Influence of the anisotropic hyperfine interaction on the 14N ENDOR and the ESEEM orientation-disordered spectra

The influence of the anisotropic hyperfine interaction on the 14N electron-nuclear double resonance/electron spin echo envelope modulation spectra is studied by approximate analytical and graphical methods for the case of the isotropic g-factor. The suggested determination of the modified characteristic directions of the magnetic field due to anisotropy enhances the insight in the structural details of the system and analytical solutions of the secular equation for these conditions are derived. The graphical method, previously used for the analysis of the orientation dependence of the 14N nuclear-transition frequencies in orientation-disordered samples for isotropic hyperfine interaction is extended to the case of arbitrary anisotropic hyperfine tensor. The above analytical and graphical methods are illustrated and tested against exact simulations in two practically important cases: (i) isotropic hyperfine interaction (hfi) exceeding other nuclear interactions in nuclear spin Hamiltonian. (ii) Cancellation of the isotropic part of the hfi.