Hot hollow cathode

The characteristic depends on the discharge gas (at a few mbar of argon i = 0.2-2 A, V = 1-2 kV at 10-20 mbar of helium i = 0.2-2 A, V < 1 kV) but also on the cathode mounting. Indeed, in a cooled cathode the characteristic is normal and the analyte volatilizes by cathodic sputtering only, whereas in the hot hollow cathode thermal evaporation also takes place, by which the characteristic especially at high currents may become normal. In this case thermal effects could even lead to a strong selective volatilization, which can be made use of analytically. The latter was shown to occur in the case of brass, as it could be demonstrated by electron probe micrographs of partly molten brass samples, the outer layers of which are less rich in Zn [467]. 

The energetically important parts of the discharge (cathode layer, dark space, and negative glow) as well as the sample are inside the cathode cavity. The volatilization results from cathodic sputtering and/or thermal evaporation. This depends on whether the whole cathode with its outer and inner wall is subjected to sputtering without any cooling (hot hollow cathode), the outer wall is shielded by a quartz... 

If the discharge is supported by thermoelectrons emitted from the hot walls, it is called a hot hollow cathode and operates in an arc mode with low voltages and high currents. 

In a hot hollow cathode source, the gas pressure in a tube is raised by having an orifice restricting the exit of gas from the tube and the thermoelectrons are trapped in the anode cavity, A high density plasma beam exits the orifice and the electrons may be used to evaporate material or ionize gases. The hot hollow cathode is capable of much higher electron and ion densities than the cold hollow cathode system. The hollow cathode electron soirrce can be used to augment plasma generation. 

Hollow cathode (plasma) A cathode with a deep cylindrical cavity or tube such that the electrons are trapped in the cavity and are effective in ionizing gases in it. The cathode can be heated to the point that there is thermoelectron emission (hot hollow cathode). The hollow cathode can be used as an electron source. 

When the intensity of a hollow cathode lamp increases because of a reduction in the shunt resistance, the profile of the emission line changes. As the central part of the cathode becomes very hot, the line is broadened for several reasons. However, vaporised atoms emitted by the cathode will reabsorb in a colder part of the lamp in the form of a very fine line. The net result is that the emission curve dips in the middle because of self-absorption. This observation is the basis of the pulsed lamp technique for correction of background absorption (Fig. 14.15). 

The use of hollow cathode lamps is sufficient for the majority of elements. The low intensity and short service life of these lamps when used for volatile elements such as As, Te, Se, Bi, etc. may, however, be a problem when determining traces of these elements. Electrodcless lamps or high intensity lamps (with auxiliary hot cathode) can be used in these cases to enhance the quality of analysis of these elements. 

Broekaert J. A. C. (1979) Some observations on sample volatilisation in a hot-type hollow cathode, Spectrochim Acta, Part B 34 11-17. 

Funnel, Buchner, 5 cm Gas diffusion tube Gloves, plastic Goggles, safety Hollow cathode, Hg Hot plate Paper, filter, 5 cm Pipet, delivery, 1 mL Pipet, Mohr, 1.0 mL Pump, diaphragm, like Neptune Dyna-pump... 

Fig. 13. Comparison of CH3 and CH densities as a function of position (distance from the cathode or filament) in the hollow-cathode (open symbols) and hot-filament (closed symbols) CVD systems. The CH3 densities in the hollow-cathode system were obtained by dividing the measured column densities by an estimated path length of 2.5 cm. (Reprinted with permission from Menningen et al., 1995a, Contrib. Plasma Phys. 35, 359, 1995 Wiley-VCH, Inc.)...

Atomic spectrometric tests because of the low volatility of zirconium compounds, it is advisable to use the hot nitrous oxide/acetylene flame. In emission mode 5 ppm zirconium can be identified at 360.12nm. By atomic absorption, using a zirconium hollow-cathode lamp the detection sensitivity... 

Figure 3.2 Wavelength-dependent spectral radiance of the xenon short-arc lamp measured in the hot-spot and at different distances from the cathode, in comparison to some selected emission lines of hollow cathode lamps...

The density of the plasma in the vicinity of the cathode can be augmented by injecting electrons from a hot filament or a hollow cathode. This increases the sputtering rate that can be attained from a magnetron source. It can also allow the sputtering discharge to be operated at a lower pressure. 

Anodic arcs can he categorized as the source of electrons. The electrons can arise from a heated thermoelectron-emitting surface, a hot or cold hollow cathode, or an arc cathode. By bending the e-beam in a magnetic held, the vaporized material may be kept from impinging on the electron source. Commercial sources for anodic arc deposition are available with most using a hollow cathode electron source. 

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