A New Q-Band EPR Probe for Quantitative Studies of Even Electron Metalloproteins

Existing Q-band (35 GHz) EPR spectrometers employ cylindrical cavities for more intense microwave magnetic fields B₁, but are so constructed that only one orientation between the external field B and B₁is allowed, namely the B B₁orientation, thus limiting the use of the spectrometer to measurements on Kramers spin systems (odd electron systems). We have designed and built a Q-band microwave probe to detect EPR signals in even electron systems, which operates in the range 2 K ≤ T ≤ 300 K for studies of metalloprotein samples. The cylindrical microwave cavity operates in the TE₀₁₁mode with cylindrical wall coupling to the waveguide, thus allowing all orientations of the external magnetic field B relative to the microwave field B₁. Such orientations allow observation of EPR transitions in non-Kramers ions (even electron) which are either forbidden or significantly weaker for B B₁. Rotation of the external magnetic field also permits easy differentiation between spin systems from even and odd electron oxidation states. The cavity consists of a metallic helix and thin metallic end walls mounted on epoxy supports, which allows efficient penetration of the modulation field. The first quantitative EPR measurements from a metalloprotein (Hemerythrin) at 35 GHz with B₁ B are presented.     

Nuclear magnetic resonance on oriented76,77,82Br in Fe

Nuclear magnetic resonance on oriented^76,77,82BrFe has been measured using recoil-implanted samples. The magnetic hyperfine splitting frequency of⁸²BrFe in a zero external magnetic field has been determined to be 201.90(3) MHz. The resonances of⁷⁶BrFe and⁷⁷BrFe were also observed in an external magnetic field of 0.2 T asv(⁷⁶BrFe)=340.9(3) MHz andv(⁷⁷BrFe)=403.5(2) MHz. With the known values of theg-factors, the hyperfine fields have been deduced:B _HF(⁸²BrFe)=81.397(27) T,B _HF(⁷⁶BrFe)=81.38(7) T. Theg-factor of⁷⁷Br was determined to be |0.6487(4)|.     

Quartet of resonance peaks of dislocation displacements in NaCl crystals during their magnetic processing in the low-frequency EPR scheme

Resonance relaxation displacements of dislocations have been studied in NaCl crystals placed in crossed ultralow magnetic fields have been studied in the electron paramagnetic resonance (EPR) scheme, i.e., in the static magnetic field B = (26–261) μT and the perpendicular radio-frequency field (with the amplitudes of 2.5 and 6 μT in the frequency range ν = (0.5–7.3) MHz). The spectrum (quartet) of equidistant resonance peaks of the dislocation mean paths l(ν) has been observed. In the most part of the studied field interval B, the frequencies of the EPR peaks correspond to the Zeeman splitting of the levels with four g-factors close to 2 and the difference of the neighboring values Δg = 0.09. The equidistance is violated only at the lowest fields, B < 50 μT, and frequencies ν < 0.7 MHz.     

Detection of vortex excitations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi₂Sr₂Ca₂Cu₃O₁₀ single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ T _c. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.     

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: I. Tunneling frequency measurements

Theory of²H-NMR spectra is developed for a ND ₄ ⁺ ion. Quadrupole and dipole-dipole interactions for four deuterons of the tetrahedral ion as well as the rotational tunneling are taken into account. Features in the single crystal spectra at the high magnetic field of 6.7 T are analyzed. Their central doublets (±5 kHz range around the Larmor frequency) are attributed to theA symmetry species and exhibit sensitivity to theA–T tunneling frequencyv _t . This allows the measurement ofv _t in a wide rangev _t <10 MHz. The shape of sidebands (±140 kHz range) is related to the T levels structure. Theoretical predictions are verified on a (ND₄)₂SnCl₆ single crystal at 44 MHz. The tunneling frequency at temperatures below 10 K equals 7.5 MHz, which is about 100 times smaller than for (NH₄)₂SnCl₆.v _t was measured up to 40 K. At still higher temperatures motional narrowing effects were observed for theA spectral components preventing the determination ofv _t .     

TDPAC measurement of quadrupole interaction at111Cd in the nearly heavy fermion system USn2.9

Quadrupole interaction at¹¹¹Cd probe has been studied in the nearly heavy fermion system USn_3– using the TDPAC method. We observe a large quadrupole interaction frequency withv _Q =89.9(10) MHz at 300 K which is much higher than the earlier reported result. The observed frequency shows a linear temperature dependence in the range 25–300 K.     

Measurements of theP 3/2-S 1/2-intervals in then=4,n=5 andn=6 states of ionized helium

TheP _3/2-S _1/2-intervals in then=4,n=5 andn=6 states of ionized helium have been measured by a radio frequency method, which permits to determine the disturbing electric fields in the interaction region and to correct their influences. The experimental results for theP _3/2-S _1/2 intervals in then=4,n=5 andn=6 states were (20,180.6±0.8) MHz, (10,332.9±1.4) MHz and (5,979.1±1.2) MHz respectively. From these intervals, the following indirect values for theS _1/2-P _1/2-Lambshifts can be deduced: (1,768.5±0.8) MHz in then=4 state, (905.0±1.4) MHz in then=5 state and (524.3±1.2) MHz in then=6 state. The results agree with the theoretical predictions. The static electric fields in the interaction region, ranging from 2 to 6 V/cm, increased with increasing electron excitation current, but were independent of the helium pressure within the range of 10 to 26 mTorr. All uncertainties are expressed as 68% confidence values.     

Multifrequency EPR spectra of molecular oxygen in solid Air

Multifrequency EPR spectra in the 94 to 550 GHz range were performed on solid air samples condensed at 5 K in the waveguide of a single pass probe. The spectra of molecular oxygen were observed and interpreted in the frame of the spin Hamiltonian model as axial S = 1 spectra with a zero field splitting parameter D = 3.572(3) cm⁻¹. The result of this study is relevant in the field of high field–high frequency EPR application in which solid air O₂ is a common paramagnetic impurity.     

Frequency dependence of the radio frequency collapse effect

A systematic study of the dependence of the radio frequency (rf) collapse effect on the frequency of the rf field which induces the fast relaxation of the hyperfine field was performed for Permalloy and amorphous Fe₄₅Ni₃₀Si₁₀B₁₅ alloy. The rf collapse was studied for the frequency range of 12 MHz to 64 MHz. The results show that the rf collapse appears gradually in both materials as the rf field frequency exceeds the Larmor frequency what is in agreement with the model predictions.     

Nuclear magnetic resonance of175Hf oriented in iron

Nuclear magnetic resonance of¹⁷⁵Hf oriented at low temperature in iron has been observed with a sample prepared by ion implantation. The centre frequency of the broad resonance line isv _L (B _ext = 0)=138.53(36)MHz. Possible origins of the large inhomogeneous line width of FWHM=11.0(1.1) MHz are discussed. A comparison with model calculations for combined magnetic and electric hyperfine interaction indicates that the centre frequency may be interpreted as the magnetic interaction frequency for¹⁷⁵Hf in unperturbed substitutional sites of the host iron. With theg-factor of¹⁷⁵Hf from literature the magnetic hyperfine field of Hf in Fe is derived asB _hf=?64.9(9.3) T fitting well into systematics.     

Superhyperfine structure of the EPR spectra of Ce<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The EPR spectra of Ce³⁺ impurity ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals have been investigated at a frequency of ∼9.3 GHz in the temperature range 5–25 K. The effective g factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other (g _‖ = 2.737, g _⊥ = 1.475 for LiYF₄:Ce³⁺). A superhyperfine structure of the EPR spectrum of Ce³⁺ ions in the LiTmF₄ Van Vleck paramagnet has been observed in the external magnetic field B oriented along the crystallographic axis c (B ‖ c). The superhyperfine structure of the EPR soectra of the Ce³⁺ ions in the LiYF₄ and LiLuF₄ diamagnetic matrices is resolved for B ⊥ c. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.     

Superhyperfine structure of the EPR spectra of Nd<Superscript>3+</Superscript> and U<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The superhyperfine structure of the EPR spectra of impurity Nd³⁺ and U³⁺ ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals has been observed and discussed. In LiYF₄: Nd (g _| = 1.987, g _⊥ = 2.554) and LiTmF₄: Nd, the superhyperfine structure is observed at the orientation of the external magnetic field B in parallel to the c axis of the crystals and consists of nine components with a splitting of ∼15.4 MHz. In LiYF₄: U (g _| = 1.149, g _⊥ = 2.508) and LiLuF₄: U, the superhyperfine structure is observed at both B | c and B ⊥ c and consists of nine and eleven components, respectively, with a splitting of ∼21.5MHz. It should be noted that the resolution of the superhyperfine structure of the EPR spectrum of LiLuF₄: U³⁺ becomes significantly higher with a deviation from the orientation B ⊥ c.     

Direct measurements of anticrossings of the electron-nuclear energy levels in LiYF4∶Ho3+ with submillimeter EPR spectroscopy

The electron paramagnetic resonance (EPR) spectra of impurity Ho³⁺ ions in monocrystals LiYF₄∶Ho³⁺ (0.1 and 1%) with the natural abundance of⁶Li (7.42%) and⁷Li (92.58%) isotopes, and in the sample⁷LiYF₄∶Ho³⁺ (0.1%) isotopically pure in⁷Li were taken at the temperature 4.2 K in the frequency range of 165–285 GHz. Resonance transitions between crystal field sublevels (the ground non-Kramers doublet and the nearest excited singlet) of the⁵I₈ term were detected. The refined set of crystal field parameters and the effective constant of the magnetic hyperfine interaction were determined from the detailed analysis of the recorded spectra at frequencies varied by 0.05 GHz. The fine structure of EPR lines with intervals of about 300 MHz observed in the sample LiYF₄∶Ho³⁺ (0.1%) can be interpreted as a result of the isotopic disorder in the Li sublattices. Direct information about energy gaps at the anticrossing points of the electron-nuclear sublevels of the ground doublet was obtained. These gaps are induced by the hyperfine interaction that mixes doublet and singlet states and by random crystal fields. Weak EPR signals from distorted single ion and pair centers of impurity Ho³⁺ ions were resolved. From a comparison of the measured and simulated spectra, estimates of spectral parameters of the dimer centers have been obtained.     

13C-NMR study of the phase transition in the layered antiferroelectric squaric acid

The temperature dependence of the¹³C-NMR spectra of undoped crystals of the layered antiferroelectric squaric acid (C₄H₂O₄; H₂SQ was measured at a resonance frequency of about 75 MHz by means of the¹H–¹³C double resonance technique. The relation between the order parameter and the chemical shift tensors at the¹³C nuclei is discussed in detail. The line splitting in the ordered phase belowT _c100°C is shown to be proportional to the order parameter. The phase transition is of first order and can be well described by a Landau expansion of the free energy. The results are compared to previous determinations of the temperature dependence of the order parameter. The spin lattice relaxation time of the¹³C system is estimated to be 5000–7500 s at 5°C and the given magnetic field.     

Cryogenic 35 GHz pulse ENDOR probehead accommodating large sample sizes: Performance and applications

The construction and performance of a cryogenic 35 GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE₁₀₂ cavity in which the radio frequency (rf) B₂-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B₂-field strength per square-root of the transmitted rf power over the frequency range 2–180 MHz. The distributions of the microwave B₁- and E₁-field, and the rf B₂-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions.     

Parametric instability in a two-hole species semiconductor-plasma

The parametric excitation of a low frequency wave has been investigated analytically in a two-hole species semiconductor-plasma in the region of kl ? 1 using the hydrodynamic model of the plasmas in the presence of a high frequency oscillatory electric field (E₀ cos ω₀t applied along the X-axis) and a d.c. magnetic field B₀ normal to the electric field (along the Z-axis), the low frequency wave propagating in the X–Z plane making a very small angle θ with the X-axis. The system supports a purely growing unstable mode. The variation of the growth rate of the unstable mode has been studied over a wide range of system parameters for the specific case of an intrinsic GaAs crystal at 300 K. The oscillatory electric field can be obtained by irradiating the crystal with a 119μm H₂O laser.     

Modulation instability of electromagnetic excitations for a Josephson unction in a finite-thickness slab

The modulation instability of finite-amplitude uniform plane waves oscillating with a Josephson frequency and experiencing a nonlinear frequency shift in a finite-thickness slab is studied in terms of the nonlocal electrodynamics of Josephson junction. A dispersion relation for the growth rate of small amplitude perturbation is derived. The domains of modulation instability for these waves are found. Modulation instability of the waves is shown to arise when the wavevectors of long-wave amplitude perturbations fall into the finite range 0 < Q < Q _B (A, D, L). In the range Q ≥ Q _B (A, D, L), the waves are stable.     

Ferromagnetic resonance in a gallium substitued garnet film

The ferromagnetic resonance at X, K and Q-band has been studied on LPE₁₁₁ films of composition Y₃Fe_4.2Ga_0.8O₁₂. From the in-plane measurement and measurement in a 11¯2 plane theg-factor,g=2-02 (K, Q),g=2·05 (X band), the total uniaxial anisotropy fieldB _A (80 K)=750 G (75 mT),B _A (300 K)=250 G (25 mT) and the cubic field 2K ₁/M have been dstermined. Besides of the main resonance with the linewidth B =15 G (1·5 mT), B =50 G (5 mT), B _min=10·5 G (1·05 mT) a linear spin-wave spectrum and some high-field modes were observed. A new approximate method of crystal orientation in the plane of the film has been used based on the analysis of the in-plane dependence of the resonance field.The authors wish to express their thanks to Dr. S.Krupika for helpful discussions.     

Proton magnetic relaxation studies of molecular diffusion in the liquid crystal 40.5

Nuclear magnetic relaxation spectroscopy is used to study the molecular dynamics in a liquid crystal butoxy benzylidene pentylaniline (40.5) in the frequency range 4 to 30 MHz and the results are compared with two other members of the same homologous series (viz 40.8 and 40.6). Spin lattice relaxation time studies indicate that molecular self diffusion (SD) and reorientation processes (R) dominate the relaxation process and their relative contributions are quantified. This contrasts with the case where order director fluctuations (ODF) effectively mediate relaxation process and all the three processes are found to be important in 40.6 in a similar frequency range.T _1D in 40.5 in the nematic phase shows temperature dependence indicating that ODF that is present at low frequencies might be diffusion assisted. These relaxation data are analysed in theS _B phase of this compound also to obtain contributions to the relaxation process. These results are also analysed to obtain different parameters associated with the above dynamical processes.