Electrons, electrical discharges

All methods of plasma production require some electrons to be present as electric-discharge initiators. For a plasma torch, the initiating electrons are introduced from a piezoelectric spark directed into argon gas flowing in the interval between two concentric quartz tubes. 

Spark (source) ionization. Occurs when a solid sample is vaporized and partially ionized by an intermittent electric discharge. Further ionization occurs in the discharge when gaseous atoms and small molecular moieties interact with energetic electrons in the intermittent discharge. It is recommended that the word source be dropped from this term. 

The method of excitation was, in the early days, by an electron beam but now a transverse electrical discharge, like that for the nitrogen laser shown in Figure 9.14, is used. Indeed such an excimer laser can be converted to a nitrogen laser by changing the gas. 

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

Static electric discharge is a serious problem ia the electronics iadustry. Electronic devices are extremely seasitive to static electric discharges. Examples of the seasitivity to electrostatic discharge (ESD) are givea ia Table 2 (26). 

The use of inadiation or electron bombardment offers an alternative approach to molecular dissociation to the use of elevated temperamres, and offers a number of practical advantages. Intensive sources of radiation in the visible and near-visible are produced by flash photolysis, in which a bank of electrical capacitors is discharged tlrrough an inert gas such as ktypton to produce up to 10 joule for a period of about 10 " s, or by the use of high power laser beams (Eastham, 1986 (loc.cit.)). A more sustainable source of radiation is obtained from electrical discharge devices usually incorporating... 

Electrical discharge machining Electrochemical milling Electron beam machining Laser beam machining Ultrasonic machining... 

Ionization The process of adding electrons to, or removing electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, and nuclear radiation can cause ionization. 

Fig. 14-4. An electric discharge tube, very low pressure. Electrons travel from the negative electrode to the positive electrode some of them pass through the triangular hole to produce a triangular spot on the fluorescent screen.

The most widespread modem use of mass spectrometers is to identify chemical substances. When a molecule is placed in a mass spectrometer, the electrical discharge strips away one of its electrons. This so-called parent ion has virtually the same mass as the neutral molecule. If the mass of the parent ion is measured with high enough accuracy, the data can provide the molecular formula of the substance. 

Electrical discharges through samples of helium gas generate He cations, some of which bond with He atoms to form Hc2 cations. These fall apart as soon as they capture electrons, but they last long enough to be studied spectroscopically. The bond dissociation energy is 250 kJ/mol, approximately 60% as strong as the bond in the H2 molecule, whose bond order is 1. 

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