ECHO PHENOMENA IN ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

This paper reviews echo phenomena observed in electron spin resonance spectroscopy. For each type of echo, a theoretical description based o n the density operator formalism is given. Except for the well-known s tandard electron spin echo experiments, we also present experimental d ata which are taken either from the literature or from recent work don e in our laboratory. First we discuss echo phenomena which can be obse rved in isolated one-spin systems. They can be classified in two categ ories, ''pancake'' echoes and ''race-track'' echoes. Then we describe coherence transfer echoes that can be observed in multispin systems. S uch echoes may be created when coherence is transferred from one pair of energy levels to another. Electron coherence transfer echoes are qu ite common in electron spin echo experiments, even if their real natur e is often not noticed. Nuclear coherence transfer echoes can be creat ed by microwave pulses only, and are exploited in modem 1D and 2D puls ed EPR techniques. Finally, we discuss experiments in which the respon se of the spin system to an extended perturbation can be considered as a stringing together of electron spin echoes.