When a molecule absorbs a photon of the appropriate energy, one of its electrons is raised to an orbital of higher energy. The resulting molecule may then take part in a reaction while in this, or an alternative, excited state. The reaction so induced may be either intramolecular – for example, rearrangement or dissociation – or intermolecular – for example, addition. Sequences which include photocyclization are used in assembling cage compounds. Isomerization can occur because the p-bond, which normally prevents it, is lost in passage to the excited state. Both alkenes and alkynes undergo photochemical addition to benzene. There is one general circumstance in which photoaddition of alkenes to carbonyl compounds fails: namely, when the energy difference between the triplet and ground state of the carbonyl compound is greater than that between the corresponding states of the alkene. The process – photodimerization – occurs with both alkenes and aromatic compounds.