[OnePound]

Hello,

Recently, I've been looking into phosphorescence, and believe I now have a handle on basic principles. However, one thing that still eludes me is why is it that only some materials phosphoresce? As I understand it, all that's required is an empty orbital for an electron to occupy; surely this criterion is met in a large number of compounds, so why is it that very few are used for phosphorescence?

Many thanks,
OnePound

[fledarmus]

That's only about half right - all that is required for a material to absorb light is an empty orbital for an electron to occupy. However, in almost all cases, that light is re-emitted almost immediately as the electron drops right back down to the ground state. In most cases, this emission is on the order of nanoseconds.

To get phosphorescence, you have to have a separate mechanism for emitting energy which does not follow the pathway for when the energy was absorbed (ie, the electron doesn't just drop right back into the ground state), and the pathway has to lead to an energy level which has only "forbidden" transitions back to the ground state. That leaves the molecule hung up in an excited state from which it can only slowly descend. The half-life of that transition can be on the order of hours, effectively allowing the molecule to store energy in the excited state. The "glow" of phosphorescence is the result of photons emitted when these hung-up molecules finally drop back down into their ground state.

[OnePound]

Thanks for tidying that part up, but I've now got another question! Why is it that only some molecules phosphoresce? I understand about the inter-system crossing and triplet states, but why can't all molecules do that?