RESUMO
We present a new way to increase the modulation amplitude of electron spin echo envelope modulation (ESEEM) experiments that are based on electron spin coherence. The method uses a train of N refocusing π-pulses where each one of them redistributes the electron spin coherence among allowed and forbidden EPR transitions. This in turn leads to a significant enhancement of the ESEEM effect, depending on the strength of the hyperfine interaction and the number of applied pulses, N. We derive analytical expressions for a general two-dimensional (2D) scheme which is based on the refocused primary echo and we explore the expected modulation enhancement of various correlation peaks as a function of k (modulation depth parameter) and N. In addition, we inspect two different one-dimensional (1D) versions of the method, namely the Carr-Purcell-Meiboom-Gill (CPMG) sequence occurring for t1=t2, and an extension of the primary echo sequence occurring for t2=0. Our study shows that these methods are particularly useful for detecting weak hyperfine couplings of magnetic nuclei having small gn factors and low natural abundances like (13)C and (29)Si. The theoretically predicted features are confirmed by experiments in disordered spin systems.