Reversal of the ionisation by the bonding of free electrons with positive ions. During recombination, the energy invested for ionisation is released again. It is emitted in the form of ultraviolet radiation. This UV radiation causes further plasma reactions, in particular the splitting of molecules into radicals. If the ions of a plasma find no reaction partner, they will recombine to form a neutral atom in a matter of split seconds. Relaxation is the return of atoms from an excited state to their basic state. Also in this process, electromagnetic radiation is radiated. It gives the plasma its characteristic colour.