Microwave power dependence of triplet state kinetics as measured with fluorescence detected magnetic resonance in zero field

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Abstract
The kinetic parameters of a triplet state, viz. the populating probabilities and the depopulating rates, can be extracted from fluorescence detected electron spin resonance experiments in zero magnetic field by monitoring the response of the system to a pulse of resonant microwaves (pulse method) or to their being switched on or off (equilibrium method). The two methods are critically evaluated. An analytic expression is given for the kinetics of the system taking into account the intensity of the driving microwave field explicitly. It is demonstrated that, for driving fields producing microwave induced transitions at a rate less than or comparable to the sublevel decay rates, the zero field transitions are not saturated, irrespective of the value of the spin lattice relaxation time. Under these conditions the equilibrium method as usually applied gives incorrect results. Our analysis is applied to kinetic experiments on the reaction centre bacteriochlorophyll triplet in photosynthetic bacteria. It is shown that the value of the molecular decay rate constants of the triplet sublevels obtained by the pulse method are to be preferred over those obtained by the equilibrium method.