Why does phosphorescence occur?

Ever wondered why the little stars glued on our rooms’ ceilings go on glowing after we’ve switched off the light? What about funny shirts or glasses which are visible at night despite darkness? Why does all this stuff happen?

Well the phenomenon which is behind this cool events is called phosphorescence and we’are about to get a little bit of insight on it!


To better understand what goes on behind the scenes when this phenomenon occurs we have to ask ourselves a simple but fundamental question. What does it happen when a material is exposed to light? Which, in a more basic form, could be reasked in the following way:  what does  happen when a molecule is exposed to light?

Nice one!

fosfo1Without getting into a too detailed analysis of the events we could simply answer the question in this way: the considered molecule absorbs the incident light. Or better, considering that a light beam consists in “little energetic packages” called photons, we should say that when a beam of photons bombs a material’s surface, the inner molecules absorb light in the form of “energetic particles”. The primary consequence of this absorption is that the molecules which were hit increase their internal energy. This energy, however, cannot be kept forever by the molecular system. In general it is quite immediately released by the molecule. This phenomenon is extremely fast and we could never appreciate it to the naked eye!

But let’s see a little bit more in detail what happens inside the molecule right after a photon absorption. There is a huge amount of extremely complex phenomena which are triggered by the absorption of light; all of them can only be explained using quantum mechanics.

fosfo3Nevertheless it is still possible to have an idea of what’s going on in the following way. We first have to accept that each molecule has only well defined accessible energy levels, which means that everything hitting the system won’t automatically be absorbed. We can imagine the reachable energy levels of a molecular system as a building’s several floors. We have also to imagine that these floors are connected one to the other by an internal lift, which lets us reach them from the bottom to the top, and that, in the meantime, we can only use the stairs to go down. That’s it? Absolutely not! There’s another complication. While going down we cannot necessarily access to each floors, as if we had a direct access from the fourth floor to the first one but in order to pass from the third to the second we found a closed keyless door. Forbidden transition there!

Our molecule can be compared to a young man living on the ground floor of this imaginary building and our absorbed photon as a sort of nutritional supplement giving the weak young man some energy to stand up and climb the building to higher floors!

Ok.. So, what does happen after a molecule has absorbed (the right amount of) energy?

Our young man can now stand up and, completely revitalized, takes the lift till the floor allowed by the amount of acquired energy. That’s exactly (more or less!) what happens to a molecular system. It absorbs a photon whose energy excites the molecule to defined level. And what about the energy release?

Our young man has to descend back to the ground floor in order to lose all he has acquired. That’s not easy at all because there is the probability for him to find a forbidden path from a floor to an other. A closed door. What then? Theoretically he should stop and stay there, hopeless. Practically he could, for instance, force the door and access the forbidden transition. Obviously it would not be so fast at all. He would need time to open a passage and finally crash the door. Probably plenty of time. But finally he would succeed and he’d be able to go back to where he began. The ground floor!

After this awesome little story we are able to answer the first real question. Ever wondered why the little stars glued on our rooms’ ceilings go on glowing after we’ve switched off the light?

fosfo2Here’s the answer: when we turn the light on, the stars begin absorbing energy. Or better the molecules inside the material start absorbing photons and get excited to a well defined molecular energy level. Immediately after they try to release this energy but it may happen that the system, attempting to go back to the ground floor, finds itself stuck at a particular energy level. Quantum mechanics reads that, in theory, there are some forbidden transitions. No way to pass! In practice, however, the molecule succeeds in forcing its passage to a lower molecular energetic level. Generally, this operation requires plenty of time, which means that our little star on the ceiling goes on glowing for minutes or hours after its first absorption.

When we switch off the room’s light we will be able to clearly see the phosphorescent star which is slowly releasing the absorbed energy  in the form of light! Phosphorescence! That’s it!

Cool isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti
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