ENDOR Coils and Related Radiofrequency Circuits

Electron-Nuclear Double Resonance (ENDOR) is an advanced electron magnetic resonance (EMR) technique that provides a much higher level of spectral resolution than conventional microwave absorption methods. The ENDOR method was introduced by Feher (1956) as a variant of the Overhauser Effect (1953) in nuclear resonance, and it entails the observation of transitions between nuclear sublevels of the electron Zeeman Effect. The electron transitions are still used as a means of detection, however, because the sensitivity of the electron resonance measurement is greater than that of the nuclear resonance. In brief, an EMR transition (Δm s = ±1, Δm I = 0, Figure 1) is saturated, which leads to the collapse of the observed EMR signal as the corresponding state populations equalize. If one now irradiates the spin system so that transitions are induced between the nuclear sublevels (i.e. Δm I = ±1, Δm s = 0), the condition of saturation in the EMR transition is lifted as the nuclear sublevel populations shift, and there is a partial recovery of the EMR signal, the so-called ‘ENDOR enhancement’ (cf. Kevan & Kispert, 1976). It is common to view the pathways among the energy levels in the spin Hamiltonian state diagram (Figure 1) as being analogous to a resistive electrical network and the spin dynamics of the ENDOR effect as a short circuit phenomenon (cf. Dwek et al., 1969; Kwiram, 1971; Möbius et al., 1982; Kurreck et al., 1988). Figure 1 Four level state diagram (m S = ±1/2; m I = ±1/2) and the transitions of an ENDOR experiment. An allowed EMR transition is saturated, and the rf frequency is swept in order to detect NMR transitions.