Quantum beats in recombination of spin-correlated radical ion pairs with equivalent protons

Quantum beats caused by the hyperfine coupling (hfc) with magnetically equivalent protons were observed in the recombination fluorescence of the singlet-correlated pairs of secondary radical ions generated in cyclohexane solutions by ionizing radiation. In agreement with theoretical predictions, the ratio I H ( t )/ I 0 ( t ) of fluorescence decay kinetics in high and zero magnetic fields showed narrow peaks for the radical cations with an even number of the equivalent protons. To generate radical pairs, p -terphenyl- d 14 was used as an electron acceptor while the solvent holes were trapped by octamethyl-1,4-cyclohexadiene, tetramethylethylene, 9,10-octalin, cis -decalin, hexamethylbenzene, durene, p -diethylbenzene, p -xylene, or benzene. When diisopropylamine was used as a hole acceptor, a trough was observed instead of the strongest peak. The position of the strongest peak (trough) is determined by the value of hfc constant only and is independent of the number of protons.