For special glasses, crystals, or gases absorb energy from an electrical current or another laser they become "excited." The excited electrons move from a lower-energy orbit to a higher-energy orbit around the atom's nucleus. When they return to their normal or "ground" state, the electrons emit photons.

I think the excited electron is always the valence electron. However In metals the valence electrons are not contained to orbitals, they exist in a sea of electrons that flow freely through the object. Does this mean that valence electrons in metals can not be excited or can not move up orbitals? or perhaps the next electron in line moves up an orbital. If so does it join the sea of electrons? (I'm just throwing out ideas here).

Maybe I'm misunderstanding what it means to be 'excited' because, as it says, energy from current can excite electrons.

Any insight is appreciated.

  • 3
    $\begingroup$ In a solid electrons are in Bloch wave functions. This leads to the band structure of the material. There are unoccupied states above the Fermi energy. Electrons can be excited from deeper levels into these levels. Or, it can leave the solid as, e.g., a photoelectron. $\endgroup$
    – Jon Custer
    Commented Nov 13, 2017 at 15:12

2 Answers 2


You seem to be misunderstanding what is a "sea of electrons". In fact, this is a metaphor upon a metaphor upon an abstraction. There is no sea. There is a huge bunch of orbitals. (Sure, the solid state people prefer to call them "states", but that's not really important.) The whole piece of metal is a giant molecule. It is not all that different from ordinary small molecules, except that it is very big, and many orbitals span the entire molecule (but then again, that's what they often do in normal molecules).

All these orbitals tend to have different energies. They are everywhere on the energy scale, very close to each other. You point your finger at any given energy, and you find an orbital with that energy. We can't really tell them apart. They kinda blend into a continuous spectrum. And that's what we metaphorically call the sea of electrons.

Electrons are not free, they are confined to some states. When a photon hits, any electron can get excited all right. It will move up to one of the empty states. In fact, it has an immense freedom compared to an electron in a small molecule. Instead of a few discrete states, it has an entire sea of them above. It can get excited to any of them, which in particular means that metal as a whole can absorb a photon of any energy. This, BTW, is also an explanation of metallic luster.

The excited electron is not always the valence electron (you may fire an X-ray photon at a substance and excite an electron from core orbitals deep down below), but that's irrelevant.

  • $\begingroup$ I think you are wrong. Only electrons near the Fermi level can get excited in a metal. Most of the states well below the Fermi level are all occupied, and due to the Pauli's exclusion principle, there cannot be more electrons with such energies. This means an electron with low energy won't be able to absorb a photon with low energy, because the resulting electron would have the same energy than an already present electron. $\endgroup$ Commented Nov 13, 2017 at 19:31
  • $\begingroup$ OK, I fixed that. $\endgroup$ Commented Nov 13, 2017 at 19:43
  • $\begingroup$ I still see the sentence "When a photon hits, any electron can get excited all right.". $\endgroup$ Commented Nov 13, 2017 at 20:01
  • $\begingroup$ Yes, and that's true (provided that photon has enough energy). $\endgroup$ Commented Nov 13, 2017 at 20:14
  • $\begingroup$ But what happens when they return to their ground state? Or do they not drop back down? $\endgroup$
    – user54812
    Commented Nov 13, 2017 at 21:36

Yes, electrons in a metal can also reach an excited state, and leave that state by emitting a photon. Metal spectra are well documented.

Doing this in a solid metal is different mainly because the density of electrons creates potential for additional collisions. Using laser ablation, we can focus energy into a very small space for a very small time. Using electricity can work for a gaseous metal (studied since early 1900's.)

The coolest way to do this is from X-ray bombardment and this technique is used to detect lead in children's toys.

With a laser, as the energy is increased, or so the length of ablation, the abundance of secondary collisions cascade through the metal and plasma is created.

Essentially, while applying electricity or radiation will excite electrons in solid metal, the effect you will notice more may be the conductivity, the heat, the incandescence, or the plasma, depending on your technique.

Penner-Hahn, James E. (2013). "Chapter 2. Technologies for Detecting Metals in Single Cells. Section 4, Intrinsic X-Ray Fluorescence". In Banci, Lucia. Metallomics and the Cell. Metal Ions in Life Sciences. 12. Springer.

Amponsah-Manager, K.; Omenetto, N.; Smith, B. W.; Gornushkin, I. B.; Winefordner, J. D. (2005). "Microchip laser ablation of metals: Investigation of the ablation process in view of its application to laser-induced breakdown spectroscopy". Journal of Analytical Atomic Spectrometry. 20 (6): 544


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