The formation of the ozone layer is also caused by photodissociation. Ozone in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O 2), splitting them into individual oxygen atoms (atomic oxygen). The atomic oxygen then combines with unbroken O 2 to create ozone, O 3. In addition, photolysis is the process by which CFCs are broken down in the upper atmosphere to form ozone-destroying chlorine free radicals.