tubes used in photon counting applications must be cooled down. 

The vacuum-tube diode, invented by Fleming24 in 1904 [2,3], works because of the relative geometrical shapes of the two concentric electrodes, the cathode and the anode. It consists of a cylindrical glass enclosure that is partially evacuated, bonded, and sealed to a metal base. It contains an inner metallic thin-wire "cathode" (negative electrode, consisting of W, oxide-covered W, or a Th-W alloy), placed along the cylinder axis. This cathode is electrically heated to 900 K or above, using an auxiliary filament circuit, typically driven by a 6.3-V power supply, to foster thermoionic emission of electrons from the cathode. This cathode is cylindrically surrounded by a metallic outer electrode, the anode or "positive electrode" or "plate," which is a hollow metallic cylinder, whose axis coincides with that of the cathode. The... 

Finally, in the solvated electron solution/vapour system electronic emission can occur in the equilibrium manner, i.e., as thermoionic emission... 

Whereas early publications have explained experimental results within the framework of the Fowler-Nordheim mechanism alone [43], recent publications [44-46] have attributed the injection of carriers to a combination of both mechanisms at low fields and high temperature, thermoionic emission is considered dominant. On the contrary, for high electric fields (typically >2MV/cm), injection would essentially occur via tunneling. 

The free passage of electrons through the film is possible at thicknesses <5 nm by tunneling. For higher thicknesses, it is assumed that the electrons surmount any charge barrier by their thermal energy, corresponding to thermoionic emission. 

Fig. 4.1 Ion source for thermoionic emission of alkali metal ions. (Reprinted with permission from ref 5. 1984, American Chemical Society)...

In desorption ionization methods, like field desorption [11], fast-atom bombardment [12], and matrix-assisted laser desorption (MALDI) [13], adding small amoimts of alkali metal salts to the sample on the emission wire (in field desorption) or target (in fast-atom bombardment and MALDI) may result in the observation of adduct ions [M + Alkali] (see also Sect. 7.2). This is generally termed doping with alkali metal salts. In principle, when a platimrm or tungsten wine, to be used in thermoionic emission, is doped with alkali metal salts, gas-phase alkali metal ions may be produced as well. It may be questioned whether this is due to a thermoionic or desorption/ionization effect. 

Campbell EEB, Eevine RD. Delayed ionization and fragmentation en route to thermoionic emission statisties and dynamies. Annu Rev Phys Chem. 2000 51 65-98. 

In the previous equation. A [A m K ] is the Richardson constant (A = 4nqm k /hp, where m is the effective mass of the charge-carrier). PAN/PEO nanofiber-diodes exhibit good rectifying behavior, and in forward conditions they roughly behave as predicted by the thermoionic emission model, especially at... 

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