Fig. 3
Simplified representations and some examples of processes that occur in photochemical reactions of natural substances: a promotion of electrons (e −) and generation of electron vacancies (holes, h +) upon irradiation of a semiconductor, which may then reduce and oxidize other substances; b excitement of organic compounds by sunlight which then directly react with other substances or are themselves altered, with examples of photochemical acidification, dissolution, and crosslinking; c photocatalysis via surface adsorption, which makes a species, here N2O, susceptible to the effect of light; d indirect generation, via a photosensitizer, of electrons and holes in a semiconductor: the difference between the highest occupied molecular orbital (HOMO) of the sensitizer and its lowest unoccupied molecular orbital (LUMO) is smaller than the band gap of the semiconductor, and therefore less energy is required to excite the sensitizer; e cooperative generation of transient reactive species by compounds that do not individually absorb sunlight; f generation of transient reactive species by light-absorbing compounds. Arrows with shadows indicate reactions induced by light (hν), asterisks (*) indicate excited species (electrons promoted to higher energy levels), single brackets (]) indicate mineral surfaces, and dotted lines (…) indicate surface adsorption. The references cited in the text offer additional, detailed explanations of these processes