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1. Order-of-magnitude SNR improvement for high-field EPR spectrometers via 3D printed quasi-optical sample holders

   https://doi.org/10.1126/sciadv.adi7412  

   Sojka, Antonín; Price, Brad D; Sherwin, Mark S (September 2023, Science Advances)

   Here, we present a rapidly prototyped, cost-efficient, and 3D printed quasi-optical sample holder for improving the signal-to-noise ratio (SNR) in modern, resonator-free, and high-field electron paramagnetic resonance (HFEPR) spectrometers. Such spectrometers typically operate in induction mode: The detected EPR (“cross-polar”) signal is polarized orthogonal to the incident (“co-polar”) radiation. The sample holder makes use of an adjustable sample positioner that allows for optimizing the sample position to maximize the 240-gigahertz magnetic fieldB₁and a rooftop mirror that allows for small rotations of the microwave polarization to maximize the cross-polar signal and minimize the co-polar background. When optimally tuned, the sample holder was able to improve co-polar isolation by ≳20 decibels, which is proven beneficial for maximizing the SNR in rapid-scan, pulsed, and continuous-wave EPR experiments. In rapid-scan mode, the improved SNR enabled the recording of entire EPR spectra of a narrow-line radical in millisecond time scales, which, in turn, enabled real-time monitoring of a sample’s evolving line shape. 

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2. Time domain measurement of electron spin relaxation at high fields and dynamic nuclear polarization at sub-millimeter wavelengths

   https://doi.org/10.1109/MWSYM.2017.8058878  

   Dubroca, Thierry; McKay, Johannes; Wang, Xiaoling; van Tol, Johan (June 2017, 2017 IEEE MTT-S International Microwave Symposium (IMS))

   We describe a 395 GHz pulsed electron paramagnetic resonance (EPR) setup, and initial results of relaxation measure-ments and cw EPR at these frequencies in samples used for liquid- and solid-state nuclear magnetic resonance enhanced by dynamic nuclear polarization (DNP). Depending on the amount of spin –orbit coupling, the spin lattice relaxation becomes significantly faster at higher fields and frequencies, which has consequences for some DNP applications at high fields and frequencies. We will dis-cuss the requirements for (sub)millimeter-wave sources and com-ponents for DNP and pulsed EPR at even higher frequencies and fields, as even higher magnetic fields will become available in the near future. 

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3. Gd ³⁺ –Gd ³⁺ distances exceeding 3 nm determined by very high frequency continuous wave electron paramagnetic resonance

   https://doi.org/10.1039/c6cp07119h  

   Clayton, Jessica A.; Qi, Mian; Godt, Adelheid; Goldfarb, Daniella; Han, Songi; Sherwin, Mark S. (January 2017, Physical Chemistry Chemical Physics)

   Electron paramagnetic resonance spectroscopy in combination with site-directed spin labeling is a very powerful tool for elucidating the structure and organization of biomolecules. Gd 3+ complexes have recently emerged as a new class of spin labels for distance determination by pulsed EPR spectroscopy at Q- and W-band. We present CW EPR measurements at 240 GHz (8.6 Tesla) on a series of Gd-rulers of the type Gd-PyMTA–spacer–Gd-PyMTA, with Gd–Gd distances ranging from 1.2 nm to 4.3 nm. CW EPR measurements of these Gd-rulers show that significant dipolar broadening of the central |−1/2〉 → |1/2〉 transition occurs at 30 K for Gd–Gd distances up to ∼3.4 nm with Gd-PyMTA as the spin label. This represents a significant extension for distances accessible by CW EPR, as nitroxide-based spin labels at X-band frequencies can typically only access distances up to ∼2 nm. We show that this broadening persists at biologically relevant temperatures above 200 K, and that this method is further extendable up to room temperature by immobilizing the sample in glassy trehalose. We show that the peak-to-peak broadening of the central transition follows the expected 1/ r 3 dependence for the electron–electron dipolar interaction, from cryogenic temperatures up to room temperature. A simple procedure for simulating the dependence of the lineshape on interspin distance is presented, in which the broadening of the central transition is modeled as an S = 1/2 spin whose CW EPR lineshape is broadened through electron–electron dipolar interactions with a neighboring S = 7/2 spin. 

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4. A modernized view of coherence pathways applied to magnetic resonance experiments in unstable, inhomogeneous fields

   https://doi.org/10.1063/5.0105388  

   Beaton, Alec A.; Guinness, Alexandria; Franck, John M. (November 2022, The Journal of Chemical Physics)

   This article presents a standardized alternative to the traditional phase cycling approach employed by the overwhelming majority of contemporary Nuclear Magnetic Resonance (NMR) research. On well-tested, stable NMR systems running well-tested pulse sequences in highly optimized, homogeneous magnetic fields, the hardware and/or software responsible for traditional phase cycling quickly isolate a meaningful subset of data by averaging and discarding between 3/4 and 127/128 of the digitized data. In contrast, the new domain colored coherence transfer (DCCT) approach enables the use of all the information acquired from all transients. This approach proves to be particularly useful where multiple coherence pathways are required, or for improving the signal when the magnetic fields are inhomogeneous and unstable. For example, the authors’ interest in the nanoscale heterogeneities of hydration dynamics demands increasingly sophisticated and automated measurements deploying Overhauser Dynamic Nuclear Polarization (ODNP) in low-field electromagnets, where phase cycling and signal averaging perform suboptimally. This article demonstrates the capabilities of DCCT on ODNP data and with a collection of algorithms that provide robust phasing, avoidance of baseline distortion, and the ability to realize relatively weak signals amid background noise through signal-averaged correlation alignment. The DCCT schema works by combining a multidimensional organization of phase cycled data with a specific methodology for visualizing the resulting complex-valued data. It could be extended to other forms of coherent spectroscopy seeking to analyze multiple coherence transfer pathways. 

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5. Terahertz electron paramagnetic resonance generalized spectroscopic ellipsometry: The magnetic response of the nitrogen defect in 4H-SiC

   https://doi.org/10.1063/5.0082353  

   Schubert, Mathias; Knight, Sean; Richter, Steffen; Kühne, Philipp; Stanishev, Vallery; Ruder, Alexander; Stokey, Megan; Korlacki, Rafał; Irmscher, Klaus; Neugebauer, Petr; et al (March 2022, Applied Physics Letters)

   We report on terahertz (THz) electron paramagnetic resonance generalized spectroscopic ellipsometry (THz-EPR-GSE). Measurements of field and frequency dependencies of magnetic response due to spin transitions associated with nitrogen defects in 4H-SiC are shown as an example. THz-EPR-GSE dispenses with the need of a cavity, permits independently scanning field and frequency parameters, and does not require field or frequency modulation. We investigate spin transitions of hexagonal ( h) and cubic ( k) coordinated nitrogen including coupling with its nuclear spin (I = 1), and we propose a model approach for the magnetic susceptibility to account for the spin transitions. From the THz-EPR-GSE measurements, we can fully determine polarization properties of the spin transitions, and we can obtain the k coordinated nitrogen g and hyperfine splitting parameters using magnetic field and frequency dependent Lorentzian oscillator line shape functions. Magnetic-field line broadening presently obscures access to h parameters. We show that measurements of THz-EPR-GSE at positive and negative fields differ fundamentally and hence provide additional information. We propose frequency-scanning THz-EPR-GSE as a versatile method to study properties of spins in solid state materials. 

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