Spectrometry by modulated dynamic nuclear polarization

A Spectrometer for Dynamic Nuclear Polarization and Electron Paramagnetic Resonance at High Frequencies

Static NMR experiments on insensitive nuclei (e.g., (13)C) have been generally difficult because of low magnetogyric ratios and/or poor natural abundance, even at the highest magnetic fields available. One method of overcoming these sensitivity limitations is dynamic nuclear polarization (DNP), which can be utilized to enhance NMR signals relative to those obtained with static NMR detection. In this study, we report a recycled-flow DNP apparatus capable of monitoring (13)C scalar-dominated enhancements 1-2 orders of magnitude greater than those obtainable using the conventional recycled-flow (13)C NMR experiment. Specifically, a mixture of benzene and several chlorocarbons were continuously "recycled" through a (13)C DNP spectrometer. Results indicate successful detection with the (13)C DNP approach in scalar-dominated cases, where analytes could not be observed with conventional static or flow NMR detection in a reasonable length of time.

Time-resolved neutron scattering has been used to study dynamically polarized protons in tyrosyl-doped bovine liver catalase. While the evolution of proton polarization and its inversion by the method of adiabatic fast passage (AFP) in a standard dynamic nuclear polarization (DNP) system with organic Cr(V) complexes can be well modelled and understood, the experiments with tyrosyl-doped catalase lead us into the world of extremely dilute paramagnets with only about 1017 unpaired electrons per cm3. In this regime, the strength of DNP is comparable to the drift of proton polarization towards its thermal equilibrium of Pe = 0.35% at T = 1 K and B = 3.5 T. Negative DNP, which counteracts this drift, is confined to protons very close to the radical site, typically within 5 Å. In contrast, AFP reverses the polarization of protons at a larger distance from the radical. The contrast of a domain of polarized close protons giving rise to neutron scattering is considerably enhanced by AFP. Moreover, the spread of proton polarization is sensitive to magnetic inhomogeneities, like the iron of the heme of the catalase molecule. In short, polarized neutron scattering from AFP-modulated polarized samples provides an excellent tool for mapping sources and sinks of proton polarization in radical proteins.

Understanding the spatial distribution of the P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as polarization sources for dynamic nuclear polarization (DNP) quantum sensing and play a significant role in the relaxation of nitrogen vacancy (NV) centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized DNP and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric line shapes. The 13C DNP frequency profile at a high magnetic field revealed a pattern that requires an asymmetric EPR line shape of the P1 clusters with electron-electron (e-e) coupling strengths exceeding the 13C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e-e couplings. We employed a two-frequency pump-probe pulsed electron double resonance technique to show cross-talk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all of the P1 populations. Direct observation of clustered P1 centers and their asymmetric line shape offers a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for 13C and other nuclear spins of analytes. We propose that room temperature 13C DNP at a high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects.

The dynamic polarization of protons in polystyrene was observed in a wide range of temperature to investigate the contribution of the electronic spin-spin interactions. The results are compared with a theory based on the rate equations. An important role of the electronic spin-spin interactions is clarified; the solid effect is dominant at room temperature, but it is quenched strongly by the spectral diffusion and the nuclear polarization is induced by the cross relaxation of the electronic spins at low temperature.

Review: 194 refs.

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent on the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization upon shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.

Rapid sequential injections of hyperpolarized [1-13C]pyruvate in vivo using a sub-kelvin, multi-sample DNP polarizer

We show that dynamic spin polarization by collective raising and lowering\noperators can drive a spin ensemble from arbitrary initial state to many-body\nsinglets, the zero-collective-spin states with large scale entanglement. For an\nensemble of $N$ arbitrary spins, both the variance of the collective spin and\nthe number of unentangled spins can be reduced to O(1) (versus the typical\nvalue of O(N)), and many-body singlets can be occupied with a population of\n$\\sim 20 %$ independent of the ensemble size. We implement this approach in a\nmesoscopic ensemble of nuclear spins through dynamic nuclear spin polarization\nby an electron. The result is of two-fold significance for spin quantum\ntechnology: (1) a resource of entanglement for nuclear spin based quantum\ninformation processing; (2) a cleaner surrounding and less quantum noise for\nthe electron spin as the environmental spin moments are effectively\nannihilated.\n

Dynamical nuclear polarization by means of shallow donors in ZnO quantum dots

We present a small angle neutron scattering study of polymers in solution. The contrast between the scattering lengths of the polymer and the solution is created by dynamic polarization of the nuclei in the sample. We wed the spectrometer PAON at G5-3 of the LLB- Saclay-reactor Orphee which is a prototype of a SANS spectrometer using dynamic nuclear polarization. The power of this method can be seen from the results obtained on two different samples: a solution of the homopolymer polystyrene in orthoterphenyl, and a mixture of two homopolymers dissolved in toluene. We observed for the polystyrene sample the decrease in contrast at positive polarization and increase in contrast for negative polarization. For the sample of the polymer mixture in solution we obtained the different structure factors by taking the SANS spectra for different polarizations. The interference term, obtained separately in the same sample, confirms the mutual interaction

The use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter.

Liquid state DNP for water accessibility measurements on spin-labeled membrane proteins at physiological temperatures

The coherent transfer of electron spin polarization to nuclei by means of a microwave pulse sequence is a promising new approach to enhancing the sensitivity of solid-state nuclear magnetic resonance (NMR). The development of pulse sequences for dynamic nuclear polarization (DNP) of bulk nuclei is far from complete, as is the understanding of what makes a good DNP sequence. In this context, we introduce a new sequence, termed Two-Pulse Phase Modulation (TPPM) DNP. We provide a general theoretical description for electron-proton polarization transfer by periodic DNP pulse sequences and find it in excellent agreement with numerical simulations. In experiments at 1.2T, TPPM DNP generates a higher gain in sensitivity than existing sequences XiX (X-inverse-X) and TOP (Time-Optimized Pulsed) DNP but does so at relatively high nutation frequencies. In contrast, we find that the XiX sequence performs very well at nutation frequencies as low as 7MHz. A combination of theoretical analysis and experimental investigation makes clear that fast electron-proton polarization transfer, due to a well-preserved dipolar coupling in the effective Hamiltonian, correlates with a short build-up time of the dynamic nuclear polarization of the bulk. Experiments further show that the performances of XiX and TOP DNP are affected differently by the concentration of the polarizing agent. These results constitute important reference points for the development of new and better DNP sequences.

Efforts towards a dynamically polarised HD-target