129Xe Dynamic Nuclear Polarization Demystified: The Influence of the Glassing Matrix on the Radical Properties

129Xe dissolution dynamic nuclear polarization (DNP) is a controversial topic. The gold standard technique for hyperpolarized xenon magnetic resonance imaging (MRI) is spin exchange optical pumping, which received FDA approval in 2022. Nevertheless, the versatility of DNP for enhancing the signal of any NMR active nucleus might provide new perspectives for hyperpolarized 129Xe NMR/MRI. Initial publications about 129Xe DNP underlined the increased complexity in the sample preparation and lower polarization levels when compared to more conventional 13C-labeled molecules, at same experimental conditions, despite very close gyromagnetic ratios. Herein, we introduce, using a Custom Fluid Path system, a user-friendly and very robust sample preparation method. Moreover, investigating the radical properties at real DNP conditions by means of LOngitudinal Detected Electron Spin Resonance, we discovered a dramatic shortening of the electron spin longitudinal relaxation time (T1e) of nitroxyl radicals in xenon DNP samples' matrices, with respect to more commonly used water:glycerol ones. Mitigating those challenges through microwave frequency modulation, we achieved over 20% 129Xe polarization without employing any deuterated solvent.

Microwave Fluorescence Detection of Spin Echoes

Counting the microwave photons emitted by an ensemble of electron spins when they relax radiatively has recently been proposed as a sensitive method for electron paramagnetic resonance spectroscopy, enabled by the development of operational single microwave photon detectors at millikelvin temperature. Here, we report the detection of spin echoes in the spin fluorescence signal. The echo manifests itself as a coherent modulation of the number of photons spontaneously emitted after a π/2_{X}-τ-π_{Y}-τ-π/2_{Φ} sequence, dependent on the relative phase Φ. We demonstrate experimentally this detection method using an ensemble of Er^{3+} ion spins in a scheelite crystal of CaWO_{4}. We use fluorescence-detected echoes to measure the erbium spin coherence time, as well as the echo envelope modulation due to the coupling to the ^{183}W nuclear spins surrounding each ion. We finally compare the signal-to-noise ratio of inductively detected and fluorescence-detected echoes, and show that it is larger with the fluorescence method.

Implementation and applications of shaped pulses in EPR

In this review, we describe the application of shaped pulses for EPR spectroscopy. Pulses generated by fast arbitrary waveform generators are mostly used in the field of EPR spectroscopy for broadband (200 MHz-1 GHz) excitation of paramagnetic species. The implementation and optimization of such broadband pulses in existing EPR spectrometers, often designed and optimized for short rectangular microwave pulses, is demanding. Therefore, a major part of this review will describe in detail the implementation, testing and optimization of shaped pulses in existing EPR spectrometers. Additionally, we review applications using such pulses for broadband inversion of longitudinal magnetization as well as for the creation and manipulation of transverse magnetization in the field of dipolar and hyperfine EPR spectroscopy. They demonstrate the great potential of shaped pulses to improve the performance of pulsed EPR experiments. We give a brief theoretical description of shaped pulses and their limitations, especially for adiabatic pulses, most often used in EPR. We believe that this review can on the one hand be of practical use to EPR groups starting to work with such pulses, and on the other hand give readers an overview of the state of the art of shaped pulse applications in EPR spectroscopy.

Twenty-three-millisecond electron spin coherence of erbium ions in a natural-abundance crystal

Erbium ions embedded in crystals have unique properties for quantum information processing, because of their optical transition at 1.5 μm and of the large magnetic moment of their effective spin-1/2 electronic ground state. Most applications of erbium require, however, long electron spin coherence times, and this has so far been missing. Here, by selecting a host matrix with a low nuclear-spin density (CaWO₄) and by quenching the spectral diffusion due to residual paramagnetic impurities at millikelvin temperatures, we obtain a 23-ms coherence time on the Er³⁺ electron spin transition. This is the longest Hahn echo electron spin coherence time measured in a material with a natural abundance of nuclear spins and on a magnetically sensitive transition. Our results establish Er³⁺:CaWO₄ as a potential platform for quantum networks.

Electron paramagnetic resonance of lanthanides

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd³⁺. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides. Magnetic resonance imaging contrast agents use the long relaxation time of the S-state Gd³⁺ ion, and NMR shift reagents use the fast relaxation of selected other lanthanides. Other than Gd³⁺ and the isoelectronic Eu²⁺ ion, spin relaxation of the lanthanides is so fast that their EPR spectra can be observed only in the liquid helium temperature range. In this chapter the EPR properties of each of the lanthanides is briefly summarized, with an emphasis on electron spin relaxation.

Hyperfine spectroscopy in a quantum-limited spectrometer

We report measurements of electron-spin-echo envelope modulation (ESEEM) performed at millikelvin temperatures in a custom-built high-sensitivity spectrometer based on superconducting micro-resonators. The high quality factor and small mode volume (down to 0.2 pL) of the resonator allow us to probe a small number of spins, down to 5 × 10 2 . We measure two-pulse ESEEM on two systems: erbium ions coupled to 183 W nuclei in a natural-abundance CaWO 4 crystal and bismuth donors coupled to residual 29 Si nuclei in a silicon substrate that was isotopically enriched in the 28 Si isotope. We also measure three- and five-pulse ESEEM for the bismuth donors in silicon. Quantitative agreement is obtained for both the hyperfine coupling strength of proximal nuclei and the nuclear-spin concentration.

Observation of an environmentally insensitive solid-state spin defect in diamond

Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid-state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV⁰). Through careful materials engineering, we achieved >80% conversion of implanted silicon to SiV⁰ SiV⁰ exhibits spin-lattice relaxation times approaching 1 minute and coherence times approaching 1 second. Its optical properties are very favorable, with ~90% of its emission into the zero-phonon line and near-transform-limited optical linewidths. These combined properties make SiV⁰ a promising defect for quantum network applications.

Enhancing quantum sensing sensitivity by a quantum memory

In quantum sensing, precision is typically limited by the maximum time interval over which phase can be accumulated. Memories have been used to enhance this time interval beyond the coherence lifetime and thus gain precision. Here, we demonstrate that by using a quantum memory an increased sensitivity can also be achieved. To this end, we use entanglement in a hybrid spin system comprising a sensing and a memory qubit associated with a single nitrogen-vacancy centre in diamond. With the memory we retain the full quantum state even after coherence decay of the sensor, which enables coherent interaction with distinct weakly coupled nuclear spin qubits. We benchmark the performance of our hybrid quantum system against use of the sensing qubit alone by gradually increasing the entanglement of sensor and memory. We further apply this quantum sensor-memory pair for high-resolution NMR spectroscopy of single ¹³C nuclear spins.

In quantum sensing, memories have been used to enhance measurement precision. Here, the authors demonstrate the use of a memory to increase sensitivity of single ¹³C nuclear spins spectroscopy by storing the full sensor state and entangling memory and sensor.

RIDME distance measurements using Gd(iii) tags with a narrow central transition

Methods based on pulse electron paramagnetic resonance allow measurement of the electron–electron dipolar coupling between two high-spin labels.

Electron Spin Coherence of Shallow Donors in Natural and Isotopically Enriched Germanium

Germanium is a widely used material for electronic and optoelectronic devices and recently it has become an important material for spintronics and quantum computing applications. Donor spins in silicon have been shown to support very long coherence times (T_{2}) when the host material is isotopically enriched to remove any magnetic nuclei. Germanium also has nonmagnetic isotopes so it is expected to support long T_{2}'s while offering some new properties. Compared to Si, Ge has a strong spin-orbit coupling, large electron wave function, high mobility, and highly anisotropic conduction band valleys which will all give rise to new physics. In this Letter, the first pulsed electron spin resonance measurements of T_{2} and the spin-lattice relaxation (T_{1}) times for ^{75}As and ^{31}P donors in natural and isotopically enriched germanium are presented. We compare samples with various levels of isotopic enrichment and find that spectral diffusion due to ^{73}Ge nuclear spins limits the coherence in samples with significant amounts of ^{73}Ge. For the most highly enriched samples, we find that T_{1} limits T_{2} to T_{2}=2T_{1}. We report an anisotropy in T_{1} and the ensemble linewidths for magnetic fields oriented along different crystal axes but do not resolve any angular dependence to the spectral-diffusion-limited T_{2} in samples with ^{73}Ge.

Advanced paramagnetic resonance spectroscopies of iron-sulfur proteins: Electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM)

The advanced electron paramagnetic resonance (EPR) techniques, electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, provide unique insights into the structure, coordination chemistry, and biochemical mechanism of nature's widely distributed iron-sulfur cluster (FeS) proteins. This review describes the ENDOR and ESEEM techniques and then provides a series of case studies on their application to a wide variety of FeS proteins including ferredoxins, nitrogenase, and radical SAM enzymes. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Correlating nuclear frequencies by two-dimensional ELDOR-detected NMR spectroscopy

ELDOR (Electron Double Resonance)-detected NMR (EDNMR) is a pulse EPR experiment that is used to measure the transition frequencies of nuclear spins coupled to electron spins. These frequencies are further used to determine hyperfine and quadrupolar couplings, which are signatures of the electronic and spatial structures of paramagnetic centers. In recent years, EDNMR has been shown to be particularly useful at high fields/high frequencies, such as W-band (∼95 GHz, ∼3.5 T), for low γ quadrupolar nuclei. Although at high fields the nuclear Larmor frequencies are usually well resolved, the limited resolution of EDNMR still remains a major concern. In this work we introduce a two dimensional, triple resonance, correlation experiment based on the EDNMR pulse sequence, which we term 2D-EDNMR. This experiment allows circumventing the resolution limitation by spreading the signals in two dimensions and the observed correlations help in the assignment of the signals. First we demonstrate the utility of the 2D-EDNMR experiment on a nitroxide spin label, where we observe correlations between (14)N nuclear frequencies. Negative cross-peaks appear between lines belonging to different MS electron spin manifolds. We resolved two independent correlation patterns for nuclear frequencies arising from the EPR transitions corresponding to the (14)N mI=0 and mI=-1 nuclear spin states, which severely overlap in the one dimensional EDNMR spectrum. The observed correlations could be accounted for by considering changes in the populations of energy levels that S=1/2, I=1 spin systems undergo during the pulse sequence. In addition to these negative cross-peaks, positive cross-peaks appear as well. We present a theoretical model based on the Liouville equation and use it to calculate the time evolution of populations of the various energy levels during the 2D-EDNMR experiment and generated simulated 2D-EDMR spectra. These calculations show that the positive cross-peaks appear due to off resonance effects and/or nuclear relaxation effects. These results suggest that the 2D-EDNMR experiment can be also useful for relaxation pathway studies. Finally we present preliminary results demonstrating that 2D-EDNMR can resolve overlapping (33)S and (14)N signals of type 1 Cu(II) center in (33)S enriched Azurin.

Disorder influenced absorption line shapes of a chromophore coupled to two-level systems

We have carried out a theoretical and numerical study of disorder-induced changes in the absorption line shape of a chromophore embedded in a host matrix. The stochastic sudden jump model is employed wherein the host matrix molecules are treated as noninteracting two-level systems (TLSs) occupying points on a three-dimensional lattice with randomly oriented dipole moments. By systematically controlling the degree of positional disorder (α) attributed to them, a perfectly crystalline (α = 0) or a glassy environment (α = 1) or a combination of the two is obtained. The interaction between the chromophore and the TLSs is assumed to be of the dipole-dipole form. With an increase in α, the broadening of the absorption line shape was found to follow a power-law behavior. More importantly, it is revealed in the long-time limit that the resultant line shape is Gaussian in the absence of disorder but transforms to Lorentzian for a completely disordered environment. For an arbitrarily intermediate value of α, the resultant line shape can be approximately fitted by a linear combination of Gaussian and Lorentzian components. The Lorentzian profile for the disordered medium is attributed to the chomophore-TLS pairs with vanishingly small separation between them.

Solid-phase polarization matrixes for dynamic nuclear polarization from homogeneously distributed radicals in mesostructured hybrid silica materials

Mesoporous hybrid silica-organic materials containing homogeneously distributed stable mono- or dinitroxide radicals covalently bound to the silica surface were developed as polarization matrixes for solid-state dynamic nuclear polarization (DNP) NMR experiments. For TEMPO-containing materials impregnated with water or 1,1,2,2-tetrachloroethane, enhancement factors of up to 36 were obtained at ∼100 K and 9.4 T without the need for a glass-forming additive. We show that the homogeneous radical distribution and the subtle balance between the concentration of radical in the material and the fraction of radicals at a sufficient inter-radical distance to promote the cross-effect are the main determinants for the DNP enhancements we obtain. The material, as well as an analogue containing the poorly soluble biradical bTUrea, is used as a polarizing matrix for DNP NMR experiments of solutions containing alanine and pyruvic acid. The analyte is separated from the polarization matrix by simple filtration.

Relaxation and modulation interference effects in two-pulse electron spin echo envelope modulation (ESEEM)

Two-pulse electron spin echo envelope modulation (ESEEM) line widths are influenced by transverse electron spin relaxation, which is in turn induced by local field fluctuations. Simultaneous analysis of the decays of the unmodulated and modulated parts of the ESEEM signal provides deeper insight into the relaxation of a spin system consisting of an electron spin 1/2 coupled to N(I) nuclei with spin 1/2. Standard two-pulse ESEEM formulas either do not account for relaxation or assume uniform relaxation for all lines. In general, the relaxation rates on allowed and forbidden transitions may not be the same. Experimental results obtained on a single crystal of Cu(II)-doped L-histidine suggest that such a difference exists. Theoretical considerations show that in such a case the product rule for two-pulse ESEEM does not extend to expressions including relaxation. Product rules in general do not properly account for relaxation in three-pulse ESEEM and HYSCORE experiments. Decay of the apparently non-oscillatory part of the two-pulse echo may be strongly affected by modulation interference. Such interference of difference frequencies of matrix nuclei may cause a rather flat initial feature, which was previously attributed solely to non-exponential phase relaxation of electron spin transitions due to spin diffusion of the matrix nuclei. In addition, the sometimes observed drastic initial decay of the time domain signal is related to modulation interference of multiple-quantum coherences that arise from a strong cross-suppression effect.

Integrated refocused virtual ESEEM: detection of nuclear transition spectra without dead time and blind spots

General expressions describing the refocused stimulated (RS) and refocused virtual (RV) electron spin echo envelope modulations (ESEEM) generated with the same basic four-pulse sequence are derived. It is shown that integration of the 3D time domain trace over the two "low-resolution" time intervals (those between the first and second and between the third and fourth microwave pulses) results in a dead time-free 1D ESEEM trace in the "high-resolution" dimension (i.e., the time interval between the second and third microwave pulses) that only contains harmonics with the fundamental frequencies of nuclear transitions. The practical implementation of the integrated RS ESEEM requires pulse swapping, which leads to unrecoverable distortions in the ESEEM traces and the resulting spectra. The integrated RV ESEEM is free from such distortions and represents a robust practical technique for obtaining dead time- and blind spots-free spectra of nuclear transitions, without homonuclear combination lines. As an application example, the integrated RV ESEEM was used to obtain the spectrum of a strongly-coupled proton of the OH ligand of the Mo(V) active center of the low-pH form of the molybdoenzyme sulfite oxidase.

Structure and dynamics of nucleic acids

In this chapter we describe the application of CW and pulsed EPR methods for the investigation of structural and dynamical properties of RNA and DNA molecules and their interaction with small molecules and proteins. Special emphasis will be given to recent applications of dipolar spectroscopy on nucleic acids.

Pulsed EPR spectroscopy: biological applications

Pulsed electron paramagnetic resonance (EPR) methods such as ESEEM, PELDOR, relaxation time measurements, transient EPR, high-field/high-frequency EPR, and pulsed ENDOR, have been used successfully to investigate the local structure and dynamics of paramagnetic centers in biological samples. These methods allow different contributions to the EPR spectra to be distinguished and can help unravel complicated EPR spectra consisting of overlapping resonance lines, as are often found in disordered protein samples. The basic principles, specific potentials, technical requirements, and limitations of these advanced EPR techniques will be reviewed together with recent applications to metal centers, organic radicals, and spin labels in proteins.

New technologies in electron spin resonance

New electron spin resonance (ESR) technologies have been developed, which have led to new and improved applications. (a) The development of two-dimensional Fourier transform (FT) ESR required spectrometers providing intense pi/2 microwave pulses of very short (3-5 ns) duration, wide bandwidths, and very short dead times. It has enabled studies that resolve sophisticated details of molecular dynamics in complex fluids. (b) Methods that produce multiple quantum coherences by pulsed ESR now enable accurate measurements of large distances (>12A). (c) One of the most important advances has been the extension of ESR to high magnetic fields and high frequencies. This has benefited from the utilization of quasi-optical methods, especially above 150 GHz. The greatly improved orientational resolution and the faster "snapshot" of motions that are provided by ESR at high frequencies enhance studies of molecular dynamics. The use of both high and lower frequencies enables one to unravel faster and slower modes from the complex dynamics of fluids and macromolecules. (d) The development of FT-ESR imaging required substantial pulsed field gradients lasting only 50-100 ns. ESR imaging is effective in studying diffusion in fluids. Areas for further development are also described.

High-field DNP and ENDOR with a novel multiple-frequency resonance structure

We describe a new triply tuned (e(-), (1)H, and (13)C) resonance structure operating at an electron Larmor frequency of 139.5 GHz for dynamic nuclear polarization (DNP) and electron nuclear double-resonance (ENDOR) experiments. In contrast to conventional double-resonance structures, the body of the microwave cavity simultaneously acts as a NMR coil, allowing for increased efficiency of radiofrequency irradiation while maintaining a high quality factor for microwave irradiation. The resonator design is ideal for low-gamma-nuclei ENDOR, where sensitivity is limited by the fact that electron spin relaxation times are on the order of the RF pulse lengths. The performance is demonstrated with (2)H ENDOR on a standard perdeuterated bis-diphenylene-phenyl-allyl stable radical. In DNP experiments, we show that the use of this resonator, combined with a low microwave power setup (17 mW), leads to significantly higher (1)H signal enhancement (epsilon approximately 400 +/- 50) than previously achieved at 5-T fields. The results emphasize the importance of optimizing the microwave B(1) field by improving either the quality factor of the microwave resonator or the microwave power level.

Principles and performance of an electron spin echo spectrometer using far infrared lasers as excitation sources

We have built an electron spin echo spectrometer operating at 604 GHz, extending the frequency limit of existing spectrometers by more than a factor of 4. In order to handle this high frequency we have used optical techniques, i.e., molecular gas lasers for the excitation pulses and far infrared techniques for the heterodyne detection system. The different components of the spectrometer are described in detail and first experimental results are given.