Pulse radiolysis in an applied magnetic field. The time dependence of the magnetic field enhancement of the fluorescence from solutions of fluorene in squalane

The effect of a magnetic field on the light emission from squalane solutions of fluorene, pulse irradiated at the center of a large electromagnet, is described. The fluorescence intensity following pulse radiolysis was found to be 40 to 50 percent higher in an applied field of 0.3 T (3000 G) and the triplet yield showed a small decrease. The enhancement of the singlet emission is attributed to an increase in the fraction of geminate solute ion recombinations leading to excited singlet fluorene. The magnetic field effect was time dependent, being undetectable during the pulse and reaching a maximum value after about 100 ns. These observations are compared with the theory of Brocklehurst which describes the singlet to triplet conversion of geminate solute ion pairs quantitatively in terms of the electron--nuclear hyperfine interactions in the radical ions. After correction for the large in-pulse fluorescence and its decay, satisfactory agreement between experiment and theory was obtained. The results emphasize the importance of ion recombination in the generation of excited states in radiolysis and confirm the role of the electron--nuclear spin interaction in determining the ratio of singlet and triplet yields.