Electron spin resonance radicals, dipolar interaction

Several pulse methods were developed for estimation distances between two slowly-relaxing spins. In a pulse electron-electron double resonance (PELDOR) technique a spin echo is created by a two-pulse sequence at one microwave frequency. The timing of a pulse at a second microwave frequency is varied (Milov et al., 1998). This method is suitable for analysis of weak dipolar interactions. 3-pulse PELDOR with all three pulses at the same microwave frequency ( 2 + 1 sequence) was proposed by Raitsimling and his co-workers (2000). A specific feature of the 2 + 1 technique is suppression of dipolar interaction of randomly distributed spins, which allows the selection of a dipolar interaction between radicals. Using a 4- pulse experiments it was possible to eliminate an inherent dead experimental deadtime that limits the magnitude of the dipolar interaction in 2 + 1 sequence and in 3-pulse ELDOR experiments (Pannier et al., 2000). 

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