Phosphorescents question?

Question

If an electron os excited into the upper energy level then how is it quantum mechanically forbidden to decay?. All it has to do is lose the right amount of energy doesn't it?. Also when the decay is said to happen slowly because it is unlikely, is it that becasue it is unlikely, only a few atoms decay at a time ,making making the emmision slow. Or is it that all of the atoms are decaying simultaneously but the decay is a very slow transition slowly decaying and releasing light?

Answer 1

For a transition to take place, two things have to be true.  The energy of the photon has the match the difference energy levels, and there has to be a way that one state can get to the other one.

Phosphorescence usually occurs when the upper state has got two electrons with their spins pointing in the same direction, and to get back to the lower energy state one of the spins has to flip, which does not happen readily.  So only a few individual atoms decay in each time interval.  But the decay when it does happen is sudden.