Quadrupole Hyperfine Interaction Research Articles

Increased sensitivity in electron–nuclear double resonance spectroscopy with chirped radiofrequency pulses

Abstract. Electron–nuclear double resonance (ENDOR) spectroscopy is an EPR technique to detect the nuclear frequency spectra of hyperfine coupled nuclei close to paramagnetic centers, which have interactions that are not resolved in continuous wave EPR spectra and may be fast relaxing on the timescale of NMR. For the common case of non-crystalline solids, such as powders or frozen solutions of transition metal complexes, the anisotropy of the hyperfine and nuclear quadrupole interactions renders ENDOR lines often several megahertz (MHz) broad, thus diminishing intensity. With commonly used ENDOR pulse sequences, only a small fraction of the NMR/ENDOR line is excited with a typical radiofrequency (RF) pulse length of several tens of microseconds (µs), and this limits the sensitivity in conventional ENDOR experiments. In this work, we show the benefit of chirped RF excitation in frequency-domain ENDOR as a simple yet effective way to significantly improve sensitivity. We demonstrate on a frozen solution of Cu(II)-tetraphenylporphyrin that the intensity of broad copper and nitrogen ENDOR lines increases up to 9-fold compared to single-frequency RF excitation, thus making the detection of metal ENDOR spectra more feasible. The tunable bandwidth of the chirp RF pulses allows the operator to optimize for sensitivity and choose a tradeoff with resolution, opening up options previously inaccessible in ENDOR spectroscopy. Also, chirp pulses help to reduce RF amplifier overtones, since lower RF powers suffice to achieve intensities matching conventional ENDOR. In 2D triple resonance experiments (TRIPLE), the signal increase exceeds 10 times for some lines, thus making chirped 2D TRIPLE experiments feasible even for broad peaks in manageable acquisition times.

Ka-Band Rotational Spectroscopy of Succinimide and N-Chlorosuccinimide.

Succinimide and its derivatives are cyclic five-membered rings that appear in a variety of natural products and are widely used in organic synthesis. From a structural standpoint, succinimide contains an NH group in the ring which interacts with two adjacent carbonyl groups, pushing the ring structure toward planarity at the expense of increasing ring strain and eclipsing interactions among the out-of-plane hydrogen atoms in the two CH2 groups. Previous quantum chemical calculations at different levels of theory have predicted both a nonplanar C2 structure and a planar C2v structure, the latter of which is the most consistent with gas-phase electron diffraction measurements. Here, we report the pure rotational spectra of succinimide and N-chlorosuccinimide in the 26.5-40.0 GHz range using chirped-pulse Fourier transform microwave spectroscopy, supported by coupled cluster and density functional theory quantum chemical calculations. The spectra were fit to Watson's A-reduced effective Hamiltonian, including both 35Cl and 37Cl isotopologues of N-chlorosuccinimide as well as the N and Cl quadrupole hyperfine interactions. On the basis of the agreement with quantum chemical calculations and the measured inertial defects, we find that the rotational spectra are consistent with a planar ring structure, with a maximum out-of-plane angle of ≤5°.

The magnetic structure and spin-reorientation of ErGa.

The magnetic structure of the intermetallic compound ErGa has been determined using high-resolution neutron powder diffraction. This compound crystallizes in the orthorhombic (Cmcm, No. 63) CrB-type structure and orders ferromagnetically at 32 (2) K, with the Er moments initially aligned along the b axis. Upon cooling below 16 K, the Er magnetic moments cant away from the b axis towards the c axis. At 3 K, the Er moment is 8.7 (3) μB and the Er magnetic moments point in the direction 31 (3)° away from the crystallographic b axis, within the bc plane. 166Er Mössbauer spectroscopy work supports this structure and shows clear signals of the spin-reorientation in both the magnetic and electric quadrupole hyperfine interactions.

14N Hyperfine and nuclear interactions of axial and basal NV centers in 4H-SiC: A high frequency (94 GHz) ENDOR study

The nitrogen-vacancy (NV) centers (NCVSi)− in 4H silicon carbide (SiC) constitute an ensemble of spin S = 1 solid state qubits interacting with the surrounding 14N and 29Si nuclei. As quantum applications based on a polarization transfer from the electron spin to the nuclei require the knowledge of the electron–nuclear interaction parameters, we have used high-frequency (94 GHz) electron–nuclear double resonance spectroscopy combined with first-principles density functional theory to investigate the hyperfine and nuclear quadrupole interactions of the basal and axial NV centers. We observed that the four inequivalent NV configurations (hk, kh, hh, and kk) exhibit different electron–nuclear interaction parameters, suggesting that each NV center may act as a separate optically addressable qubit. Finally, we rationalized the observed differences in terms of distinctions in the local atomic structures of the NV configurations. Thus, our results provide the basic knowledge for an extension of quantum protocols involving the 14N nuclear spin.

The Parameters of the Hyperfine Structure of 57Fe Nuclei in FeBO3 and Fe0.91Ga0.09BO3 Single Crystals

The parameters of the hyperfine interaction of 57Fe nuclei in single crystals of iron borate FeBO3 and its isostructural solid solution Fe0.91Ga0.09BO3 have been determined by Mössbauer spectroscopy. A theoretical model has been developed to describe resonant transitions of iron nuclei in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction, taking into account the statistical distribution of Ga and Fe over octahedral positions in the Fe0.91Ga0.09BO3 crystal. It has been established that even a low Ga concentration leads to a significant change in the hyperfine structure of the 57Fe nuclei inFeBO3, which manifests itself in the appearance of additional components in the Mössbauer spectra of the Fe0.91Ga0.09BO3 single crystal.

Ab initio calculations of the hyperfine structure of 109Cd, 109Cd+ and reevaluation of the nuclear quadrupole moment Q(109Cd)

Large scale ab initio calculations of the electric contribution (i.e. the electric field gradient (EFG)) to the electric quadrupole hyperfine interaction constants B for the state of 109Cd+ ion and the states of neutral 109Cd atom were performed. To probe the sensitivity of B to different electron correlation effects, six sets of variational multi-configuration Dirac–Hartree–Fock and relativistic configuration interaction calculations employing different strategies were carried out. The calculated EFGs, together with experimental values of B, allow us to extract a new value of the nuclear electric quadrupole moment Q(109Cd) = 0.634(22) b which is about 5% larger than the recommended value (Stone 2016 At. Data Nucl. Data Tables 111–112 1; Pyykkö 2018 Mol. Phys. 116 1328). Efforts were made to provide a realistic theoretical uncertainty for the final Q value based on statistical principles and correlation with the magnetic dipole hyperfine interaction constant A.

Magnetic and electric hyperfine parameters of antiferromagnet FеBO3 intended for monochromatization of synchrotron radiation

In this work, Mössbauer spectroscopy and X-ray diffraction were used to determine the precision values of the hyperfine interaction parameters and the crystal structure of FeBO3 single crystals in a wide temperature range, including the region of magnetic phase transitions (TN). A theoretical model has been developed to describe nuclear resonance transitions in iron atoms in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction. A technique for correcting the Mössbauer spectra, considering the effective thickness of the absorber, has also been elaborated.It is shown that the appearance of two additional resonance lines in the hyperfine structure significantly affects the shape of the FeBO3 spectra near the N é el temperature (ТN). The characteristic Debye temperatures for Fe and B cations in the iron borate structure have been determined. The developed technique and the results obtained are extremely important for the use of FeBO3 crystals in new high-tech branches of science and technology, including optoelectronics and synchrotron technologies.

Imaging and sensing of pH and chemical state with nuclear-spin-correlated cascade gamma rays via radioactive tracer

Single-photon-emission computed tomography (SPECT) and positron-emission tomography (PET) are highly sensitive molecular detection and imaging techniques that generally measure accumulation of radio-labeled molecules by detecting gamma rays. Quantum sensing of local molecular environment via spin, such as nitrogen vacancy (NV) centers, has also been reported. Here, we describe quantum sensing and imaging using nuclear-spin time-space correlated cascade gamma-rays via a radioactive tracer. Indium-111 (111In) is widely used in SPECT to detect accumulation using a single gamma-ray photon. The time-space distribution of two successive cascade gamma-rays emitted from an 111In atom carries significant information on the chemical and physical state surrounding molecules with double photon coincidence detection. We propose and demonstrate quantum sensing capability of local micro-environment (pH and chelating molecule) in solution along with radioactive tracer accumulation imaging, by using multiple gamma-rays time-and-energy detection. Local molecular environment is extracted through electric quadrupole hyperfine interaction in the intermediate nuclear spin state by the explicit distribution of sub-MeV gamma rays. This work demonstrates a proof of concept, and further work is necessary to increase the sensitivity of the technique for in vivo imaging and to study the effect of scattered radiation for possible application in nuclear medicine.

Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

The hydrogen bond plays a key role in weak directional intermolecular interactions. It is operative in determining molecular conformation and aggregation, and controls the function of many chemical systems, ranging from inorganic, organic to biological molecules. Although an enormous amount of spectroscopic information has been collected about hydrogen-bond formation between molecules with closed-shell electronic configuration, the details of such interactions between open-shell radicals and closed-shell molecules are still rare. Here we report on an investigation of hydrogen-bonded complexes between pyrroline-type as well as piperidine-type neutral nitroxide radicals and an alcohol, i.e., 2-propanol. These nitroxide radicals are commonly used as EPR spin labels and probes. To obtain information on the geometry of the complexes and their electronic structure, multi-resonance EPR techniques at various microwave frequencies (X-, Q-, W-band, 244 GHz) have been employed in conjunction with DFT calculations. The planar five-membered ring system of the pyrroline-type nitroxide radical was found to form exclusively well-defined in-plane σ-type hydrogen-bonded complexes with one 2-propanol molecule in the first solvation shell in frozen solution. The measured hyperfine parameters of the hydrogen-bridge proton and the internal magnetic parameters describing the electron Zeeman and the electron-nuclear hyperfine and nuclear quadrupole interactions are in good agreement with values predicted by state-of-the-art DFT calculations. In contrast, multi-resonance EPR on the non-planar six-membered ring system of the piperidine-type nitroxide radical (TEMPOL) reveals a more complex situation, i.e., a mixture of a σ-type with, presumably, an out-of-plane π-type complex, both present in comparable fraction in frozen solution. For TEMPOL, the DFT calculations failed to predict magnetic interaction parameters that are in good agreement with experiment, apparently due to the considerable flexibility of the nitroxide and hydrogen-bonded complex. The detailed information about nitroxide/solvent complexes is of particular importance for Dynamic Nuclear Polarization (DNP) and site-directed spin-labeling EPR studies that employ nitroxides as polarizing agents or spin labels, respectively.

Measurements of the Hyperfine Structure of Atomic Energy Levels in Co ii

Abstract Analysis of hyperfine structure constants of singly ionized cobalt (Co II) were performed on cobalt spectra measured by Fourier transform spectrometers in the region 3000–63,000 cm−1 (33333 – 1587 Å). Fits to over 700 spectral lines led to measurements of 292 magnetic dipole hyperfine interaction A constants, with values between −32.5 mK and 59.5 mK (1 mK = 0.001 cm−1). Uncertainties of 255 A constants were between ±0.4 mK and ±3.0 mK, the remaining 37 ranged up to ±7 mK. The electric quadrupole hyperfine interaction B constant could be estimated for only one energy level. The number of Co II levels with known A values has now increased tenfold, improving and enabling the wider, more reliable, and accurate application of Co II in astronomical chemical abundance analyses.

Isolated Ti(III) Species on the Surface of a Pre-active Ziegler Natta Catalyst

The nature of Ti(III) species, introduced in working models of industrial Ziegler Natta catalyst precursors, consisting of MgCl2/TiCl4 binary systems, eventually containing different Lewis basis, are studied by a combination of X- and Q-band CW and pulse EPR spectroscopy. In Ziegler Natta catalysts, Ti(III) play the double role of active catalytic species and unconventional spin probes. On the binary system, two dominant Ti(III) species, characterized by distinctively different EPR spectra, are observed. 35,37Cl Q-Band HYSCORE spectra allow estimating the hyperfine and nuclear quadrupole interactions of directly coordinated Cl, characterized by a hyperfine dipolar contribution of the order of 5 MHz and nuclear quadrupole interactions of the order of e2qQ/h = 9 MHz. Interestingly, the two dominant EPR active species are selectively suppressed by the presence of different Lewis bases, indicating the possibility to address the long standing issue of the influence of Lewis bases in driving specific morphological configurations and influencing the catalytic properties of Ti(III) active sites.

Hyperfine and quadrupole interactions for Dy isotopes in DyPc2 molecules

Nuclear spin levels play an important role in understanding magnetization dynamics and implementation and control of quantum bits in lanthanide-based single-molecule magnets. We investigate the hyperfine and nuclear quadrupole interactions for 161Dy and 163Dy nuclei in anionic DyPc2 (Pc = phthalocyanine) single-molecule magnets, using multiconfigurational ab initio methods (beyond density-functional theory) including spin–orbit interaction. The two isotopes of Dy are chosen because the others have zero nuclear spin. Both isotopes have the nuclear spin I = 5/2, although the magnitude and sign of the nuclear magnetic moment differ from each other. The large energy gap between the electronic ground and first-excited Kramers doublets, allows us to map the microscopic hyperfine and quadrupole interaction Hamiltonian onto an effective Hamiltonian with an electronic pseudo-spin that corresponds to the ground Kramers doublet. Our ab initio calculations show that the coupling between the nuclear spin and electronic orbital angular momentum contributes the most to the hyperfine interaction and that both the hyperfine and nuclear quadrupole interactions for 161Dy and 163Dy nuclei are much smaller than those for the 159Tb nucleus in TbPc2 single-molecule magnets. The calculated separations of the electronic-nuclear levels are comparable to experimental data reported for 163DyPc2. We demonstrate that hyperfine interaction for the Dy Kramers ion leads to tunnel splitting (or quantum tunneling of magnetization) at zero field. This effect does not occur for TbPc2 single-molecule magnets. The magnetic field values of the avoided level crossings for 161DyPc2 and 163DyPc2 are found to be noticeably different, which can be observed from the experiment.

Structure of the 11/2− isomeric state in La133

We report measurement of the $g$-factor for the ${11/2}^{\ensuremath{-}}$ isomeric state at 535 keV in $^{133}\mathrm{La}$, employing the time differential perturbed angular distribution technique (TDPAD). This isomer was populated in the reaction $^{126}\mathrm{Te}(^{11}\mathrm{B}$, $4n)^{133}\mathrm{La}$ at beam energy of 52 MeV. From the observed nuclear spin precession, analysed through combined, magnetic dipole and electric quadrupole hyperfine interactions, we obtain the $g$ factor for the $11/{2}^{\ensuremath{-}}$ state as $g=1.16\ifmmode\pm\else\textpm\fi{}0.07$. In addition, this analysis provides the spectroscopic quadrupole moment $|Q|=1.71\ifmmode\pm\else\textpm\fi{}0.34\phantom{\rule{3.33333pt}{0ex}}b$, yielding the deformation parameter $\ensuremath{\beta}=0.28\ifmmode\pm\else\textpm\fi{}0.10$. Further, we have performed theoretical calculations using the large-scale shell model and the Monte Carlo shell model. The results successfully describe the low-lying levels and the band structures of $^{133}\mathrm{La}$, and the calculated $g$ factor compares well with the values obtained from our experiment. The dominant configuration of $11/{2}^{\ensuremath{-}}$ isomeric state in $^{133}\mathrm{La}$ is inferred to be $\ensuremath{\pi}({h}_{11/2})\ensuremath{\bigotimes}^{132}\mathrm{Ba}({0}^{+})$.

Nature of Hyperfine Interactions in TbPc2 Single-Molecule Magnets: Multiconfigurational Ab Initio Study.

Lanthanide-based single-ion magnetic molecules can have large magnetic hyperfine interactions as well as large magnetic anisotropy. Recent experimental studies reported tunability of these properties by changes of chemical environments or by application of external stimuli for device applications. In order to provide insight onto the origin and mechanism of such tunability, here we investigate the magnetic hyperfine and nuclear quadrupole interactions for a 159Tb nucleus in TbPc2 (Pc = phthalocyanine) single-molecule magnets using multiconfigurational ab initio methods including spin-orbit interaction. Since the electronic ground and first-excited (quasi)doublets are well separated in energy, the microscopic Hamiltonian can be mapped onto an effective Hamiltonian with an electronic pseudospin S = 1/2. From the ab initio calculated parameters, we find that the magnetic hyperfine coupling is dominated by the interaction of the Tb nuclear spin with electronic orbital angular momentum. The asymmetric 4f-like electronic charge distribution leads to a strong nuclear quadrupole interaction with significant transverse terms for the molecule with low symmetry. The ab initio calculated electronic-nuclear spectrum including the magnetic hyperfine and quadrupole interactions is in excellent agreement with the experiment. We further find that the transverse quadrupole interactions significantly influence the avoided level crossings in magnetization dynamics and that the molecular distortions affect mostly the Fermi contact terms as well as the transverse quadrupole interactions.

Revealing the hidden hyperfine interactions in ε -iron

Herein, evidence for the long-sought finite hyperfine interaction in the high-pressure hexagonal close-packed $\ensuremath{\epsilon}$-iron is gained through synchrotron radiation perturbed angular correlation spectroscopy. This method yields an energy splitting of $3.5(5)\phantom{\rule{4.pt}{0ex}}\text{neV}$ between the ${m}_{{I}_{\text{e}}}=\ifmmode\pm\else\textpm\fi{}1/2$ and ${m}_{{I}_{\text{e}}}=\ifmmode\pm\else\textpm\fi{}3/2$ nuclear sublevels of the iron-57 $14.412\text{-keV}$ nuclear excited state at $30(1)\phantom{\rule{4.pt}{0ex}}\text{GPa}$ and room temperature. This energy splitting is related to a nuclear quadrupole hyperfine interaction with an electric field gradient of $eq=1.2(2)\ifmmode\times\else\texttimes\fi{}{10}^{16}{\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm}}^{2}$. However, there is still a possibility that the splitting of the iron-57 nuclear levels is related to a modest magnetic hyperfine interaction of ca. $0.40(5)\phantom{\rule{4.pt}{0ex}}\text{T}$.

Experimental TDPAC and Theoretical DFT Study of Structural, Electronic, and Hyperfine Properties in (111In → )111Cd-Doped SnO2 Semiconductor: Ab Initio Modeling of the Electron-Capture-Decay After-Effects Phenomenon

In this paper we investigate the effect of Cd doping at ultralow concentrations in SnO2 both experimentally, by measuring the temperature dependence of the electric quadrupole hyperfine interactions with time-differential γ–γ perturbed angular correlation (TDPAC) spectroscopy using 111Cd as probe nuclei, and theoretically, by performing first-principles calculations based on the density functional theory. TDPAC spectra were successfully analyzed with a time-dependent on–off model for the perturbation factor. These results show combined dynamic plus static interactions whose electric-field-gradients were associated in this model to different stable electronic configurations close to the Cd atoms. The dynamic regime is then originated in fast fluctuations between these different electronic configurations. First-principles calculation results show that the Cd impurity introduces a double acceptor level in the top of the valence band of the doped semiconductor and produces isotropic outward relaxations of the nearest oxygen neighbors. The variation of the calculated electric-field gradient tensor as a function of the charge state of the Cd impurity level shows an interesting behavior that explains the experimental results, giving strong support from first-principles to the electron-capture after-effects proposed scenario. The electron-capture decay of the parent 111In to 111Cd as well as the double acceptor character of the 111Cd impurity and the electric nature of the host are shown to contribute to the existence of these types of time-dependent hyperfine interactions.

Dynamic quadrupole interactions in semiconductors

The time differential perturbed angular correlation, TDPAC, technique has been used for several decades to study electric quadrupole hyperfine interactions in semiconductors such as dynamic quadrupole interactions (DQI) resulting from after-effects of the nuclear decay as well as static quadrupole interactions originating from static defects around the probe nuclei such as interstitial ions, stresses in the crystalline structure, and impurities. Nowadays, the quality of the available semiconductor materials is much better, allowing us to study purely dynamic interactions. We present TDPAC measurements on pure Si, Ge, GaAs, and InP as a function of temperature between 12 K and 110 K. The probe 111In (111Cd) was used. Implantation damage was recovered by thermal annealing. Si experienced the strongest DQI with lifetime, τg, increasing with rising temperature, followed by Ge. In contrast, InP and GaAs, which have larger band gaps and less electron concentration than Si and Ge in the same temperature range, presented no DQI. The results obtained also allow us to conclude that indirect band gap semiconductors showed the dynamic interaction, whereas the direct band gap semiconductors, restricted to GaAs and InP, did not.

Theoretical study of the hyperfine-interaction constants and the isotope-shift factors for the 3s21S0–3s3p3,1P1o transitions in Al+

We calculated the magnetic dipole and the electric quadrupole hyperfine interaction constants of 3s3p $^{3,1}P^o_1$ states and the isotope shift, including mass and field shift, factors for transitions from these two states to the ground state 3s$^2~^1S_0$ in Al$^+$ ions using the multiconfiguration Dirac-Hartree-Fock method. The effects of the electron correlations and the Breit interaction on these physical quantities were investigated in detail based on the active space approach. It is found that the CC and the higher-order correlations are considerable for evaluating the uncertainties of the atomic parameters concerned. The uncertainties of the hyperfine interaction constants in this work are less than 1.5\%. Although the isotope shift factors are highly sensitive to the electron correlations, reasonable uncertainties were obtained by exploring the effects of the electron correlations. Moreover, we found that the relativistic nuclear recoil corrections to the mass shift factors are very small and insensitive to the electron correlations for Al$^{+}$. These atomic parameters present in this work are valuable for extracting the nuclear electric quadrupole moments and the mean-square charge radii of Al isotopes.

High-resolution magneto-optical spectroscopy ofLiYF47:Er3+167,Er3+166and analysis of hyperfine structure of ultranarrow optical transitions

We performed high-resolution magneto-optical spectroscopy of the hyperfine transitions from $^{4}I_{15/2}$ to the $^{4}I_{13/2}$ and $^{4}I_{9/2}$ multiplets of $^{167}\mathrm{Er}^{3+}$ and $^{166}\mathrm{Er}^{3+}$ in an isotopically purified $^{7}\mathrm{LiYF}_{4}$ crystal in various external magnetic fields up to 0.7 T. The obtained experimental results are interpreted in the framework of the generalized theoretical approach. The derived model successfully explains all the experimentally observed optical hyperfine transitions by using a single set of basic parameters found for the crystal-field interaction, magnetic dipole and electric quadrupole hyperfine interactions, together with Zeeman interactions at different orientations of the external magnetic field. A number of the studied quantum transitions appears to be promising for use in Raman quantum storage at optical telecommunication wavelengths.

Ligand Binding to Chlorite Dismutase from Magnetospirillum sp.

Chlorite dismutase (Cld) catalyzes the reduction of chlorite to chloride and dioxygen. Here, the ligand binding to Cld of Magnetospirillum sp. (MaCld) is investigated with X-ray crystallography and electron paramagnetic resonance (EPR). EPR reveals a large heterogeneity in the structure of wild-type MaCld, showing a variety of low- and high-spin ferric heme forms. Addition of an axial ligand, such as azide or imidazole, removes this heterogeneity almost entirely. This is in line with the two high resolution crystal structures of MaCld obtained in the presence of azide and thiocyanate that show the coordination of the ligands to the heme iron. The crystal structure of the MaCld-azide complex reveals a single well-defined orientation of the azide molecule in the heme pocket. EPR shows, however, a pH-dependent heme structure, probably due to acid-base transitions of the surrounding amino-acid residues stabilizing azide. For the azide and imidazole complex of MaCld, the hyperfine and nuclear quadrupole interactions with the close-by (14)N and (1)H nuclei are determined using pulsed EPR. These values are compared to the corresponding data for the low-spin forms observed in the ferric wild-type MaCld and to existing EPR data on azide and imidazole complexes of other heme proteins.