Heavy Element Photophysics and Photochemistry

ODNMR: Probing Interactions where Conventional NMR Fails

Conventional spectroscopic methods such as NMR and Raman spectroscopy fail in determining the immediate local structure of f-element ions doped into solids. We now have demonstrated that our optically detected nuclear magnetic resonance (ODNMR) apparatus provides effective methods for studying nuclear magnetic interactions of paramagnetic f-element ions in solids. Our ODNMR experiments begin with a single-frequency laser that induces an optical transition between two electronic states of an f-element ion. An applied radio frequency (rf) field is swept through nuclear magnetic resonances of the sample.

This image shows the angular dependence of the observed (symbols) and calculated (solid and dashed line curves) ODNMR frequencies of I_Z= 1/2« 3/2 transitions for ¹⁵¹Eu³⁺ in a LaF₃ crystal at two magnetically inequivalent sites termed A and B. The external magnetic field of 0.26 tesla was always in a plane containing the c-axis and perpendicular to the a-axis. With a potential to reveal minute structure differences in detail, we expect to apply this ODNMR technique to fundamental studies on actinide-doped materials to determine, among other factors, the influence of radiation damage on the local environment surrounding both ions that have undergone nuclear decay and those that have not.

We optically detect resonances either via coherently emitted light whose frequency is modulated by the resonance or via isotropic fluorescence whose intensity changes at resonance. In effect, ODNMR arises from optical and rf double resonance. The optical resonance involves only an f-element ion; the rf resonance may involve the nucleus of the f-element ion or that of its surrounding ligands through superhyperfine interaction. The first ODNMR work performed in our laboratory was on Eu³⁺ doped into LaF₃. This material was chosen due its challenging nature. Previous studies by others on this system failed to provide interpretable data. Analysis of our ODNMR spectra, using an effective nuclear quadrupole Hamiltonian and the measured quadrupole splittings, provided values for the quadrupole interaction parameters.

Subsequently, the angular-dependence of Zeeman splitting of the ODNMR spectra for the ⁷F₀ ground state of ¹⁵¹Eu³⁺ in LaF₃ was measured in the presence of an external static magnetic field. The observed additional splitting when the static field was tilted away from the crystal axis is attributed to magnetically inequivalent Eu³⁺ sites, as was found previously for ¹³⁹La in LaF₃ using conventional NMR. Anisotropic magnetic interaction results in different nuclear Zeeman splitting for metal ions on different locations. In analysis of our data, the orientation of the principal axes of the interaction tensor and the anisotropic magnetic shielding factors were determined for Eu³⁺ on different sites. This work showed that none of the principal axes coincides with the crystallographic axes. However, the magnetic interaction tensors for different Eu³⁺ sites are related to each other by C₂ rotation symmetry.

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