How Lasers Work Pt. 2

PART II: PHOSPHORESCENCE

We now move on to the slightly more interesting phosphorescence. In some cases, an intermediate energy state may be located between our two energy levels we've been discussing. So let's label our three states S0, S1, and S2 with energy gaps equal to E01, E02, and E12. Now if we use light of energy E02, we will excite ground state (S0) atoms to the second excited state (S2). These atoms may make a very fast transition to the first excited state (S1) without emitting a photon (a so-called radiationless transition, with the energy, E12, released in another form, perhaps heat). Now, if the transition from S1 to S0 is really unfavorable (a so called forbidden transition, usually implying a violation of spin conservation), then the relaxation from S1 to S0 will take a long time. Thus, we pump atoms up to a really excited state, they quickly come down to an intermediate state, and then slowly relax back down to the ground state, emitting light in the process - note: the energy of the light given off, E01, will not be the same as the energy we put in, E02, and in this way we can get cool colors of light from our atom. This describes phosphorescence: we shine light on our stuff, it starts giving off light of a different color, and even after we turn off our light, it continues giving off light for some time (because atoms are still in the intermediate excited state S1, and are taking a long time to come back down).

This is how some glow-in-the-dark materials work, namely those which have to be sitting in light for a while. The light 'charges' the material, getting atoms into excited (but forbidden) states, and then when you take the light away, all these atoms slowly relax back to the ground state, releasing light in the process. This process is especially useful for making really sweet stickers to put in your stickerbook.