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Photodissociation is a process in which a molecule absorbs a photon of light and breaks down into smaller fragments. On the other hand, photoionization is the process where an atom or molecule absorbs a photon of light and loses an electron, forming a positively charged ion.

For a molecule to undergo photodissociation or photoionization, it must absorb a photon that has enough energy to break the chemical bond between its atoms or ionize it by removing an electron. The energy requirement for photodissociation of oxygen (\(O_2\)) is lower than that of photoionization. Specifically, photodissociation requires an energy in the ultraviolet (UV) light range (~ \(240 \mathrm{~nm} \)), while photoionization demands a much higher energy, in the extreme ultraviolet (EUV) light range (~ \(20 \mathrm{~nm} \)).

At altitudes below 90 km, the Earth's atmosphere consists of a higher proportion of neutral molecules like oxygen, which makes photodissociation more likely to occur. Moreover, the atmosphere absorbs the majority of the high-energy EUV photons that would cause photoionization before they can reach these lower altitudes. This absorption is primarily due to the interaction of EUV photons with atomic and molecular nitrogen in the atmosphere. As a result, there are fewer EUV photons available for photoionization of oxygen, making photodissociation the more significant process at altitudes below 90 km. In summary, the primary reason behind the importance of photodissociation over photoionization at altitudes below 90 km is the lower energy requirement for photodissociation and the decreasing availability of high-energy EUV photons at these altitudes due to atmospheric absorption.