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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-
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