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My book states that when very high voltages are present air can become ionized. Any "odd free" electron can be accelerated to sufficient kinetic energy to knock electrons out of O2 and N2 molecules. After this breakdown when electrons recombine with their molecules light is emitted (lighting). My question is which electrons recombine? The ones knocked out or does the electron that was accelerated by the potential difference take its place? Also, the book doesn't specify this but is light emitted because the electrons now occupy a lower energy state after recombining, therefore, emitting a photon?

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Electrical breakdown involves the ionization of molecules by electrons that are accelerated in the high electric field. Usually, an avalanche process occurs where electrons knock out additional electrons from neutral molecules and they again knock out more electrons from neutral molecules and so forth. Thus plasma of positively charged molecules and negatively charges electrons is created which is highly conductive. In this process, free electrons of this plasma or any other free electron, e.g. provided by a negative electrode, can recombine with ionized molecules producing a neutral molecule and emit the lost energy as light. An electron accelerated towards an ionized molecule is not necessary for that.

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  • $\begingroup$ Thank you. But just to clarify the electrons return to a lower energy state? Does light emission always involve electrons going to a lower energy state? $\endgroup$ – user177470 Mar 26 '18 at 22:44
  • $\begingroup$ @Idan - Yes, light emission in a gas discharge is always related to electron transitions from higher to lower energy states. This happens when electrons recombine with the ionized molecules. Light is emitted and the molecules become neutral in their lowest energy state. It can also happen when electrons still bound to the molecules are only excited by the impact of energetic electrons and then return to their lowest state via light emission. $\endgroup$ – freecharly Mar 26 '18 at 23:00

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