Describing Phosphorescence

The excited electrons in the glow-in-the-dark toothbrush do not lose all their energy at once. Instead these electrons lose enough energy to nearby atoms to make the transition from the conduction band to the impurity band. In the computer representation a dashed downward arrow represents this transition. After this transition occurs, the conduction band turns gray and the impurity band turns black. This change indicates that electrons have lost some energy and now have energies associated with the impurity band. The change in energy is small and is generally in the thermal energy range.

 

In fluorescent materials electrons have energies in the impurity state band for a very short time (10-9 to 10-6 seconds). Then, they emit light as their energy changes to an energy in the valence band. As a result, fluorescent materials will only glow while light of sufficient energy shine on them. In phosphorescent objects, the electrons remain in the impurity band for awhile. After this time delay the electrons emit light as their energy changes. Thus, phosphorescent materials emit light using energy that was absorbed at an earlier time. When all energy is converted to light, the object stops glowing in the dark.

 

The electrons remain with energy in the impurity band because the physical situation “forbids” a direct transition from impurity to valence band. To change energy to that of the valence band the electrons must first gain back the thermal energy that they lost when they made the transition to the impurity band. Because the energy is small, it can generally be provided by the ambient energy in the air. (See the figures below).