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I rolled back the edit because I really am looking for the following:

I am looking for an element or compound that would have a bright green emission when electrically excited and is a gas at between 1 torr and 4 torr. If such a compound could be nontoxic, that would be beneficial.

The pressure range is important.

I'm familiar with the emissions spectra of all the noble gases, but none of them work for me. I want to find such a compound to use in a Geissler tube, which will be excited with about 50kVac at 300 to 500kHz. I am capable of pulling a vacuum down to 0.2 torr, and I normally work between 1 torr and 4 torr.

I know of a Xenon/nitrogen mix that produces the desired color, but it's too fussy for me to achieve, and Xenon is too expensive for this purpose.

Relatedly, I was wondering why the Xenon/nitrogen mixture emits a green light. Neither Xenon or Nitrogen has much green at the appropriate pressure, but somehow the combination, within a narrow range of Nitrogen partial pressure, produces a bright green. What causes this color when the two compounds are mixed together, and how can I predict the color of a given mixture of gases at a given pressure?

I know of the bohr model and balmer series etc, but I don't understand how to apply that to compounds or mixtures of compounds. Is there some other way to predict the spectra produced by mixtures of gases?

It's not easy gettin' green! (Apologies to Kermit)

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    $\begingroup$ Mother nature produces green northern lights using oxygen atoms and nitrogen molecules. I wonder if this has been ever done in a laboratory. $\endgroup$ – aventurin Jun 7 '16 at 18:23
  • $\begingroup$ I'm aware. One question would be whether the actual composition of the atmosphere is different at that altitude. The other is, when pumping oxygen through a vac pump using oil, "Do ya feel lucky punk?" $\endgroup$ – user103218 Jun 7 '16 at 21:24
  • $\begingroup$ It may be difficult to control a xenon/nitrogen mixture, but the reason why it is used may be because, all things considered, it is the best way to achieve the result you want. If there were cheaper or easier ways to do it, those who want a green glow would already use it. I'd be interested to see whether there are some cheap but poorly performing alternatives, though. $\endgroup$ – matt_black Jun 10 '16 at 9:27
  • $\begingroup$ That would assume that the solution space has been thoroughly explored, and that this information had been preserved. I don't believe either of those is really true. $\endgroup$ – user103218 Jun 11 '16 at 17:06
  • $\begingroup$ I did find this which explains the xenon/nitrogen/oxygen mix: quora.com/Is-xenon-the-perfect-element $\endgroup$ – user103218 Jun 15 '16 at 19:23
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While it is not a gas, copper and barium compounds come to mind. If you could introduce a volatile copper compound, you could get something remotely like a mercury vapor lamp, except with copper.

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  • $\begingroup$ Many commercial discharge lamps do something like this but they work at high pressures using metal halide compounds of, for example, rare-earth halides with narrow emission lines. $\endgroup$ – matt_black Jul 2 '16 at 13:52
  • $\begingroup$ Also, if it is possible to use copper electrodes, then it might be a very low-tech solution for introducing copper ions into the discharge. No idea about the pressures and gas mixtures required though. $\endgroup$ – uLoop Jul 2 '16 at 13:56
  • $\begingroup$ My pieces don't have electrodes as such. The excitation is AC coupled through the glass, so the temperature inside is not much elevated over ambient. $\endgroup$ – user103218 Jul 5 '16 at 12:37
  • $\begingroup$ Is there somewhere an index of the emission lines/bands for compounds? Seems like this should exist. If I can find out what compounds will give me the right emission bands, then I can sort for which will be in the gas state at the appropriate temperature and pressure. $\endgroup$ – user103218 Jul 5 '16 at 12:39
  • $\begingroup$ I am not aware of such an index, but I never looked for one so I would suggest googling. But I just had another idea: boron compounds also have green emission lines. You could try trimethylborate, although it could break down and form deposits. Diborane could be more stable, since it cannot form soot or boron oxides, but it is toxic and extremely flammable. $\endgroup$ – uLoop Jul 11 '16 at 19:49

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