Types of Ionization

Types of Ionization

Q. Explain the terms Ionization  and hence explain different ionization processes with reference to breakdown in gases ?

1. Ionization by collision
2. Photo ionization
3. Secondary Ionization process
4. Electron decay and attachment process
   

1. Ionization by Collision :

• The process of liberating an electron from a gas molecule with the simultaneous production of a positive ion is called ionization.
• In this process, collision of a free electron takes place with a neutral gas molecule hence a new electron and positive ion is formed. Consider Figure an electric field E is applied across a low pressure gas column between plane parallel electrodes. The electron travel is from cathode to anode. During its travel, from cathode, it accelerates due to successive collisions with other gas molecules till it reaches anode. The electron is accelerating during the travel i.e. it gains energy, if this gained energy (∑) exceeds the ionization potential V_i’ which is the energy required to dislodge an electron from its atomic shell, then ionization takes place. This process is represented by following equation,

Where , A is the atom, 

A⁺is positive ion and 

e^– is the electron

• A few of the electrons produced at the cathode by some external energy e.g. ultravioletlight falling on the cathode causes ionization of neutral gas particles producing positive ions and additional electrons. The additional electrons, then, themselves make collisions causing ionization and thus the process continues. Thus there is an increase in the electron current. The number of electrons reaching the anode per unit time is greater than those liberated at cathode. In addition, the positive ions also reach the cathode causing bombardment on the cathode. This gives rise to secondary electrons.

2. Photo Ionization

• This involves interaction of radiation with matter. Photo-ionization process takes place when the amount of radiation energy absorbed by an atom or molecule is more than the ionization potential of that molecule. 
• The radiation is absorbed by atoms or molecules by processes like :

(a) Excitation of atoms to a higher energy state.

(b) Dissociation of diatomic molecule or direct ionization etc.

• An excited atom emits radiation when the electron returns to the lower state, and when an atom absorbs radiation the reverse process takes place. This can be described by

hV + A ⇌ A^.

Ionisation occurs when λ  ≤ C. h/V_i

Where,

h is the Planck’s constant

C is the velocity of light 1.

λ is the wavelength of the incident radiation

V_i is ionization energy of the atom Substituting for h and C, we get

λ ≤ [1.27/V_i]
x 10^-6 cm

Where V_i is in electron volts (eV)

• Greater the ionization energy, the lesser will be the wavelength of the radiation that causes ionization . It is experimentally proved that a radiation with a wavelength of 1250 A is sufficient to cause photo-ionization of almost all gases.

3. Secondary Ionization Process

 The secondary Electrons are produced by secondary ionization. These initiate a sustained discharge . The sustained discharge is due to ionization by collision and photo ionization

Electron Emission Due to Positive Ion Impact

Ionization by collision of photo-ionization causes formation of positive ions. These travel towards the cathode. When a positive ion approaches a metallic cathode it causes emission of electrons from the cathode. In this process the positive ice gives away its kinetic energy. If the ionization energy is greater than twice the work function of the metal, then one electron will be ejected and a second electron will The probability of this process is measured as y_i which is called the Townsend’s secondary ionization co-efficient due to positive ions.

y_i is defined as the net yield of electrons per incident positive ions.

y_i increases with ion velocity and depends on the type of gas and electrode material neutralize the ion used

Electron Emission Due to Photons

If electron is to be dislodged from the outer shell, it should be given enough energy so that it overcomes the surface potential barrier. The external energy can be supplied in the form of a photon of UV light of suitable frequency.

The electron emission from a metal surface takes place at a critical condition given by

h . v ≥ Φ

where Φ is the work function of metallic electrode. The frequency (v) is given by the relationship

v = Φ/h

is known as threshold frequency.

For a clean nickel surface with  = 4.5 eV, the threshold frequency will be that correspond to a wavelength λ = 2755 A.

Since Φ is typically a few electrons volts, the threshold frequency lies in the far ultraviolet region of the electromagnetic radiation spectrum.

Electron Emission Due to Metastable and Neutral Atoms

A metastable atom or molecule is an excited particle. The lifetime of such metastable atom is very large (10^-3 s) compared to the lifetime of an ordinary particle (10^-8 s). The electrons can be made to eject from the metal surface by means of an impact of an excited (meta-stable) atom. The total energy should be sufficient to overcome the work function. The process described is commonly observed metastable atoms since the life time of other excited states is very short so there cannot reach the cathode surface and thus cannot cause electron emission. The emission may take place in such cases only if these are very near to cathode. Neutral atoms in the ground state can also cause secondary electron emission. The kinetic energy required however should be high.

4. Electron Attachment Process

• Some gases are electronegative in nature. They have affinity for electrons. Attachment collision is a process in which electrons may become attached to atoms or molecules to form negative ions. Electron attachment process depends on the energy of the electron and the nature of the gas. All electrically insulated gases, such O₂, CO₂,
  CI₂, F₂, C₂F₆, C₃F₃,
  C₄F₁₀, CCI₂F₂ and SF₆ exhibit this property. An electron attachment process can be represented as : +
• Atom + e^– + k  → negative atomic ion + [Ea – K]

• The energy liberated in this process is in the form of the electron affinity (Ea) plus K.E. (K).The outermost orbit of attaching sues or insulating gases has vacancies. Thus these are supposed to process affinity for electrons. The free electrons present in the ionized gas are absorbed due to the vacancies.
• This process takes place in the circuit breakers. Due to separation of circuit breaker contacts, the space between circuit breaker contacts is ionized and the dielectric strength between these contacts should be rapidly built up using suitable medium like insulating gas. If it is done, the are does not restrike. Thus are interruption process involves electron attachment process This process is of due importance in switchgears.