Selective detection of EchoEPR signals due to naturally substituted C-13 radicals in a single crystal of ammonium tartrate

Echo-detected electron paramagnetic resonance (echoEPR) profiles for irradi ated deuterated ammonium tartrate single crystals depend strongly on the de lays between pulses of the echo sequence. This is mainly due to instantaneo us and spectral diffusion that plays a crucial role in determining the deca y of the echo at every field position: the dephasing rate 1/T-M depends on the number of spins excited by the pulses and on the total number of intera cting spins. A rigorous simulation of the echoEPR profiles at different del ays requires the evaluation of the modulation pattern (ESEEM) and of the de phasing processes at every field position. From the simulations, informatio n on the microscopic radical concentration, and on the electron-electron fl ip-flop rates of the single radical species can be obtained. Natural isotop e C-13 substitution generates low-concentration radicals with relaxation pr operties different from the equivalent C-12-substitued radicals. The differ ent behavior is discussed.