He-Ne Laser -- Basic Questions

  • #1
Hello,

It is said, that the Helium part of the He-Ne-Laser is excited by gas discharge, using an external E-Field.
First question: How should that work? gas discharge means ionisation, and ionisation is not exactly the same as excitement.

Second:
Why is it not possible, to directly excite the Neon states? So why does "gas discharge" work with Helium, but not with Neon?
 

Answers and Replies

  • #2
blue_leaf77
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How should that work? gas discharge means ionisation, and ionisation is not exactly the same as excitement.
The excitation of He is facilitated by electron impact.
Why is it not possible, to directly excite the Neon states? So why does "gas discharge" work with Helium, but not with Neon?
It's possible and does happen during the lasing process. However, with the addition of He, this atom can be excited by electron impact to 2S states (21S and 23S), which are metastable, i.e. they have long lifetime. Efficient resonant energy transfer can then be mitigated between 2S states of He and 5s and 4s of Ne. The point is, exciting He leads to more efficient pumping than exciting Ne because 5s and 4s states of Ne have shorter lifetime than the 2S states of He.
 
  • #3
What does "efficient pumping" mean?
 
  • #4
blue_leaf77
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What does "efficient pumping" mean?
Population inversion can be achieved with lower pump rate.
 
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  • #5
Oh yes, of course. Thank you.
 
  • #6
blue_leaf77
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I'm not quite familiar with the process of electrical discharge, but many sources like https://en.wikipedia.org/wiki/Electric_discharge_in_gases suggest that the initial electrons may be formed when radiation from environment like cosmic rays hit the atoms in the gas, ionizing it to produce electrons. These electrons are then accelerated by the voltage difference between the cathode and anode such that they may trigger other ionization events when hitting the atoms. So in this sense, there do exist some portion of the gas which is ionized.
 

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