Photochemistry (Encyclopedia of Science)
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:
O2 + hν O + O
(The expression hν 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 (O3):
O + O2 O3
Steps in photochemical processes
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 hν.
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:
(The entire section is 505 words.)
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