Theory of Saturation and Double Resonance Effects in ESR Spectra. III. rf Coherence and Line Shapes

Abstract

The theory of saturation in the electron spin resonance spectra of dilute solutions of free radicals has been employed to analyze in detail several possible effects in ENDOR spectra that are due to the coherent nature of the applied nuclear rf and microwave fields. These effects are implicitly contained in a ``coherence matrix,'' although their observation also depends on the details of the relaxation terms. It is shown that (1) strong nuclear rf fields can split ESR lines, a phenomenon similar to ``spin‐tickling'' in NMR double resonance, (2) strong microwave fields in the presence of weaker, but saturating, nuclear rf fields can split ENDOR lines, and this latter effect is demonstrated experimentally. A subtle coherence effect requiring nuclear spins of I>½, or more than one equivalent spin of I=½, and dependent upon nuclear rf field strength is carefully analyzed. It is predicted to significantly affect only the ENDOR line obtained from saturating the M = 0 hyperfine line for two or four equivalent protons. The theory for this effect appears to be in very good agreement with detailed experimental studies on Coppinger's radical. Other radicals are also found to exhibit such coherence effects. The ability to distinguish and analyze them is deemed important in applying ENDOR as an analytical tool. Computer simulations of the line shapes in the presence of coherence effects are also presented.