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.