I Why do only low-pressure gases emit a line spectrum?

[Jason Ko]

I'm recently studying Bohr's model. My textbook claims that only low-pressure gases emit a line spectrum while solids, liquids and high-pressure gases emit light with a continuous range but why?

 

[mfb]

Nearby atoms influence the energy levels in atoms, lowering or raising them a bit. In a low pressure gas this can be negligible, in solids and liquids it's very important.

 

[Jason Ko]

Nearby atoms influence the energy levels in atoms, lowering or raising them a bit. In a low pressure gas this can be negligible, in solids and liquids it's very important.

How exactly does the change of energy level result in a continuous spectrum? Could you please explain it in detail?

 

[Vanadium 50]

Suppose one atom has an energy level ay E, Bring another one nearby and you have a level at E + δ and a level at E - δ where δ is a small number. Add more atoms and you get more levels, until it looks like a band, not a line.

 

[sophiecentaur]

only low-pressure gases emit a line spectrum

The Pauli Exclusion Principle says that no two atoms can have precisely the same quantum levels and we never look at just one atom. The precise energy level of a system will be affected by more than the 'simple' fields that the theory suggests for a single atom; nearby atoms affect each other and the more and the closer they are means a (albeit narrow) range of energy states exists and lines 'broaden'.
Even in a low pressure gas, the line spectrum could have a width. In a high pressure gas, the interaction is higher and at high temperatures, there can be doppler shift too. In 'condensed matter' you cannot get line spectra because there is a continuum of energy states and you get bands.

 

[Baluncore]

The Pauli Exclusion Principle says that no two atoms can have precisely the same quantum levels and we never look at just one atom.

And there I was, thinking that the Pauli Exclusion Principle dictated that no two electrons (generally fermions), within a quantum system (one atom), could share the same quantum state at the same time.

Photons emitted by a solid, or by a dense gas or plasma, do not tend to escape to be seen.

 

[sophiecentaur]

And there I was, thinking that the Pauli Exclusion Principle dictated that no two electrons (generally fermions), within a quantum system (one atom), could share the same quantum state at the same time.

The point is (imo) that there are more "quantum systems" than just single atoms. As soon as two atoms get within range of each other you have a quantum system. We were told about Pauli in the simple context of the Hydrogen atom and that system can be described by just four quantum numbers. Bring two or more atoms near each other and the total number of quantum numbers needed to describe the system 'fully' increases as the potential energy between them becomes significant. Molecular Hydrogen has a much more complicated spectral line structure.
When the pressure is high enough, the nearby molecules have a mutual effect and, still under the influence of Pauli the basic atomic energy states 'squeeze each other apart'. In solids, the interaction between atoms is much higher so the lines spread out into bands.
And, of course, Pauli accounts for electron degeneracy pressure in stars.

Photons emitted by a solid, or by a dense gas or plasma, do not tend to escape to be seen

Just cos you can't see it, doesn't mean it's not happening. Glass is a transparent solid and you can see photons released inside it.