After lights-out tonight, my daughter was admiring the “glow in the dark” stars on her bedroom walls and asked me how they worked. I told her that certain chemicals in the stars had been absorbing some of the light in her bedroom and were now giving it back out. She was happy with this explanation, but I sure wasn’t.
Am I right in viewing phosphorescence as analogous to emission by an excited electron as it falls back to a lower energy state within an atom? If so, what in phosphorescence corresponds to the excited state? Where and how is the incoming light energy stored until it is re-emitted as ‘day-glo’?
Well, here we go.
To understand it requires some background info on fluorensence–which basically occurse when light is absorbed (the molecules shift to an excited electronic state), and then re-emitted. Basically, think of a blacklight. Of course, when the light goes away, the glow stops almost instantly too.
Here’s where phosphorescence comes from. If there’s an excited state–lower than the one you excited the molecules to originally–which “crosses” (I can be more specific if you want) the original one, the molecules will drop to that state before dropping down to the ground state. This, however, is a semi-allowed transition, so it doesn’t just happen instantly to all the molecules–hence, the continued glow once the light source is removed.
Sorta. The idea is that there’s an intermediate state that you don’t get to in the intial excitation–it skip over that for reason you probably don’t care about. However, it is available from the excited state, so instead of going straight down to the ground state (fluorescence), it sorta makes a “detour” into the intermediate, then drops back down to ground (phosphorescence–which takse some time).
I love the way my physical chemistry book descibes the emmission step in phosphorescence. It calls the step “approximately” forbidden. So when a physical chemist says something is forbidden, you got to ask him how forbidden it is.
Anyway, Myrr21 has been correct in his description, but if you want more details here are some websites.