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I think a substance which emits light when it gets vaporized could have numerous applications, for example in display technology or in leakage detection.

The only problem is: I'm struggling to find a explanation how this should work.

Light emission from atoms stems from the electrons changing there discrete energy states inside the atom. On the other hand phase change is connected to inter-atom forces and thermo-dynamics. (This should apply to molecules as well).

Is there a phenomenon, which somehow connects these two mechanism, so that a phase change is always accompanied with photon emission? Or is there a material / compound / element with this property?

As photon emission is always also energy emission I'd suspect this is more likely when changing from solid to liquid or liquid to gaseous. Optical photons and (more-or-less) standard conditions would be preferable, but I'm mainly interested whether such thing exists or could exist at all.

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    $\begingroup$ This seems incompatible with the principle that phase transitions are reversible. The free energy difference is zero. $\endgroup$ – Bert Barrois Jun 26 '18 at 11:45
  • $\begingroup$ Probably a lot easier to accomplish the goals you mention through other means. $\endgroup$ – user93146 Jun 26 '18 at 13:10
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There is heat emitted or absorbed during phase changes. This is infrared rather than visible light. This is because phase changes involve intermolecular interactions with lower energies than the atomic interactions associated with electrons jumping between different levels in atoms. So there are photons involved in phase changes but they are just not visible.

Just because these photons are emitted does not mean that they are easily observed (see the comment by @AndersSandberg that links evidence of observation of these phase transition photons nevertheless). In a mixed phase medium the photons emitted when bonds are broken may be almost immediately absorbed leading to kinetic energy increases by the free phase.

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