I was reading about gas discharge tubes and it said that when a high voltage is applied between the cathode and anode, electrons get pulled off the gas atoms. My question is how does this happen simply because of a high voltage? I thought you needed electrons to collide with the gas atoms and knock off electrons, but I never heard about them being pulled off gas atoms by a high voltage. Can someone explain how this works?


up vote 1 down vote accepted

You're correct that it's not the high voltage itself which ionizes the atoms, but rather the free electrons accelerated by this electric field.

In any system at finite temperature, there is a non-zero probability for some atoms to be ionized at any given time. Applying a strong electric field causes the free electrons to accelerate. Collisions with bound atoms then result in electrons being ejected from their orbitals, which are accelerated by the field and go on to ionize yet more atoms. The resulting electron avalanche can lead to near-complete ionization of the entire gas discharge.

  • Hello thanks. Where do the free electrons come from? I'm just wondering because in the book I'm using it says that thermionic emission is a different way of creating an electron beam than discharge tubes, so I'm guessing the free electrons in the discharge tube aren't produced by "boiling off" electrons from the cathode. – Awesome Academist Jun 13 '15 at 19:06
  • The electrons are falling in the electric field from the cathode. – daniel Azuelos Jun 13 '15 at 19:48
  • @AwesomeAcademist: that's correct, thermionic emission is one way to produce free electrons but in a discharge tube the background ionization is provided by any one of a number of processes. Part of it is due simply to the collisions between atoms in the gas; this is where the finite temperature comes in, as at a given temperature collisions occur with a range of energies. Another cause of ionization is background radiation (from terrestrial or cosmic sources). The process can be assisted with the use of a powerful UV flash-lamp too, if desired. – tok3rat0r Jun 13 '15 at 20:54
  • (I should add that thermionic emission also plays a part in initiating breakdown in cold cathode discharge tube, but that this process is superseded by secondary electron emission from the cathode once a sufficient number of ions have been produced). – tok3rat0r Jun 13 '15 at 21:07

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