Photochemistry is the study of chemical changes made possible by light energy. The production of ozone in Earth's upper atmosphere is an example of such a change. Light from the Sun (solar energy) strikes oxygen molecules in the stratosphere, causing them to break down into two oxygen atoms:
O 2 + h ν → O + O
(The expression h ν is commonly used to represent a unit of light energy known as the photon.)
In the next stage of that reaction, oxygen atoms react with oxygen molecules to produce ozone (O 3 ):
O + O 2 → O 3
The excited state. A photochemical change takes place in two steps. Imagine that a light beam is shined on a piece of gold. The light beam can be thought of as a stream of photons, tiny packages of energy. The energy of the photon is expressed by means of the unit h ν .
When a photon strikes an atom of gold, it may be absorbed by an electron in the gold atom. The electron then becomes excited, meaning that it has more energy than it did before being hit by the photon. Chemists use an asterisk (*) to indicated that something is in an excited state. Thus, the collision of a photon with an electron (e) can be represented as follows:
e + h ν → e*
Once an electron is excited, the whole atom in which it resides is also excited. Another way to represent the same change, then, is to show that the gold atom (Au) becomes excited when struck by a photon:
Au + h ν → Au*
Emission of energy. Electrons, atoms, and molecules normally do not remain in an excited state for very long. They tend to give off their excess energy very quickly and return to their original state. When they do so, they often undergo a chemical change. Since this change was originally made possible by absorbed light energy, it is known as a photochemical change.
The formation of ozone is just one example of the many kinds of photochemical changes that can occur. When solar energy breaks an oxygen molecule into two parts, one or both of the oxygen atoms formed may be excited. Another way to write the very first equation above is as follows:
O 2 + h ν → O* + O
The excited oxygen atom (O*) then has the excess energy needed to react with a second oxygen molecule to form ozone:
O* + O 2 → O 3
Another way for an excited atom or molecule to lose its energy is to give it off as light. This process is just the reverse of the process by which the atom or molecule first became excited. If the atom or molecule gives off its excess energy almost immediately, the material in which it is contained glows very briefly, a process known as fluorescence. If the excess energy is given off more slowly over a period of time, the process is known as phosphorescence. Both fluorescence and phosphorescence are examples of the general process of light emission by excited materials known as luminescence.