Matter interacts with electromagnetic radiation (such as ultraviolet and visible light) through the processes of absorption and emission. The internal structure of atoms and molecules is such that absorption and emission of electromagnetic radiation can occur only between distinct energy levels. If the atom is in its lowest energy level or ground state, it must absorb the exact
amount of energy required to reach one of its higher energy levels, called excited states. Likewise an atom that is in an excited state can only emit radiation whose energy is exactly equal to the difference in energies of the initial and final states. The energy of electromagnetic radiation is related to its wavelength as follows; shorter wavelengths correspond to greater energies, longer wavelengths correspond to lower energies.
In addition to emitting radiation, atoms and molecules that are in excited states can give up energy in other ways. In a gas, they can transfer energy to their neighbors through collisions which generate heat. In liquids and solids, where they attached to their neighbors to some extent, they can give up energy through vibrations. The observation of fluorescence in gases at low pressure comes about because there are too few neighboring atoms or molecules to take away energy by collisions before the radiation can be emitted. Similarly, the structure of certain liquids and solids permits them to exhibit strong fluorescence.
If the wavelength of the radiation that was absorbed by the fluorescent material is equal to that of its emitted radiation, the process is called resonance fluorescence. Usually though, the atom or molecule loses some of its energy to its surroundings, so that the emitted radiation will have a longer wavelength than the absorbed radiation. In this process, simply called fluorescence, Stoke's Law says that the emitted wavelength will be longer than the absorbed wavelength. There is a short delay between absorption and emission in fluorescence that can be a millionth of a second or less. There are some solids, however, that continue to emit radiation for seconds or more after the incident radiation is turned off. In this case, the phenomenon is called phosphorescence.
As an example of fluorescence, consider the energy level diagram for the gemstone ruby in Figure 1. Ruby is a crystalline solid composed of aluminum, oxygen, and a small amount of chromium, which is the atom responsible for its reddish color. If blue light strikes a ruby in its ground state, it is absorbed, raising the ruby to an excited state. After losing some of this energy to internal vibrations the ruby will settle into a metastable state—one in which it can remain longer than for most excited states (a few thousandths of a second). Then the ruby will spontaneously drop to its ground state emitting red radiation whose wavelength (longer than the blue radiation) measures 6,943 angstroms. The fluorescent efficiency of the ruby—the ratio of the intensity of fluorescent radiation to the intensity of the absorbed radiation—is very high. For this reason the ruby was the material used in building the first laser.