Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation

Area of Science:

• Magnetic Resonance
• Quantum Mechanics
• Materials Science

Background:

• Dynamic Nuclear Polarization (DNP) is crucial for spin orientation and hyperpolarization in MRI.
• Existing DNP theories often rely on the high temperature approximation, which is inadequate for highly polarized systems.
• Previous work on thermal mixing DNP was limited by assumptions of fast spectral diffusion.

Purpose of the Study:

• To extend cross-relaxation theory beyond the high temperature approximation for DNP.
• To apply the extended theory to spectral diffusion in glasses with paramagnetic centers.
• To provide a more accurate theoretical framework for DNP mechanisms like the cross-effect.

Main Methods:

• Extended Provotorov's cross-relaxation theory to low temperatures.
• Applied the formalism to spectral diffusion in glasses with anisotropic g-tensors.
• Investigated DNP of proton and 13C spins using TEMPO-doped samples.

Main Results:

• The extended theory accurately describes DNP via the cross-effect mechanism.
• Results align with thermal mixing predictions in the fast spectral diffusion limit.
• Slow spectral diffusion leads to hole burning in the ESR signal, consistent with simpler models.

Conclusions:

• The developed theory provides a more comprehensive description of DNP, particularly under conditions not covered by the high temperature approximation.
• This advancement is critical for optimizing DNP applications in magnetic resonance imaging and polarized targets.
• The study offers new insights into spectral diffusion dynamics in paramagnetic glasses.