Dynamic Processes as Followed by Electron Paramagnetic Resonance

Abstract

The use of Electron Paramagnetic Resonance (EPR) for the study of dynamic processes has received increasing attention in recent years. In fact, time dependent processes and their influence on EPR line shapes can be treated in close analogy with the case of NMR. In a qualitative way, it can be said that the observed spectra depend upon the time scale of the dynamic process under consideration, with respect to some characteristic time scale of the method which is related to a difference in resonance frequencies for localized states. However due to the large shifts in frequencies arising from g factor anisotropics or hyperfine splittings, the spectral shapes can become quite complex and the notion of a fixed time scale vanishes upon further scrutiny. This is because each process leading to a shift in resonance frequencies can lead to a different “EPR time scale”. In addition, lineshapes are strongly dependent upon relaxation rates which themselves can be modulated by a dynamic process. This extends considerably the range of “time scales” which can be probed by EPR.