Module 2 : Gaseous Dielectrics
Lecture 6 : Properties of vacuum, Related ionization Process

Photoionization and Interaction of Metastables with Molecules

 
  • The impact ionization process is possible only when the energy exchanged in collision is more than the energy required for ionization (eUI). The electrons having energy lower than the ionization energy may excite the gas molecules to higher energy states on collision. Under this condition, an electron is raised from a lower energy level to a higher one.
  • On recovering from the excited state of electron in 10-7 to 10-10 sec, a molecule radiates a quantum of energy of photon   (h).
  • This energy in turn may  ionize another molecule whose ionization potential energy is equal to or less than the photon energy.
  • This process is known as photoionization and may be represented as A+hA+ + e , where A represents a neutral atom or molecule in the gas and h the photon energy.
  • For the photoionization to occur,
                                                                                                
or the photon wavelength λ is,
                                                                                                 
as                                                                                           

where c0 is the velocity of light ( c0= 2.998 X 108 m/s), h is Planck's constant (h = 6.63 x 10-34 J.s) and v is the frequency of light.

  • Only a very strong radiation of light quantum (photon) having a short wave-length of less than 65 to 100 nm, can cause photoionization of a gas.
  • The basic requirement for photoionization to occur is that the quantum energy of electromagnetic radiation must be greater than the ionization energy of the gas.
  • If the photon energy is less than (eUI), it may still be absorbed by the molecule and raise it to a higher energy level. This process is known as photoexcitation.
  • For certain gas molecules, the lifetime in the excited state may extend to a few tens of milli seconds. This is known as 'Metastable state' and the molecules under this state are referred as 'Metastables'.
  • Metastables have a relatively higher potential energy and, therefore, ionize neutral particles on collision.
  • However, the photons released by this reaction have too Iow energy to cause ionization in pure gases, but they may release electrons on striking the cathode. This process is known as the cathode emission or secondary ionization also   - process as described by Townsend.
    Since the ionization caused by metastable interaction is accompanied with a time delay, it has been observed that these reactions are responsible for longer time-lags than usual in some gases.
 
1