How do electrons in glow in the dark compounds gain and release energy?

[pzona]

I have a basic understanding of how the "glow in the dark" idea works. From what I can tell, electrons gain energy from light, "charge" up and move to a higher energy level, and when they no longer receive energy from the light (are in the dark), the electrons fall back toward a lower level and release photons. I know there are other glow in the dark compounds involving radioactivity, but my question is about the type involving energized electrons.

When the lower level electrons are energized by the light, doesn't that leave the lower energy levels empty? And if so, doesn't this violate Hund's rule? I considered the idea that maybe the original high energy electrons dropped down to fill the lower levels, but if this were the case, what would make glow in the dark compounds different from anything else? And why wouldn't they glow in normal conditions? Or maybe they do glow, and it's harder to observe because the environment is already light? If anyone could satisfy my curiosity, I'd appreciate it.

 

[ggucinski]

You have a good grasp of the photoelectric effect: electrons absorb energy from light and move up to a higher "excited" state, leaving lower energy levels unoccupied. The process of dropping back down to the ground state releases energy in the form of a photon. Hund's rule, however, only applies to the ground state.

The glow-in-the-dark phenomena, phosphorescence, can be explained using quantum mechanics. Generally, absorption of a photon results in an excited electron with the same spin quantum number, yet in rare cases the spin is inverted (i.e. from -1/2 to +1/2). The transition from this state back to the ground state is significantly slower, hence we observe the emission of photons for a time after the initial charging. You can see why keeping glow sticks in the freezer can extend their lifetime.

To indulge your curiosity even further, the wikipedia article on phosphorescence is pretty good: http://en.wikipedia.org/wiki/Phosphorescence"

 

[quicknote]

I can confirm that your understanding of how glow in the dark compounds work is correct. The phenomenon is known as phosphorescence and it involves the absorption of energy by electrons in a material, causing them to move to a higher energy level. When these electrons fall back to their original energy level, they release the excess energy in the form of light, giving the material its characteristic glow.

To address your question about the lower energy levels being left empty, it is important to note that electrons are constantly moving and shifting between energy levels in an atom. When they are energized by light, they simply move to a higher energy level temporarily and then return to their original energy level once the light source is removed. This does not violate Hund's rule, which states that electrons will occupy different orbitals within a subshell before pairing up in the same orbital.

As for why glow in the dark compounds only exhibit this phenomenon when energized by light, it is because the energy from light is needed to excite the electrons in the first place. Without this energy, the electrons would remain in their original energy level and there would be no excess energy to release as light. Additionally, the environment being already light may make it difficult to observe the glow, but it does not affect the phenomenon itself.

I hope this answers your questions and satisfies your curiosity about glow in the dark compounds. If you have any further inquiries, please do not hesitate to ask. As scientists, we are always happy to share our knowledge and insights with others.