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I am aware that low pressure/density gases produce an emission spectrum as there are specific energy transitions that the electrons can make, emitting certain frequencies of EM waves.

However high pressure gases produce a continuous blackbody body spectrum. I am not sure how it is able to emit the entire range of the EM spectrum if only certain transitions are possible. Could someone please explain this?

Further confusing me, I have read that certain high pressure gases still produce an emission spectrum rather than a continuous spectrum. For example, this is observed for hydrogen gas. Is this true and if so could someone also explain this?

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It has been explained to me that as the gas molecules become closer together, due to the Pauli Exclusion Principle, the energy levels slightly change in value, allowing for a full range of electron transitions. However correct if I am wrong, but doesn't the PEP basically states that you can't have more than 2 electrons in an orbital. If that is correct, then how does moving two molecules together violate the PEP.

From a chemistry point of view, the changing of energy levels is due to atomic orbitals forming molecular orbitals which form a continuum as the number of atoms reach millions. This makes much more sense to me. How does this explanation fit in with the explanation using Pauli Exclusion Principle?

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High pressure gases do not produce a true continuum, and they do not become closer to the blackbody case with increasing pressure.

The individual emission lines only become broader with pressure and make the emission spectrum appear continuous because the lines start to overlap.

The number of molecular transitions does not change with pressure. There is a line shift and line width change as well as change in shape of the line with increasing pressure.

For very high densities (and also pressure) other absorption effects are also observed such as collision induced absorption or absorption from dimers. I think that there probably are also dual effects generating emission.

However, the latter effect is really caused by a new species which introduces new transitions. The same applies to a plasma as compared to a gas consisting of molecules. Plasmas are also much more blackbody like because there are free electrons which create a continuum of transitions (which is necessary for true blackbody radiation).

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  • $\begingroup$ Thanks for the answer. So high pressure gases produce a continuous spectrum, but don't produce a blackbody curve, right? Also are you saying when atoms collide with each other, they lose some energy as EM radiation of varying frequency? $\endgroup$
    – Nanoputian
    Commented May 17, 2016 at 11:20
  • $\begingroup$ Yes (at least if the gas is not a plasma) and Yes. $\endgroup$
    – Andreas H.
    Commented May 18, 2016 at 18:54
  • $\begingroup$ I don't really understand how the Pauli Exclusion Principle causes the energy levels to change in energy because doesn't it just say that only 2 electrons can fit in each orbital? From a chemistry point of view, the changing of energy levels is due to atomic orbitals forming molecular orbitals which form a continuum as the number of atoms reach millions. How does this explanation fit in with the explanation using PEP? $\endgroup$
    – Nanoputian
    Commented May 30, 2016 at 7:01

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