5-and 6-pulse electron spin echo envelope modulation (ESEEM) of multi-nuclear spin systems

In 3-pulse ESEEM and the original 4-pulse HYSCORE, nuclei with large modulation depth (k approximate to 1) suppress spectral peaks from nuclei with weak modulations (k approximate to 0). This cross suppression can impede the detection of the latter nuclei, which are often the ones of interest. We show that two extended pulse sequences, 5-pulse ESEEM and 6-pulse HYSCORE, can be used as experimental alternatives that sufrer less strongly from the cross suppression and allow to recover signals of k approximate to 0 nuclei in the presence of k approximate to 1 nuclei. In the extended sequences, modulations from k approximate to 0 nuclei are strongly enhanced. In addition, rmulti-quantum transitions are absent which simplifies the spectra. General analytical expressions for the modulation signals in these sequences are derived and discussed. Numerical simulations and experimental spectra that demonstrate the higher sensitivity of the extended pulse sequences are presented. (c) 2007 Elsevier Inc. All rights reserved.