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The active material in a helium-neon laser is Neon. Thus the laser frequency matches energy levels in the Neon atom. This also means the Neon atoms will absorb this frequency well. I imaging that this effective absorption may lead to a noticeable trace when the laser is shone through a tube filled with Neon.

Did anyone ever made this experiment (it’s easy to do when you are working in appropriately equipped lab)? Or does anybody know how to perform the necessary calculation.

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  • $\begingroup$ @Akerai when you look at pictures, the laser tube shines quite brightly but the neon atoms are excited by collisions with helium atoms, not by absorption. In addition only a fraction of the radiation is allowed to escape. All in all I cannot arrive at some intuition about the fraction of Neon atoms that will be excited when they are illuminated by the beam outside of the laser cavity. $\endgroup$ Commented Jun 14, 2020 at 13:34
  • $\begingroup$ The lasing level that you see is between two excited states of Ne. They will not be populated by shining a HeNe laser through neon. So, no. $\endgroup$
    – Jon Custer
    Commented Jun 14, 2020 at 15:57
  • $\begingroup$ @JonCuster yes, I see, I think you are right. Why don’t you make it an answer so I can give you the credit. $\endgroup$ Commented Jun 14, 2020 at 16:03

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The visible 632nm output of a HeNe laser is between the 5S and 3P levels in Ne. These are both excited states, and are not normally populated in the gas without pumping (electrical discharge in a lamp, transfer from He in a HeNe). So, no, you will not absorb and re-emit visible 632nm radiation by just shining a HeNe laser beam through neon gas. (In general, recall that 2-level systems are not amenable to making lasers out of - you usually need a 3-level system or better to get the population inversion).

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  • $\begingroup$ Great. Now obviously, there is a follow up question: is there any, preferably gaseous, material with a an energy level corresponding to 632nm from the ground state? But I’ll leave if with that. Thanks! $\endgroup$ Commented Jun 14, 2020 at 16:18
  • $\begingroup$ @HartmutBraun what is your goal exactly? nist is your friend If your goal is just to do a excitation/fluorescence experiment then just buy a cheap bluray (405nm) laser and shine it into things to your content. Works exceptionally well with tonic water. $\endgroup$ Commented Jun 14, 2020 at 16:42
  • $\begingroup$ @JoséAndrade I didn’t think of any practical application. But I reckoned it could be quite a spectacular view, just for fun or physics education. I saw a video by physics girl on YT shining a laser into jelly. Looked already nice but maybe it could be substantially improved. $\endgroup$ Commented Jun 14, 2020 at 16:48

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