Electron Spin-Echo Envelope Modulation at S-Band. 3: Analysis of Nuclear Quadrupole Interaction Effects

In the present paper the nuclear modulation of electron spin echo signals at S-band is investigated in the case of interacting nuclei with a quadrupole moment high enough to cause nuclear quadrupole couplings not negligible with respect to the nuclear Zeeman and dipolar hyperfine couplings. Both the two-pulse and three-pulse electron spin echo envelope modulation (ESEEM) due to (27)Al and (14)N are simulated at different values of the nuclear quadrupole coupling by numerical diagonalization of the nuclear Hamiltonians. The behavior of their amplitude and periods is discussed on the basis of the ratios between the strengths of the nuclear quadrupole interaction and the nuclear Zeeman and the dipolar hyperfine interactions. The interpretation of their trends in terms of the eigenfunctions and eigenvectors of the nuclear Hamiltonians is carried out by using analytical equations obtained by perturbation approaches. First order perturbation treatments for integer and half-integer nuclear spin quantum numbers are developed when the nuclear quadrupole coupling is the main interaction. A discussion On the limits of the interpretation based on the perturbation approach is also given by comparing the magnitude Fourier transform of the patterns calculated by exact diagonalization and analytical equations.