Electronegative gas

A plasma potential that is positive with respect to electrode potentials is primarily a consequence of the greater mobility of electrons compared with positive ions. When there are many more negative ions than electrons in the plasma (e.g., in highly electronegative gases), plasma potentials are below electrode potentials, at least during part of the rf cycle (19b). 

Dissociative electron attachment collisions are important in electronegative gases, and the corresponding cross section has the following general shape ... 

We may suppose that it is essential to use a proper model for the metal. The adsorption of cesium on tungsten, a particularly simple case since the cesium ionizes completely on the metal surface, has been treated in this way most successfully by Langmuir (40) and de Boer (41). Catalysis requires a similar treatment for electronegative gases on metals, where the bond formed is largely homopolar, and not ionic as in the case of cesium. 

Actual losses of charged particles in electronegative gases are mostly due to ion-ion recombination, which involves neutralization of positive and negative ions in binary or three-body collisions. Ion-ion recombination can proceed by a variety of different mechanisms, which dominate at different pressure ranges, but all of them are usually characterized by very high rate coefficients. 

Numerically, the Child law sheath can be of the order of 100 Debye lengths in conditions of typical low-pressure discharges applied for surface treatment. More details regarding sheaths, including collisional sheaths, sheaths in electronegative gases, radioftequency plasma sheaths, and pulsed potential sheaths can be found, in particular, in the book of Lieberman and Lichtenberg (1994). 

If the moderate or high-pressure gas is not electronegative, the volume balance of electrons and positive ions can be reduced to the simple ionization-recombination balance. In electronegative gases, however, two qualitatively different self-sustained regimes can be achieved (at different effectiveness of electron detachment) one controlled by recombination and another controlled by electron attachment. 

In this case, actnal losses of charged particles are also due to electron-ion recombination, in the same way as non-electronegative gases. Such a situation can take place, in particular. 

The parameter 5- = k /k shows the detachment ability that compensates for electron losses dne to attachment. If 5- 1, the attachment inflnence is negligible and kinetic equation (4-21) becomes eqnivalent to one for non-electronegative gases. The kinetic equation inclndes the effective rate coefficients of ionization, kf = kj + g, and recombination, k f = kf + gk. Eqnation (4-21) describes electron density evolution to the steady-state magnitnde of the recombination-controlled regime ... 

Electrical discharges in O2 and other electronegative gases have not received much attention, apart from the identification of the ionic... 

Stoflfels E, Stoffels WW, Tachibana K. Electron attachment mass spectrometry as a diagnostics for electronegative gases and plasmas. Rev Sci Instrum. 1998 69 116-22. 

Examples of the various types of observed uniform field behavior of the breakdown voltage (Vs)mix of binary gas mixtures with respect to those (Vs)a,b of the individual components A, B as a function of gas composition are shown in Fig. 4. Figure 4a shows the behavior of (Vs)mix for binary mixtures of electronegative gases whose A a(( )) is independent of gas number density N. The (Vs)mix is nearly equal to the sum of the partial-pressure-weighted... 

The statistical time lag 4 can vary from nanoseconds to milliseconds (or longer) depending on the time the initiatory electron becomes available when V > Vs. Initiatory electrons can be produced by cosmic radiation, natural radioactivity from materials, field emission from the cathode surface, or collisional detachment from negative ions in the case of electronegative gases. In experimental apparatus they can be produced intentionally by, for instance, an ultraviolet source, in which case ts is reduced considerably. 

In experiments aiming at a search for emitters, which surface-ionize organic compounds more efficiently [20] (based on the understanding that the work func tion of the surface is greatly increased by the chemisorption of electronegative gases [21]), a considerable increase in alkali metal ion currents is always observed, when chlorine (or fluorine) is introduced into a mass spectrometer with a thermal ionization source. This phenomenon is well interpreted as the so-called stimulated surface ionization [22] in which the exothermic reaction on the surface is responsible for the ionization. This is another interesting example of RSI. 

Flashover voltage can be increased by raising the gas pressure, by utilizing electronegative gases like SFe with a high electric strength, and by immersion in... 

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