TIME-RESOLVED AND MODULATION METHODS IN THE STUDY OF THE EFFECTS OF MAGNETIC-FIELDS ON THE YIELDS OF FREE-RADICAL REACTIONS

In an attempt to understand the apparently disparate results reported in magnetic field effect studies on the same reaction system from diff erent experimental groups, two different techniques, time-resolved met hods and field-modulated ones, have been applied to systems comprising pyrene and either dicyanobenzene or dimethylaniline. It is shown that these yield different results whose origin lies less in the detection method than in the light sources which produce the radicals. The resu lts from the time-resolved experiments, in which the effects of spin r elaxation in a singlet-correlated pair are apparent, have been analyze d using a novel approach applicable to more complex reaction systems t han have been studied in the past. An original experiment is introduce d to observe the effects of relaxation at field strengths where the hy perfine-driven spin mixing has reached its asymptotic rate. The discre pancy between the results originates in a much shortened radical pair lifetime, resulting from a very low radical concentration under the co nditions used in the modulation experiment as opposed to the time-reso lved one. This lifetime effect has been confirmed by using degenerate electron-exchange reactions to affect the chemistry in the period of o bservation in each technique. Low-field minima in the field effect cur ves observed using the modulation method are shown to originate in hyp erfine and effects and not to be due to J-resonances.