Multi-Frequency and High Field ESR

As mentioned in Chapter 1 high field ESR gives a better spectral resolution of anisotropic g-factors, and allows simpler analysis than is possible at X-band of spectra with large zero-field (another term is fine structure) and hyperfine splittings. These features, (l)-(3) below, are the most important in general applications, but some other characteristics of high field ESR are also of relevance [2]  

The frequency bands are often denoted by one-letter symbols as reported in Table 4.1. 

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Multi-frequency and high field studies are at present mostly carried out in CW-mode. The chapter is concerned with examples of CW ESR applications, mostly in rigid amorphous matrices, e.g. frozen solutions, multiphase systems and in polycrystalline samples where the additional information from multifrequency and/or high field measurements are the most crucial for accurate characterisation. 

The advances in technology in the last years led to the development and optimization of new ESR methods such as high field/multi-frequency ESR [29-31], double resonance [32, 33], and pulse methods [34-36]. The improvements in resolution brought by these methods have made possible analysis and interpretation of more complex systems and detection and characterization of transient paramagnetic intermediates inaccessible before. 

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