phosphorescent substances
Once excited, an atom or molecule will usually spontaneously de-excite quickly. The electrons raised to higher levels are attracted to lower ones by the positive charge of the nucleus. Spontaneous de-excitation has a very short mean lifetime of 8 typically about 10 s . However, some levels have significantly longer lifetimes, ranging up to milliseconds to minutes or even hours. These energy levels are inhibited and are slow in de-exciting because their quantum numbers differ greatly from those of available lower levels. Although these level lifetimes are short in human terms, they are many orders of magnitude longer than is typical and, thus, are said to be metastable, meaning relatively stable. Phosphorescence is the de-excitation of a metastable state. Glow-in-the-dark materials, such as luminous dials on some watches and clocks and on childrens toys and pajamas, are made of phosphorescent substances. Visible light excites the atoms or molecules to metastable states that decay slowly, releasing the stored excitation energy partially as visible light. In some ceramics, atomic excitation energy can be frozen in after the ceramic has cooled from its firing. It is very slowly released, but the ceramic can be induced to phosphoresce by heatinga process called thermoluminescence. Since the release is slow, thermoluminescence can be used to date antiquities. The less light emitted, the older the ceramic. See Figure 30.35.