I have read this question:
Where niels nielsen says:
Furthermore, it is those outermost electrons that are responsible for giving color to some substances and either transparency or opacity to others.
Now wiki says:
Similar to an electron in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon.
So one says that only valence electrons can emit visible light, the other one says core electrons do have the ability to absorb and emit photons. This is a contradiction (for visible light). Or it might be the case that visible light can only be emitted by valence electrons, and core electrons can only emit non visible light, but this needs a clarification.
Now I do understand, that core electrons (or at least a valence electron relaxing to the core shell) can determine some part of the emission spectrum. This is described by core electron spectroscopy.
A core electron can be removed from its core-level upon absorption of electromagnetic radiation. This will either excite the electron to an empty valence shell or cause it to be emitted as a photoelectron due to the photoelectric effect. The resulting atom will have an empty space in the core electron shell, often referred to as a core-hole. It is in a metastable state and will decay within 10−15 s, releasing the excess energy via X-ray fluorescence (as a characteristic X-ray) or by the Auger effect.
But these are much higher energy photons released by these core electrons (actually they relax from the valence shell to the core shell, so it is not even really core electron photon emission). But this does not talk about visible light.
What this does not describe is whether it is possible to excite a core electron, that is, can a core electron absorb a photon, move to a higher energy level as per QM, and then relax and emit a photon at all? And does this emission include visible light photons?
There could be, that core electrons can only absorb higher energy photons. OK, but they still could cascade down in multiple steps emitting visible. Why would the visible matching gap only be available for valence electrons? Why are they so special? They are special, since they are the outermost. But visible is lower energy, then what the core electrons can absorb. So why can core electrons only absorb higher energies, what disallows the lower gaps for them?
- Can core electrons emit visible light (is the visible part of the emission spectrum determined in part by core electrons)?