Electron spin resonance magnetic field gradients

ESR spectroscopy can be transformed into an imaging method, ESRI, for samples containing unpaired electron spins, if the spectra are measured in the presence of magnetic field gradients. In an ESRI experiment the microwave power is absorbed by the unpaired electrons located at point x when the resonance condition, Equation (10), is fulfilled. 

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

Quadrupolar nuclei constitute most of the magnetic nuclei within the Periodic Table of the elements. However, the lack of suitable instrumentation as well as the misconception of the deleterious nature of these nuclei have impeded a more widespread utilization of their resonances. Quadrupolar relaxation resulting from the interaction of the nuclear quadrupolar moment with finite electric field gradients is the principal source of nuclear relaxation in nearly all compounds. However, albeit generally eonsidered a nuisance, the phenomenon may as well be exploited to the experimenter s advantage. In contrast to spin-j nuclei whose relaxation behaviour is principally dictated by the dynamics of the molecules in solution, structural and electronic effects play the key role in the relaxation process of quadrupolar nuclei. 

If the electric field gradient (EFG) is non-zero, for instance due to a non-cubic valence electron distribution and/or non-cubic lattice site symmetry, electric quadrupole interaction as visualized by the precession of the quadrupole moment vector about the field gradient axis sets in and splits the degenerate I = 3/2 level into two substates with magnetic spin quantum numbers mj = 3/2 and 1/2 (Fig. 2.4). The energy difference between the two substates A q is observed in the spectrum as the separation between the two resonance fines. These two resonance lines in the spectrum refer to the two transitions between the two substates of the... 

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