- 1.Excitation processes
Electron impac! ionization (Fig. 3): e- + A → e- + e- + A+ Electrons with sufficient energy can remove an electron from an atom and produce one extra electron and an ion. This extra electron can again be accelerated to gain enough energy and ionize another atom. This multiplication process leads to a continuous generation of ionized species and the plasma is sustained. The ionization processes generally have the highest energy barriers, on the order of 10 eV.
Electron impact excitation (Fig. 4): e- + A → e- + A* Electrons with sufficient energy can also excite the electrons of an atom from the lower energy level to a higher energy level. This process produces an excited neutral species whose chemical reactivity towards the surface could be quite different from the ground state atoms. The threshold energy needed to produce excited species can vary greatly, depending on the molecule and the type of excitation.
Some excited atoms have very long lifetimes (~ 1–10 msec) because the selection rules forbid its relaxation to the ground state. These excited atoms are thus called metastables. All noble gases have metastable states.
Electron impact dissociation of diatomic molecules (A2) (Fig. 5): e- + A2 → e- + A + A Electrons with sufficient energy can also break the chemical bonds of a molecule and produce atomic species. These atomic species could gain enough energy and be at a higher energy level than the ground state atoms. Dissociative processes usually have lower threshold energies than ionization processes. Dissociative threshold energies vary from 0 to above 10 eV, depending upon the strength of the bond that is broken and the mechanism by which the process occurs.
This process is mostly responsible for the production of chemically active radicals in most of the plasmas.
Electron metastable ionization (Fig. 6): e- + A* → e- + e- + A+ Electrons with sufficient energy can also remove an electron from a metastable atom and produce one extra electron and an ion. Since the metastable atom is already excited, less energy is required here to ionize the atom.
Metastable-neutral ionization (Fig. 7): A* + B → A + e- + B+ Metastable atom can collide with a neutral and ionize it if the ionization energy of the neutral (B) is less than the excitation energy of the metastable (A*). This is also called the Penning Ionization process.
KeywordsMetastable Atom Extra Electron Electron Impact Excitation Ground State Atom Rutherford Back Scattering
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