Nuclear Magnetic Hyperfine Research Articles

Observation of dynamic nuclear polarization echoes.

It is demonstrated that the time evolution of the electron-nuclear polarization transfer process during pulsed dynamic nuclear polarization (DNP) can be reversed on a microsecond timescale, leading to the observation of DNP echoes. The DNP echoes are induced by consecutive application of two pulse trains that produce effective Hamiltonians that differ only in the sign of the effective hyperfine coupling. The experiments have been performed on a frozen solution of trityl radicals in water/glycerol on a homebuilt X-band electron paramagnetic resonance/DNP spectrometer at 80 kelvins. We envisage that DNP echoes will play an important role in future development of pulsed DNP for sensitivity-enhanced nuclear magnetic resonance, hyperfine spectroscopy, and quantum sensing.

On the relationship between magnetic moment and nuclear magnetic hyperfine field of 57Fe

On the relationship between magnetic moment and nuclear magnetic hyperfine field of 57Fe

Collective radiation spectrum for ensembles with Zeeman splitting in single-photon superradiance

In an ensemble of identical atoms, cooperative effects like sub- or superradiance may alter the decay rates and the energy of specific transitions may be shifted from the single-atom value by the so-called collective Lamb shift. While such effects in ensembles of two-level systems are by now well understood, realistic multi-level systems are more difficult to handle. In this work we show that in a system of atoms or nuclei under the action of an external magnetic field, the collective contribution to the level shifts can amount to seizable deviations from the single-atom Zeeman or magnetic hyperfine splitting picture. We develop a formalism to describe single-photon superradiance in multi-level systems in the small sample limit and quantify the parameter regime for which the collective Lamb shift leads to measurable deviations in the magnetic-field-induced splitting. In particular, we show that this effect should be observable in the nuclear magnetic hyperfine splitting in M\"ossbauer nuclei embedded in thin-film x-ray cavities.

Electron Paramagnetic Resonance (EPR) studies on the photo-thermo ionization process of photo-thermo-refractive glasses

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive sodium alumino silicate glass, containing NaF and KBr additives, along with cerium, silver, tin and antimony oxide dopants. UV-exposed regions of this glass produce NaF nanocrystals upon heating, giving rise to a permanent localized refractive index change. In this article we examine the initial stages of this crystallization process by continuous-wave and pulsed X-band electron paramagnetic resonance (EPR) spectroscopy. UV exposure of PTR glass produces unpaired electrons whose EPR spectrum is characterized by pronounced peak splitting arising from nuclear magnetic hyperfine interactions with spin-5/2 and spin-7/2 nuclei suggesting close proximity of the unpaired electrons with 121Sb and 123Sb nuclei. These results indicate that the Sb2O3 dopant plays a key role in the initial stages of the crystallization mechanism. Upon thermal annealing, leading to the crystallization of NaF, these species disappear, indicating their transient nature. A number of other unpaired electron species identified in the dopant free matrix appear to be unrelated to the crystallization process. These results clearly challenge the classical mechanism proposed decades ago to explain the complex crystallization process of PTR glass. Together with more recent results from optical spectroscopy they support the Nikonorov model involving (1) photoionization of Ce3+, (2) transfer of this electron to Sb5+ species to create a Sb4+ species, (3) upon annealing electron transfer from Sb4+ to Ag+ ions, producing silver atoms, (4) coalescence of these species into Ag clusters, which (5) serve as nucleation catalysts for NaF nanocrystals.

Taming the Electronic Structure of Lead and Eka-lead (Flerovium) by the Relativistic Coupled Cluster Method

Theoretical investigations of the superheavy elements (SHEs) are extremely challenging and are often the sole source of useful chemical information. Relativistic Fock-space multireference coupled cluster (RFS-MRCC) computations have been carried out for evaluating the ionization potential (IP), excitation energies (EE), nuclear magnetic hyperfine constant (A), lifetime (τ), and Landé g factor of singly ionized eka-lead (Fl II). To judge the accuracy of Fl II results, similar calculations are performed for Pb II, which shows a nice and consistent agreement with known experimental values. Thus, we believe that our predictions for Fl are reliable and useful for the simulation of experimental behavior. To the best of our knowledge, no prior theoretical and/or experimental information is available for A, τ, and g-factor of this SHE. The higher IPs and EEs of Fl II, with respect to Pb II, indicate the former to be more inert and less metallic than Pb. This is contingent on the effects of the relativistic stabilization of the 7s and 7p1/2 orbitals. The present analysis demonstrates the influence of higher-body cluster operators on atomic properties. The close agreement with the experiment (having an estimated error within 1-2%) indicates that the FS-MRCC method is a reliable predictive tool in cases where the experimental results are not readily available, such as the SHEs. The remaining source of error possibly stems out from the omission of the full-blown triple virtual excitations and the absence of Breit interaction.

Nuclear hyperfine level pattern and hyperfine specific heat of frustrated Gd-pyrochlores Gd2 M 2O7 (M = Ti, Sn, Hf, Zr)

The nuclear contribution to the specific heat originates from the combination of a nuclear electric quadrupole interaction and a nuclear magnetic hyperfine interaction of the magnetic atoms embedded in the crystalline lattice structure. We have estimated the nuclear hyperfine level patterns of the nuclear ground state \(I_{g} = 3/2^{-}\) and the first excited state \(I_{ex} = 5/2^{+}\) at 86.5 keV of Mossbauer-active Gd155 nucleus of Gd-based pyrochlores Gd2M2O7 (M = Ti, Sn, Hf, Zr). The nuclear quadrupole and hyperfine interactions are scaled in accordance with the ratios of quadrupole and magnetic moments of \(I_{g} = 3/2^{-}\) state of Gd155 and Gd157 nuclei to get the contribution from Gd157 nucleus of Gd2M2O7 to the hyperfine specific heat Chf appropriate to its relative isotopic abundance. It is found that Chf(T) shows a Schottky hump around 6–8 mK for these pyrochlores. At temperatures much higher than the hyperfine level splitting, Chf behaves as AT − 2 with A ∼ 1–2 × 10 − 4 JK/mole-Gd which is consistent with experimental results.

Effects of O18 isotopic substitution on the rotational spectra and potential splitting in the OH–OH2 complex: Improved measurements for O16H–O16H2 and O18H–O18H2, new measurements for the mixed isotopic forms, and ab initio calculations of the A2′-A2″ energy separation

Rotational spectra have been observed for (16)OH-(16)OH(2), (16)OH-(18)OH(2), (18)OH-(16)OH(2), and (18)OH-(18)OH(2) with complete resolution of the nuclear magnetic hyperfine structure from the OH and water protons. Transition frequencies have been analyzed for each isotopic form using the model of Marshall and Lester [J. Chem. Phys. 121, 3019 (2004)], which accounts for partial quenching of the OH orbital angular momentum and the decoupling of the electronic spin from the OH molecular axis. The analysis accounts for both the ground ((2)A(')) and first electronically excited ((2)A(")) states of the system, which correspond roughly to occupancy by the odd electron in the p(y) and p(x) orbitals, respectively (where p(y) is in the mirror plane of the complex and p(x) is perpendicular to p(y) and the OH bond axis). The spectroscopic measurements yield a parameter, rho, which is equal to the vibrationally averaged (2)A(')-(2)A(") energy separation that would be obtained if spin-orbit coupling and rotation were absent. For the parent species, rho = -146.560 27(9) cm(-1). (18)O substitution on the water increases /rho/ by 0.105 29(10) cm(-1), while substitution on the OH decreases /rho/ by 0.068 64(11) cm(-1). In the OH-OH(2) complex, the observed value of rho implies an energy spacing between the rotationless levels of the (2)A(') and (2)A(") states of 203.76 cm(-1). Ab initio calculations have been performed with quadratic configuration interaction with single and double excitations (QCISD), as well as multireference configuration interaction (MRCI), both with and without the inclusion of spin-orbit coupling. The MRCI calculations with spin-orbit coupling perform the best, giving a value of 171 cm(-1) for the (2)A(')-(2)A(") energy spacing at the equilibrium geometry. Calculations along the large-amplitude bending coordinates of the OH and OH(2) moieties within the complex are presented and are shown to be consistent with a vibrational averaging effect as the main cause of the observed isotopic sensitivity of rho.

Sub-Doppler millimetre-wave spectroscopy of DBS and HBS: accurate values of nuclear electric and magnetic hyperfine structure constants

The unstable thioborine molecule and its deuterated variant have been produced by a high-temperature reaction between hydrogen sulfide and crystalline boron at 1100 degrees C in a flow system. Five rotational transitions from J = 2 <-- 1, to J = 6 <-- 5 have been recorded with sub-Doppler resolution for the vibrational ground state of H10/11BS and D10/11BS using the Lamb-dip technique. The hyperfine structure due to the electric quadrupole interaction of deuterium nucleus has been resolved yielding the first experimental determination of the deuterium quadrupole coupling constant in thioborine, which is 0.1403(75) MHz in D11 BS and 0.1360(38) MHz in D10BS. Fairly accurate values of 10/11B spin-rotation coupling constants and of the hydrogen-boron spin-spin coupling constants have also been determined. Additionally, the hyperfine structure of the rotational lines for the nu2 = 1 excited state has been investigated, thus obtaining information on the asymmetry of the electric field gradient at the B nucleus in the bending state.

Hyperfine‐Induced f↔f Transitions: Effective Operator Formulation

The direct perturbing influence of the nuclear magnetic and electric multipole hyperfine interactions upon the amplitude of the f↔f electric dipole transitions is analyzed. In this approach, the role of the forcing mechanism is played by an interaction other than the crystal field potential, which is the origin of all existing theoretical models. In particular, new effective operators of the second order that result from the electric dipole hyperfine interactions and compete with the standard Judd–Ofelt terms are introduced. In addition, the tensorial structure of the third‐order effective operators that contribute to the transition amplitude is discussed, and attention is directed to the possibility and necessity of the introduction of a new parameterization scheme of f‐spectra that would be applicable for the description of the hypersensitivity and such transitions that are highly forbidden by the standard selection rules.

Relativistic coupled cluster method

Relativistic coupled cluster method (CCM) is applied to compute the low lying excited and ion states of strontium and ytterbium atom. The resulting excitation and ionization energies are in excellent agreement with experimental data and with other correlated calculations. The nuclear magnetic dipole hyperfine constants (A) and electric quadrupole hyperfine constants (B) of excited states are also evaluated and are in accord with experiment. We further address the basis set dependency of the computed properties.

Relativistic effective valence shell Hamiltonian method: Excitation and ionization energies of heavy metal atoms

The relativistic effective valence shell Hamiltonian H(v) method (through second order) is applied to the computation of the low lying excited and ion states of closed shell heavy metal atoms/ions. The resulting excitation and ionization energies are in favorable agreement with experimental data and with other theoretical calculations. The nuclear magnetic hyperfine constants A and lifetimes tau of excited states are evaluated and they are also in accord with experiment. Some of the calculated quantities have not previously been computed.

Saturated absorption spectroscopy of Xe using a GaAs semiconductor laser

Doppler-free absorption spectrum from 5P 56S to 5P 56P transitions of Xe in a hollow cathode discharge was measured in the wavelength region from 810 to 910 nm. The saturated dip signals detected by a frequency modulated probe beam were doubly AM-modulated by a pump beam chopped using a conventional mechanical chopper. The Doppler background was greatly reduced by detection using a second lock in amplifier synchronous to the chopping signal. The isotope shifts of even mass 136−128Xe in natural abundance were resolved for the five transitions from the metastable states of J=2 and 0, while in the transitions from the J=1 states were partially resolved. The nuclear magnetic and quadrupole hyperfine coupling constants of 131Xe and 129Xe determined are listed with those reported previously. The specific mass shifts and the field shifts were estimated using the relation given by King's diagram and compared with those obtained from the changes in the mean square-radii of the nuclear charge distribution.

High resolution molecular beam study of the origin bands of the à 2Π–X̃ 2Σ+ and Ã″ 2Π1/2–X̃ 2Σ+ systems of yttrium imide (Y14NH, Y15NH, and Y14ND)

The (0,0,0)–(0,0,0) bands of the à 2Π–X̃ 2Σ+ and Ã″ 2Π1/2–X̃ 2Σ+ systems of three isotopomers of yttrium imide (Y14NH, Y15NH, and Y14ND) have been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The previously studied B̃ 2Σ+−X̃ 2Σ+ (0,0,0)–(0,0,0) bands of the three isotopomers have been reanalyzed. Global fits of all observed bands, in which the ground state has been fitted to a Hamiltonian model, while the excited states have been represented by term values, have been performed for the three isotopomers. Subsequently, the individual bands have been fitted. The ground state parameters have been fixed at the values obtained in the global fits, while the upper states have been fitted to the Hamiltonian models. The (0,0,0) à 2Π state of Y14NH, Y15NH, and Y14ND is severely perturbed. Even though the nature of these perturbing states can only be speculated upon, the introduction of effective perturbers made it possible to deperturb the state successfully. The Ã″ 2Π1/2 state is unperturbed. The spectra can be reproduced to better than 120 MHz (0.004 cm−1). The analyses confirm that the molecule is linear in all four states with the nuclear arrangement Y–N–H. The bond lengths (r0 structure) in the X̃ 2Σ+ ground state and the Ã″ 2Π1/2, à 2Π, and B̃ 2Σ+ excited states have been derived to be rYN=0.187785(17) nm, rNH=0.10039(14) nm; rYN=0.1927(1) nm, rNH=0.081(1) nm; rYN=0.19013(56) nm, rNH=0.1032(54) nm; and rYN=0.18848(52) nm, rNH=0.1236(46) nm, respectively. The electronic configurations for the X̃ 2Σ+ ground state and the à 2Π, Ã″ 2Π1/2, and B̃ 2Σ+ excited states are discussed and compared with ab initio calculations whenever possible.

High resolution molecular beam study of the origin band of the B̃ 2Σ+–X̃ 2Σ+ system of yttrium imide

The (0,0,0)–(0,0,0) band of the B̃ 2Σ+–X̃ 2Σ+ system of three isotopomers of yttrium imide (Y14NH, Y15NH, and Y14ND) has been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The B̃ 2Σ+(0,0,0) state of Y14NH, Y14ND, and Y15NH is severely perturbed below J=30.5 by eight, three, and two vibronic states, respectively. Although, the nature of these perturbing states can only be speculated upon, their symmetries are either Σ2 or Π2, and this has made it possible to deperturb the B̃ 2Σ+ state successfully. The spectra can be reproduced within 140 MHz (0.0047 cm−1). The analyses confirm that the molecule is linear in both states with the nuclear arrangement Y–N–H. The bond lengths in the ground X̃ 2Σ+ state and the B̃ 2Σ+ state have been derived to be rY–N=1.877 57(13) Å, rN–H=1.0067(10) Å, and rY–N=1.8839(43) Å, rN–H=1.242(30) Å, respectively. The results are compared with the values of ab initio calculations on YNH and YN, and the experimental data on YN and YO. The atomic character of the unpaired electron in the ground state is 58% Y + 5s and 42% Y + 5p. The electron configurations for the ground X̃ 2Σ+ state and the B̃ 2Σ+ state are discussed and compared with ab initio calculations whenever possible.

14N Hyperfine Structure in the Pure Rotational Spectrum of24Mg14N12C

The pure rotational spectrum of24Mg14N12C, between 11.9 and 23.9 GHz, has been measured using a pulsed jet Fourier transform microwave spectrometer. The hyperfine structure due to the14N nucleus has been measured, and the nuclear quadrupole coupling, Fermi contact, and dipole-dipole interaction constants have been determined. The ionic character of the metal-isocyanide bond has been investigated through the nuclear quadrupole and magnetic hyperfine parameters.

Laser spectroscopy and density functional calculations on niobium monocarbide

A survey of the jet-cooled Nb12C and Nb13C radicals has been carried out between 13 500 and 18 000 cm−1 using laser-induced fluorescence and resonant two-photon ionization spectroscopy. Several vibronic bands belonging to at least six band systems have been identified. Three of these systems appear to belong to 2Π1/2–2Δ3/2 transitions in which the lower 2Δ3/2 state is the ground electronic state of the molecule. The other three systems also terminate to the same three 2Π1/2 upper states, but originate from a state lying 830 cm−1 above the X 2Δ3/2 state. This state is assigned as the A 2Σ+ state. The ionization potential has been determined to be 56 402±15 cm−1 or 6.9929±0.0018 eV using two-color photoionization efficiency spectroscopy. This value, combined with the ionization potential of Nb and the bond energy of NbC+, yields an improved bond energy of 5.39±0.15 eV for NbC. The (4,0) band of the B 2Π1/2–X 2Δ3/2 system has been studied at a resolution of approximately 0.005 cm−1 using laser-induced fluorescence spectroscopy. The nuclear magnetic hyperfine structure has been resolved in both states, and an analysis confirms that the 2Δ ground state arises from the σ2δ1 electron configuration in which the unpaired δ electron is a pure Nb 4d electron associated with the 4F term arising from the excited 5s24d3 electron configuration. Density functional calculations have been carried out on the lowest 2Δ, 2Σ+, 4Δ, 2Π, and 4(Π,Φ) states of the neutral and the 1Σ+, 3Δ, and 3(Π,Φ) states of the cations. These calculations fully support the experimental evidence for the ground state.

Rotational and hyperfine analysis of the (0,0) band of the B2Sigma+-X2Sigma+ system of indium monoxide

The InO molecule has been produced in a molecular beam using a laser vaporization source, and its (0,0) band, near 428.2 nm, has been studied by laser-induced fluorescence at sub-Doppler (~0.005 cm-1) resolution. The spectrum is characterized by impressive nuclear magnetic hyperfine structure due to the 11.5 In nucleus. Analysis ofthe rotational and hyper fine structure in the band indicates that the transition is B2Sigma+ (case b beta s) - X2 Sigma+ (case b beta J) in which there is a large hyperfine splitting in the upper state and a predominantly spin-rotation splitting in the lower (ground) state of the molecule. The band origin, nu(0,0), lies at 23332.296 cm-1, and derived constants, in cm-1, for the two v = 0 levels are as follows: (table omitted) Dispersed fluorescence experiments and photographic emission spectroscopy using microwave excitation, hollow cathode and dc arc sources have been employed to obtain vibrational data.

Quantum beat study of the nuclear hyperfine structure of OD and Ar⋅OD in their A 2Σ+ electronic states

The nuclear hyperfine structure of OD and Ar⋅OD in their A 2Σ+ electronic states has been studied by quantum beat spectroscopy. The very cold transient species were produced in a supersonic expansion using a pulsed discharge nozzle. Coherent excitation of hyperfine (hf) states, arising from one fine structure (OD) or rotational (Ar⋅OD) level, created quantum beats on the fluorescence decay. The beat frequencies, which correspond to energy separations between hf levels, could be measured to ±75 kHz. The splitting of the hf levels into their Zeeman components was investigated in a weak magnetic field. A fit of the zero field and Zeeman data yielded the relevant constants for the nuclear magnetic and electric quadrupole hyperfine interactions as well as the pertinent g-factors in each species. In the case of OD, the hf parameters agree well with those reported previously but are more accurately defined. For Ar⋅OD the previously unknown hyperfine and spin-rotation parameters of the A 2Σ+ state were determined. A comparison of the hf parameters in the two systems allowed assessment of the effect of van der Waals complex formation on the electron distribution. Thus complexation is found to reduce the unpaired electron density on the deuteron by 7% which is indicative of significant chemical bonding between the Ar atom and the OD moiety in the A 2Σ+ state of Ar⋅OD. For both systems, the g-factors gS and gl obtained suggest an admixture of other, possibly quartet, electronic states into the A 2Σ+ state.

An Interpretive Basis of the Proton Nuclear Magnetic Resonance Hyperfine Shifts for Structure Determination of High-Spin Ferric Hemoproteins. Implications for the Reversible Thermal Unfolding of Ferricytochromec‘ fromRhodopseudomonas palustris

An NMR approach to determining the solution molecular structure of a high-spin ferric hemoprotein, 13 kDa ferricytochrome c‘ from Rhodopseudomonas palustris (Rp), has been investigated. In parallel with the use of appropriately tailored 1D and 2D experiments to provide scalar and dipolar correlations for the strongly relaxed and hyperfine-shifted heme cavity residues, we explore an interpretive basis of the large hyperfine shifts for noncoordinated residues which could provide constraints in solution structure determination for high-spin ferric hemoproteins. It is shown that the complete heme can be uniquely assigned in spite of the extreme relaxation properties (T1s 1−8 ms). Sufficient scalar connectivities are detected for strongly relaxed protons (T1 ≥ 4 ms) to uniquely assign residues on both the proximal and distal sides of the heme. The spatial correlations indicate that the structure is homologous to the four-helix bundle observed for other cytochromes c‘. The pattern of large hyperfine shifts for noncoordinated residues is shown to be qualitatively reproduced by the dipolar shifts for a structural homolog based on an axial zero-field splitting of ∼12 cm-1. It is concluded that, when this approach is combined with more conventional 2D methods for the diamagnetic portion of the protein, a complete structure determination of a five-coordinate ferric hemoprotein should be readily attainable. It is shown that the ferricytochrome c‘ unfolds reversibly at high temperature and that there exists at least one equilibrium intermediate in this unfolding that is suggested to involve helix separation from the heme.

14N quadrupole coupling constants and14N and19F spin–rotation coupling constants of nitrosyl fluoride, FNO, and nitryl fluoride, FNO2

High-resolution pure rotational spectra of nitrosyl fluoride, FNO, and nitryl fluoride, FNO2, have been measured using a cavity pulsed microwave Fourier-transform spectrometer. The samples were prepared using a pulsed electric discharge through precursor substances, with the discharge apparatus mounted as part of the pulsed nozzle of the spectrometer. Nuclear quadrupole and magnetic hyperfine structures due to 14N and 19F have been resolved for both molecules, and precise values for the nuclear quadrupole and spin–rotation coupling constants have been obtained. The 14N quadrupole coupling constants of FNO2 have been rationalized in terms of a σ-withdrawal of electron density from NO2 towards F. This view is corroborated by the magnetic shielding estimated from the F spin–rotation constants.