A two-step two-laser fluorescence spectroscopy method was employed to enable time-resolved detection of magnetic field effects (MFEs) on radical pairs (RPs). In the photoinduced hydrogen abstraction reaction of benzophenone (BP) in sodium dodecyl sulfate (SDS) micellar solution, fluorescence from the BP ketyl radical (BPH•) was observed under various magnetic fields; BPH• forms RPs with hydrogen-abstracted SDS radicals (•SDS) in the micelles. Kinetic parameters of the RP dynamics such as the spin relaxation rate, the generation rate, and the escape rate were determined from the simple analysis. The MFEs on BPH• observed from the fluorescence measurements were explained by the spin relaxation mechanism proposed by Hayashi and Nagakura. In this study, we demonstrate that two-step two-laser fluorescence spectroscopy in combination with a magnetic field provides valuable information about photochemical reactions in which RPs play important roles. ■ INTRODUCTIONRadicals and radical pairs (RPs) are common short-lived intermediates in various photochemical reactions. A magnetic field can alter the reactivity of a RP, thereby affecting the yields of photochemical products. A unique aspect of such magnetic field effects (MFEs) on RPs is that the magnetic field can change the rate of RP reactions that proceed via the activated states of the reactions, although the magnetic energy of the RP is much smaller than the thermal energy. For this reason, MFEs on photochemical reactions involving RPs have attracted considerable attentions since the mid 1970s and have been studied by various quantitative measurements such as product analysis, steady-state and transient absorption spectroscopy, and fluorescence spectroscopy. 1−4 The mechanisms of MFEs including the hyperfine coupling mechanism, the Δg mechanism, the level-crossing mechanism, the triplet mechanism, the spin relaxation mechanism, and low-field effects have been well established by reliable experimental measurements and through the development of theory of spin dynamics, which are also associated with the generation of chemically induced magnetic polarizations. 1,4 It has been suggested and demonstrated that RP dynamics in the absence and presence of magnetic fields are rich in information not only regarding the mechanisms of MFEs 5−7 but also regarding spatial RP motions in various media such as uniform solutions, 8,9 micelles, 10−13 proteins, 14 and ionic liquids. 15,16 Transient absorption spectroscopy, in which the absorption of transient radicals can be observed with high time resolution, is surely the first choice for the direct observation of RP dynamics under magnetic fields. In transient absorption measurements, however, the absorption of the photoexcited states, which are precursors of the radicals, is often overlapped with the absorption of the radicals. In such situations, it is difficult to distinguish the RP dynamics from the excited-state ones. To measure complete RP dynamics, it is necessary to choose an adequate photochemical reaction in which the absor...