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.[4]


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?


  1. Can core electrons emit visible light (is the visible part of the emission spectrum determined in part by core electrons)?
  • $\begingroup$ Well, the least tightly bound core electron is hydrogen. Look at the difference between a 1s and 2s energy level in hydrogen. It isn't in the visible. Or do you mean core electrons in a more general 'anything not the outermost electron' sense? $\endgroup$
    – Jon Custer
    Commented Oct 24, 2019 at 21:15
  • 2
    $\begingroup$ So this one says that core electrons do have the ability to absorb and emit photons. This is a contradiction. Where's the contradiction? Are X-rays not made up of photons too? $\endgroup$
    – Gert
    Commented Oct 24, 2019 at 21:41
  • $\begingroup$ @Gert niels nielsen says valence only. That one says cores too. But I will make it more clear. $\endgroup$ Commented Oct 24, 2019 at 21:43
  • $\begingroup$ @JonCuster I mean in a more general way, anything but the outermost. $\endgroup$ Commented Oct 24, 2019 at 21:47
  • 1
    $\begingroup$ Hmm... to me it's clear. Mess with valence electrons and emission VIS-photons result. Mess with core electrons and you get X-rays or Auger-effect. $\endgroup$
    – Gert
    Commented Oct 24, 2019 at 21:48

2 Answers 2


My2cts is right. the essential feature here is the idea that the wavelength of the light emitted when an electron orbit decays from an excited state depends on the energy difference between the two states. if this energy difference is of order ~a few eV (as for the valence electron levels and the unoccupied level just above them) then the emitted photon is in the wavelength range for infrared, visible, or UV light. If it is of order ~tens of keV (as it would be for a valence electron jumping down to occupy an empty level deep in the core of a metal atom like iron or copper) then the photon is in the x-ray range.


There is no contradiction. Core electrons can absorb high energy photons but not visible light. A core electron can only emit light if an electron missing from an even deeper orbital. This light will also be high energy light.

  • $\begingroup$ This might be the answer, but can you please elaborate? There has to be an explanation, in the electron configuration, or in the QM allowed energy gaps for the valence and for the core electrons, and then for core electrons the available gaps are not in the visible wavelength spectrum, but why? There must be a reason why only the valence electron can move to a higher energy level that matches the visible spectrum? $\endgroup$ Commented Oct 24, 2019 at 21:53
  • $\begingroup$ I could imagine that the valence electron has the most energy available for gaps, but then why are core electrons absorbing and emitting higher energy but not smaller? $\endgroup$ Commented Oct 24, 2019 at 21:56
  • $\begingroup$ And based on your description, which might be right, core electrons could still cascade down in multiple steps emitting visible? $\endgroup$ Commented Oct 24, 2019 at 21:57
  • $\begingroup$ Visible light may well be emitted in some process of repopulatimg an empty core orbital. It would be a stretch of words to characterise this as a core electron emitting visible light. $\endgroup$
    – my2cts
    Commented Oct 24, 2019 at 22:17

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