Polarized Neutron Scattering from Polarized Nuclei Near Paramagnetic Centers

Polarized neutron scattering from dynamically polarized targets has been used for the study of hydrogenous materials in several laboratories. A new variant, which is less dependent on specific deuteration, is proposed. This is based on the observation that a radio frequency (RF) field swept over a selected part of an NMR line depolarizes the spins depending on their spatial distribution in and around paramagnetic molecules. An RF field swept over a narrow frequency interval has little effect on polarized nuclear spins in the paramagnetic centers, whereas remote nuclear spins can be depolarized. When applying the sweep over a much larger frequency range, all spins are depolarized. This was observed both by NMR and by polarized neutron scattering after selective depolarization of the proton spins in a solid solution of protonated EHBA-Cr^V, Na(C₁₂H₂₀O₇Cr^V).D₂O, in a mixture of deuterated glycerol and water. The decay time of proton spin polarization in domains associated with the paramagnetic centers varies between 5 h in a deuterated solvent and less than 1 min in a protonated solvent. The size of such a domain is barely larger than 1 nm. The same observation is also made when the nuclear polarization is reversed by adiabatic fast passage. If the RF sweep is limited to frequencies close to the central peak of the proton NMR line, the proton spins far away from the paramagnetic centers are reversed, whereas those of EHBA-Cr^V and close to the centers maintain their original polarization. This method may become a useful tool in neutron scattering once the paramagnetic centers can be fixed as spin labels to well defined sites in a macromolecule, preferably in a crystal lattice.