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"