Photochemistry

The energy of an absorbed photon may be sufficient to break molecular bonds (path i), creating two or more atomic or molecular fragments. An important example of photodissociation is found in the photochemistry of stratospheric ozone. Ozone (O₃) is produced in the stratosphere from molecular oxygen (O₂) through the following pair of reactions: O₂ + hν → O + O and O + O₂ → O₃ where hn represents the energy of a photon of ultraviolet light with a wavelength less than 260 nm. Ozone is also dissociated by short-wavelength ultraviolet light (200-300 nm) through the reaction: O₃ + hν → O₂ + O. The oxygen atom formed from this reaction may recombine with molecular oxygen to regenerate ozone, thereby completing the ozone cycle. The great importance of stratospheric ozone is that it absorbs harmful short-wavelength ultraviolet light before it reaches the Earth's surface, thus serving as a protective shield.

In recent years, the effect of chlorofluorocarbons, commonly known as Freons or CFCs, on the ozone cycle has become of great concern. CFCs rise into the stratosphere where they are dissociated by ultraviolet light, producing chlorine atoms (Cl) through the reaction: CFC + hν → Cl + CFC(minus one Cl). These chlorine atoms react with ozone to produce ClO and molecular oxygen: Cl + O₃ → ClO + O₂. ClO reacts with the oxygen atoms produced from the photodissociation of ozone in reaction 5 to produce molecular oxygen and a chlorine atom: ClO + O → O₂ + Cl. Therefore, the presence of CFCs interrupts the natural ozone cycle by consuming the oxygen atoms that should combine with molecular oxygen to regenerate ozone. The net result is that ozone is removed from the stratosphere while the chlorine atoms are regenerated in a catalytic process to continue the destructive cycle.