Definition of ion pair return

The recombination of a pair of ions R+ and Z- formed from ionization of RZ.

If the ions are paired as a tight ion pair and recombine without prior separation into a loose ion pair this is called "internal ion-pair return":

R+Z- (tight ion pair) RZ (covalent molecule)

It is a special case of "primary geminate recombination".

If the ions are paired as a loose ion pair and form the covalent chemical species via a tight ion pair, this is called "external ion-pair return":

R+||Z- (loose ion pair) R+Z- (tight ion pair) RZ (covalent molecule)

It is a special case of "secondary geminate recombination".

When the covalent molecule RZ is reformed without direct evidence of prior partial racemization or without other direct evidence of prior formation of a tight ion pair, (e.g., without partial racemization if the group R is suitably chiral) the internal ion-pair return is sometimes called a "hidden return".

External (unimolecular) ion-pair return is to be distinguished from "external (bimolecular) ion return", the (reversible) process whereby dissociated ions are converted into loose ion pairs:

R+ + Z- R+||Z-