Photodegradation is the process by which an organic compound is broken down into smaller molecules or even mineralized due to the absorption of light, particularly ultraviolet UV radiation. In this case, we will discuss the photodegradation mechanism of benzene.Benzene C6H6 is an aromatic hydrocarbon with a planar hexagonal ring structure and alternating single and double carbon-carbon bonds. The photodegradation of benzene occurs through a series of photophysical processes, which can be summarized as follows:1. Absorption of light: The first step in the photodegradation process is the absorption of light by the benzene molecule. When benzene absorbs UV light, it gets excited from its ground state S0 to a higher-energy excited state S1 or S2 . This process is known as electronic excitation.2. Internal conversion and vibrational relaxation: After the initial excitation, the benzene molecule undergoes internal conversion, a non-radiative process in which the molecule transitions from the higher excited state S2 to a lower excited state S1 without emitting a photon. Following internal conversion, the molecule undergoes vibrational relaxation, where it loses excess vibrational energy and reaches the lowest vibrational level of the excited state S1 .3. Intersystem crossing: In some cases, the excited benzene molecule can undergo intersystem crossing, a process in which the molecule transitions from the singlet excited state S1 to a triplet excited state T1 by changing its spin multiplicity. This process is relatively slow but can lead to the formation of reactive triplet species.4. Photochemical reactions: Once the benzene molecule is in an excited state S1 or T1 , it can undergo various photochemical reactions, such as: a. Homolytic cleavage: The excited benzene molecule can undergo homolytic cleavage of one of its carbon-carbon bonds, leading to the formation of two phenyl radicals C6H5 . These radicals can further react with other molecules in the environment, such as oxygen, to form reactive oxygen species ROS or other degradation products. b. Hydrogen abstraction: The excited benzene molecule can also abstract a hydrogen atom from a neighboring molecule, such as water or another benzene molecule, to form a phenyl radical C6H5 and a hydroxyl radical OH or a benzyl radical C6H5CH2 , respectively. These radicals can further participate in various reactions, leading to the formation of different degradation products.5. Formation of degradation products: The radicals and reactive species formed during the photochemical reactions can undergo further reactions, leading to the formation of various degradation products. Some common products formed during the photodegradation of benzene include phenol, catechol, hydroquinone, and benzoquinone, among others. These products can be further degraded by photolysis or other degradation processes, such as biodegradation or chemical oxidation, ultimately leading to the mineralization of benzene into carbon dioxide and water.In summary, the photodegradation of benzene involves the absorption of light, electronic excitation, internal conversion, vibrational relaxation, intersystem crossing, and various photochemical reactions, ultimately leading to the formation of degradation products and mineralization.