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Dielectric Gases 359

the dielectric behavior of the gas by their effect(s) on the Processes 5a and 5b are the main reactions which de-
number density and energies of the free electrons present plete the electrons present in the dielectric, producing par-
in the electrically stressed gas. Both the numbers and the ent 5a and fragment 5b negative ions. In this way the elec-
energies of the free electrons depend on the gas itself and trons are prevented from causing ionization of the gas.
the density-reduced electric field E/N (E is the applied To this end, besides the electron-attachment cross sec-
electric field and N the gas number density). Let us briefly tion or the electron-attachment rate constant as a func-
look at the processes in Table I and their expected effect(s) tion of electron energy, the binding energy of the attached
on the dielectric properties of the gas. For convenience, electron (otherwise known as the electron affinity) must
we distinguish three groups of interactions: (1) electron– be large to prevent electron detachment (i.e., release of
molecule, (2) photon–molecule, and (3) “secondary.” In the attached electron). Process 5 has been studied under
∗
Table I, AB represents an unexcited and AB an excited “isolated conditions” (i.e., very low pressures) and un-
diatomic or polyatomic molecule, and the double arrows der “multiple collision conditions” (i.e., high pressures) in
indicate that the reaction can produce a multiplicity of which the effect of the medium can often be significant. It
products. is a resonant process occurring in the energy range from 0
to ∼20 eV, depending on AB. It crucially affects the break-
down voltage and other properties of the gas dielectric be-
1. Electron–Molecule Interactions
cause of its dominant role on the number density of the
Processes 1 and 2 in Table I are direct elastic and in- electrons. Gases with large electron-attachment cross sec-
elastic electron scattering, respectively. Along with pro- tions are called electronegative or electron-attaching; the
cess 5c (indirect elastic and inelastic electron scattering, cross sections for the electron-attachment resonances de-
whereby the colliding electron is temporarily captured by crease with increasing energy, and hence low-energy free
the molecule and then released), they crucially determine electrons can be more efficiently removed from the dielec-
the energies of the free electrons present in the stressed tric (via this process) than can higher energy electrons. Not
gas. Their cross sections depend on the electron energy all gases are electronegative, however, but the good gas di-
itself and the details of the molecular (atomic) electronic electrics are, or else contain electronegative additives.
structure. The inelastic electron scattering processes in- The last process in group A in Table I, process 6, occurs
volve excitation of rotational, vibrational, and electronic athigherenergiesthan5,andalthoughitproducesnegative
states, while the elastic scattering does not change the ions, it does not deplete electrons, it slows them down.
internal energy of the molecule (atom). Although direct In the energy range of interest, its cross section is not
electron scattering is nonresonant (i.e., it occurs over a appreciable, and its effect on the gas dielectric properties
wide range of electron energies), indirect electron scatter- is thought not to be significant.
ing is resonant; it usually is very efficient at low ( 20 eV)
energies and rather significant in establishing the “elec-
2. Photon–Molecule Interactions
tron slowing-down” properties of the dielectric gas. Obvi-
ously, polyatomic molecules are, as a rule, more efficient In the second group of reactions are those between di-
in slowing down the electrons than are small molecules or electric gas molecules and photons, produced by deexci-
∗
atoms. tation of excited species at high E/N (such as AB and
∗
Processes 3a and 3b represent the dissociation of B in Table I and more likely, excited species produced
molecules by electrons. They can proceed via a multi- by recombination processes). Here, three types of pro-
plicity of channels and thus produce a variety of neutral cesses can increase the number of free electrons in the gas
∗
fragments, some of which, such as B in 3b, may be ex- dielectric: photoionization of AB (processes 8a and 8b),
cited and posses sufficient internal energy to ionize an photoionization of the negative ions (photodetachment,
impurity species C present in the dielectric (process 11b) process 10), and Penning ionization (process 11a or 11b,
∗
and in this way to increase electron production (B can due to excited species produced via 7 or 3b, respectively).
also eject electrons when it collides with a surface). Pro- Of course, photons can collide with the electrodes and
cesses 3a and 3b slow down the electrons present in the inject new electrons into the gas dielectric in this way.
gas, as do processes 1, 2, and 5c, but in addition they pro-
duce free radicals which can change the number density
3. “Secondary” Interactions
and the composition of the dielectric gas.
Processes4aand4baretheprincipalwaysbywhichnew The number density of electrons and ions, and thus the
electrons are generated by electron–molecule collisions associated space charge effects in nonuniform fields, can
(and by which existing ones are slowed down); through be further affected by what can be termed secondary re-
process 4b a multiplicity of positive ions can be produced. actions. These can deplete electrons and positive ions