A photon of UV "light" hits an oxygen molecule. The energy from the photon breaks the molecule apart. It becomes two separate oxygen atoms. This is an example of photodissociation. Click on image for full size (24 Kb JPEG) Images courtesy COMET.

Photons are tiny bits of light and other kinds of electromagnetic radiation. Photons can sometimes break apart molecules. When this happens, it is called photodissociation.

When a photon runs into a molecule, it adds energy to the molecule. Molecules have chemical bonds that hold the atoms in them together with each other. If the chemical bond breaks, the molecule falls apart. Sometimes photons have enough energy to break the bonds in a molecule. A photon of ultraviolet (UV) "light" has more energy than a photon of visible light. UV photons can cause photodissociation more easily than visible light photons can.

Photodissociation happens a lot in Earth's atmosphere. There are lots of chemical reactions in our air. Photodissociation provides the energy for many of those reactions. For example, photodissociation helps make smog. It also helps create ozone.