MAGNETIC-FIELD EFFECTS ON CHEMICAL-REACTION YIELDS ARISING FROM AVOIDED LEVEL-CROSSINGS IN MOLECULAR TRIPLET-STATES

The unexpectedly large magnetic field effect found for continuously illuminated reaction centres of the photosynthetic bacterium Rhodobacter sphaeroides at cryogenic temperatures is discussed in terms of the magnetic field-induced mixing of sub-levels mechanism. The pronounced maximum in the steady state concentration of the triplet state of the primary donor at a field strength close to 20 mT is shown to arise from the avoided level-crossing of the triplet spin states when the magnetic field is almost along the Z axis of the zero-field splitting of the triplet. The dramatic change in the spin eigenstates as the field is varied through the avoided crossing leads to changes in triplet-to-ground-state intersystem crossing rates, and hence the triplet concentration. The predicted 350% change in triplet concentration is similar to that observed experimentally.