Photons of the correct energy promote atomic and molecular electrons from ground state AOs and MOs to excited state orbitals. Photoexcited species may adsorb more photons until the species photoionises to the cation and electron, or it may relax by one of many possible mechanisms. Reaction chemists are particularly interested in photoinduced radical fragmentation. For more information look in the Chemogenesis
webbook section on photochemistry.