Transient EPR at 240 GHz of the excited triplet state of free-base tetra-phenyl porphyrin

The electron paramagnetic resonance (EPR) spectrum measured at 240 GHz of the lowest excited triplet state of the free-base tetra-phenyl porphyrin (H₂-TPP) molecule is presented. The high frequency that is employed allows for the full resolution of theg-anisotropy (2.00354(2), 2.00272(15), and 2.00194(5) forx, y, andz zero-field axes respectively), and the obtained values are compared to those of other porphyrin-based systems. These results are one of the first addressing theg-anisotropy in the photoexcited state of porphyrins. The usefulness of high-field transient EPR in elucidating not only the principal values of theg-tensor but also the orientation of theg-tensor principal axes with respect to those of the zero-field splitting tensor in porphyrin disordered systems is also discussed.     

High-field/high-frequency EPR/ENDOR — A powerful new tool in photosynthesis research

In photosynthesis research the elucidation of the spatial and electronic structures of the electron donor and acceptor ion radicals is very important for an understanding of the light-induced electron transfer process. Recent 3 mm (W-band, 95 GHz) high-field EPR and ENDOR studies on the primary donor cation radicals P^+. (bacteriochlorophyll dimer), the acceptor anion radicals Q^+. (quinones), and the charge-separated radical pair (P^+.Q^?.) in photosynthetic bacteria and biomimetic model systems are presented. From single crystals of, for instance,Rb. sphaeroides R-26 reaction centers both the hyperfine tensors of various protons and theg-tensor of P₈₆₅ ^+. have been determined and compared with calculated tensor values based on recent X-ray structure data. The results consistently reveal a breaking of the local C₂ symmetry of the electronic structure at the primary donor side of the reaction center. This is of particular interest since it might be relevant for the vectorial electron transfer along the protein complex. Among the quinone radical anions studied are frozen solutions of the electron acceptors of bacterial and plant reaction centers (ubiquinone and plastoquinone, respectively). The increased electron Zeeman interaction in high-field EPR leads to almost completely resolvedg-tensor components even in disordered samples. Theg-tensors and component linewidths are sensitive probes for specific anisotropic interactions with the environment. In the case of the transient correlated coupled radical pair P₈₆₅ ^+.-Q_A ^?. ofRb. sphaeroides (Fe replaced by Zn) the spin-polarized high-field EPR spectra allow an unambiguous determination of the relative orientation of theg-tensors of the donor and acceptor parts. Thereby high-precision structure information is obtained on the electron transfer pigments after light-induced charge separation.     

Towards an identification of chemically different flavin radicals by means of theirg-tensor

Theg-tensors of two chemically different flavin mononucleotide (FMN) radicals, one of which is covalently bound via N(5) of its 7,8-dimethyl isoalloxazine moiety, and the other one non-covalently bound to mutant LOV domains of the blue-light receptor phototropin, LOV1 C57M and LOV2 C450A, respectively, have been determined by very high microwave frequency and high magnetic field electron paramagnetic resonance (EPR) performed at 360 GHz and 12.8 T. Due to the high spectral resolution of the frozen-solution continuous-wave EPR spectra, the anisotropy of theg-tensors could be fully resolved. By least-squares fittings of spectral simulations to expermental data, the principal values ofg have been established:g _X=2.00554(5),g _Y=2.00391(5), andg _Z=2.00247(7) for the N(5)-alkyl-chain-linked FMN radical in LOV1 C57M-675, andg _X=2.00427(5),g _Y=2.00360(5), andg _Z=2.00220(7) for the noncovalently bound FMN radical in LOV2 C450A-605. By a comparison of these values to the ones from the flavin adenine dinucleotide radicals in two photolyases, the radical in LOV2 C450A-605 could be clearly identified as a neutral FMN radical, FMNH. In contrast, LOV1 C57M-675 exhibits significantly shifted principal components ofg, the differences being caused by spin-orbit coupling of the nearby sulfur from the reactive methionine residue, and the modified chemical structure due to the covalent attachment at N(5) of the radical to the apoprotein. The results clearly show the potential of using theg-tensor as probe of the global electronic and chemical structure of protein-bound flavin radicals.     

Electron paramagnetic resonance of some lanthanide ions in yttrium and lutetium vanadate

The electron paramagnetic resonance (EPR) parameters have been determined for the trivalent ions neodymium, erbium and ytterbium, at low abundance in lutetium vanadate. Crystal field splittings produce ground states which are Kramers doublets, and measurements were made of theg-values both parallel and perpendicular to the tetragonal axis; they are compared with previous measurements in yttrium vanadate. Two of the crystal field splitting parameters of the trivalent gadolinium ion (S = 7/2) in LuVO₄ show noticeable difference from those in YVO₄.     

A D-band (130 GHz) EPR study of the primary electron donor triplet state in photosynthetic reaction centers ofRhodobacter sphaeroides R26

A high-field (D-band, 130 GHz) electron spin echo-detected spectrum of the primary electron donor triplet state,³P, in quinone-depleted photosynthetic reaction centers from the bacteriumRhodobacter sphaeroides R26 is obtained. It shows a significantg-anisotropy, which is larger than that of the primary donor oxidized state, P^+?. Simulation gives the tripletg-tensor principal values of 2.0037, 2.0028, and 2.0022 (precision ±0.0001), assuming that theg-tensor is coaxial to a zerofield splitting tensor. The³P spectral lineshape reveals an orientational anisotropy of the triplet quantum yield. We explain this anisotropy as arising from the difference in the main values and relative orientations between theg-tensors of P^+? and I _A ^?? in the primary radical pair (the triplet state’s precursor).     

States inN=80140Nd populated in the decay of140Pm

The level structure of theN=80,¹⁴⁰Nd has been investigated through the beta decay of^140m Pm and^140g Pm, whose decay half-lives were measured to be 5.95±0.05 min and 9.1±0.5 sec, respectively. Decay schemes for the two isomers have been constructed fromγ-ray measurements. With the help of the beta decayft values and internal conversion electron measurements, spins and parities were deduced for levels in¹⁴⁰Nd. Among the levels a 7^? state at 2222.0 keV and a 5^? state at 2273.3 keV are identified as two-neutron quasiparticle states. The (ie233-01) for^140m Pm and^140g Pm have been deduced as 7^? and 1⁺, respectively. From beta end-point measurements, the decay energies of the two isomers have been determined asQ _EC=6.48±0.07 MeV^140m (Pm) andQ _EC=6.08±0.10 MeV (^140gPm). The results are discussed in the light of nuclear systematics of theN=79 and 80 isotones.     

EPR of a fullerene-molecule-derived paramagnetic center as a mesoscopic conducting object

Discussion of theg-factor value of fullerene is based on the model of itinerant electrons restricted to the surface of the fullerene molecule C₆₀. The Ag shift, i.e., the difference between the experimentalg-factor and theg-factor of free electron Δg = g ? 2.0023 for C ₆₀ ^?1 is negative as for a very small metallic conducting particle.g-factor value is proportional to the interaction between itinerant electrons in the conduction band, thus the Δg is negative for C ₆₀ ^?1 and C ₆₀ ^?3 having less than half filled conduction band, while Δg is positive for C ₆₀ ⁺ where the conduction band is almost filled.     

Probing electronic and structural properties of the reduced [2Fe−2S] cluster by orientation-selective1H ENDOR spectroscopy: Adrenodoxin versus rieske iron-sulfur protein

The¹H electron-nuclear double resonance (ENDOR) spectra in frozen buffer solutions of the reduced [2Fe?2S] clusters in adrenodoxin (Adx) and in the “Rieske” iron-sulfur protein (ISP) from the bovine mitochondrial bc1 complex were measured at low temperatures (5–20 K) and analyzed by spectra reconstruction. A single paramagnetic species with iron valence states (II) and (III) connected uniquely to the cluster irons was found in both proteins. For Adx, the experimental spectra from 23 field positions across the nearly axial (g _max=2.0241,g _int=1.9347, andg _min=1.9331) electron paramagnetic resonance (EPR) spectrum were analyzed. Four larger hyperfine couplings were assigned to the cysteine β-protons near the Fe(III) ion. Transfer into the crystal structure showed that the Fe(III) ion was coordinated to the residues Cys55 and Cys92. The spin density was estimated as +1.60 for the Fe(III) and ?0.6 for the Fe(II) ion, respectively. Theg-tensor direction with respect to the cluster showed strong similarities with the earlier assignment inArthospira platensis ferredoxin (Canne C., Ebelshauser M., Gay E., Shergill J.K., Cammack R., Kappl R., Hüttermann J.: J. Biol. Inorg. Chem. 5, 514, 2000). An Adx mutant (T54A) exhibiting a change (70 mV) in redox potential showed no significant influence at the [2Fe?2S] cluster. The Rieske ISP was subjected to the same analysis. The ENDOR spectra from 35 field positions across the rhombic (g _max=2.028,g _int=1.891, andg _min=1.757) EPR spectrum were simulated. Three major proton contributions were identified from the orientation behavior. Two were assigned to cysteine β-protons and one to a β-proton of His141. In contrast to Adx, the direction of theg _max-component was found to lie roughly in the FeS-core plane and the largest proton coupling occurred alongg _int. The spin population was estimated as about +1.6 for the oxidized and ?0.55 for the reduced iron.     

Precision measurements of the nuclear magnetic dipole moments of6Li,7Li,23Na,39K and41K

Using the atomic beam magnetic resonance method and using the technique of separated oscillatory fields in combination with the triple resonance method the ratiosg _I /g _J of the stable alkali isotopes Li, Na and K in the²S_1/2-ground state have been measured at magnetic fields of about 3 kOe. Frequency shifts analogous to the Bloch-Siegert-shift, expected from theory, were observed and eliminated by extrapolation to vanishing rf-field strength. The results, uncorrected for atomic diamagnetism, are:⁶Li:g _I /g _J =?0.22356978(10)·10^{?3 7}Li:g _I /g _J =?0.59042719(10)·10^{?3 23}Na:g _I /g _J =?0.40184406(40)·10^{?3 39}K:g _I /g _J =?0.07088613 (6)·10^{?3 41}K:g _I /g _J =?0.03890837 (4)·10^?3 Furthermore, the hyperfine structure splitting constants in the² S _1/2-ground states have been determined. Using the most precise absoluteg _J -values available so far one can calculate absolute values for the uncorrectedg _I - factors. The results are:Δν/MHz ?g _I·10^{3 6}Li: 228.2052590(30) 0.4476540 (3)⁷Li: 803.5040866(10) 1.1822130 (6)²³Na: 1771.6261288(10) 0.8046108 (8)³⁹K: 461.7197202(14) 0.14193489(12)⁴¹K: 254.0138720(20) 0.07790600 (8) By comparing the absoluteg _I - factors with measurements received by the NMR-method, the chemical shifts of the NMR-frequencies caused by the hydrate surrounding of the alkali ions in the NMR-probe are determined.     

Optical detected ESR in the4 S 3/2 excited state of Er:YCl3

The electron spin resonance in the⁴ S _3/2 excited state of Er³⁺ in yttrium trichloride was studied by optical detection techniques. From angular dependence of the resonance field the principle value of theg-tensor in direction of the twofold crystal axis was deduced to beg∥=3.350±0.004 and the perpendicular valueg⊥ in the crystallographica-b-plane was extrapolated to beg⊥=2.857±0.004. The lifetime of the excited state is found to be temperature independent with τ _r =(1.62±0.02)·10^?3 sec and the spin lattice relaxation timeT ₁ was determined in the temperature region 1.5 to 2.1 °K by observing the recovery of the fluorescent light signal after a microwave saturation pulse was switched off.T ₁ is found to follow a direct process with \(T_1^{ - 1} = k \cdot cth\left( {\frac{{\rlap{--} h\omega }}{{2kT}}} \right)\) .     

Investigation of the two quasiparticle rotational band at 1172 keV in172Yb and determination of theg R -factor of the 3+ 3 band head

Investigations of the rotational band based on the 3⁺ 3 two quasiparticle state in¹⁷²Yb have shown that some of its properties can be well described by the collective model of Bohr and Mottelson. As a result of these investigations we have derived the ratio (g _k – g _R )/Q _o =? (0.0136 ± 0.0007). The magneticg-factor of the band head has also been determined by an integral angular correlation measurement perturbed by an external magnetic field. As a result we foundg=0.201 ± 0.030. Using this value and our result of (g _k – g _R )/Q _o we have calculated theg _R -factor of the two quasiparticle state asg _R =0.283 ± 0.018 which is out of the errors smaller than theg _R -factor of the ground state rotational band. An analysis of our angular correlation experiments gave for the mixing parameter δ of theK-forbidden 1094 keV intraband transition: δ=? (3.63 _?0.06 ^+0.14 ) in agreement with the results of other authors.     

Half-life andg-factor of the 11/2− isomeric states in115Sb and117Sb

Using the time-differential perturbed angular distribution method (TDPAD) theg-factors of the 11/2^? isomeric states in¹¹⁵Sb and¹⁷Sb have been measured to beg(¹¹⁵Sb, 11/2^?)=0.97 ±0.10 andg(¹¹⁷Sb, 11/2^?)=1.02 ±0.07. The experimental half-lives and theg-factors are compared to the single-particle estimates.     

Elektronenspin-Resonanz von Stickstoffzentren in Alkalihalogenid-Kristallen

Paramagnetic nitrogen centers are produced in nitrate doped single crystals of KCl, KBr, KJ, and NaCl by introduction of F-centers. The electron spin resonance is studied at low temperature. The hyperfine structure of the lines is caused by interaction with two N¹⁴ nuclei. The angle dependence of the resonance spectra is measured by rotating the crystals about several crystallographic axes. The results can be fitted to a spin-Hamiltonian with orthorhombic symmetry. The components of theg-tensor and theA-tensor are determined. The centers are believed to be N ₂ ^? -ions in negative ion vacancies.     

The beta branching ratio and conversion coefficients in the137Cs decay

The decay of¹³⁷Cs has been reinvestigated using several precision counting methods. The emission rate ofβ-particles plus internal conversion electrons was measured by the 4π-proportional counter method using vacuum evaporated sources free of self-absorption and checked by the liquid scintillation method. TheK-conversion coefficient was determined by the electron X-ray coincidence method using a magnetic spectrometer and a high resolution Si(Li) detector. TheK/(L+M+...) conversion ratio and a second less accurate value for the β-branching ratio were obtained from the recorded electron spectra. Theγ-ray emission rate of all sources was determined to within ±0.14%, on the average, with a calibrated NaI(Tl) crystal detector. As results the intensity of theβ-decay to the ground state of¹³⁷Ba could be determined to (5.4±0.3)% of the¹³⁷Cs decays, theK-conversion coefficient to 0.0916±0.0004, and theK/(L+M+...) conversion ratio to 4.41±0.04. From these values the γ-ray emission intensity is (85.1±0.4)% of the¹³⁷Cs decays and theK X-ray emission intensity is (8.13±0.10)% of the emittedγ-rays. All errors are discussed in detail.     

Angle-selective measurements of spin soliton in ladder polydiacetylene by pulsed 94 GHz EPR

Angle-selection experiments of a spin soliton in randomly oriented ladder polydiacetylene were carried out by pulsed electron paramagnetic resonance (EPR) at W-band. EPR measurement using 94 GHz microwaves increased the difference in the resonance field due tog anisotropy of the spin soliton to allow the orientation dependence of transient nutation, electron nuclear double resonance (ENDOR) and spin relaxations to be investigated. The shape of theg anisotropy-resolved nutation spectrum was discussed on the basis of the EPR transition moments and the differences between spin relaxation times. Reliable assignments of hyperfine couplings to the β protons (H_β) of the alkyl side chains were achieved with the support of W-band ENDOR measurements. No significant orientational dependence in theT ₁ andT ₂ processes was found in terms of isotropy of the H_β-hyperfine interaction.     

Hyperfine structure measurements in the4 I 9/2 ground state of141Pr

Using the atomic beam magnetic resonance method, the five hyperfine structure separations in the 4f ³ 6s ^{2 4}I_9/2 ground state of ₅₉ ¹⁴¹ Pr have been measured. The results are:F F′ E _FF′ ^* /h (MHz) 7 6 6477.913423(17) 6 5 5556.359848 (6) 5 4 4633.023306 (2) 4 3 3708.201146 (5) 3 2 2782.190601(15) From these quantities, the multipole interaction constantsA _k,k=1, 2, 3, 4 between the nucleus and the electron shell have been calculated.A ₄ especially then served to give the following limit for the intrinsic hexadecapole moment: ¦Q ₄₀¦<0.4eb ². Furthermore, theg _J -factors of the⁴ I multiplet have been measured at magnetic fields of 300 Oe. The results are:g _J(⁴ I _9/2)=0.7310371(15)g _J(⁴ I _11/2)=0.9651476(20)g _J(⁴ I _13/2)=1.1063197(40)g _J(⁴ I _15/2)=1.197963 (30) Small corrections due to perturbations by neighbouring fine structure levels are included.     

Observation of knight shifts in metallic tungsten and tantalum with the 6.2 keV181Ta Mössbauer resonance

Completely resolved Mössbauer spectra for the 6.2 keV¹⁸¹Ta transition were measured at R.T. in external magnetic field of 2.8 and 4.4 kOe for¹⁸¹Ta impurities in W- and Tametal respectively. A more precise mean value for theg-factor ratiog _9/2/g _7/2 of 1.797(3) was obtained. The magnetic moment of the 6.2 keV-state was determined to be +5.47 (2) n.m. From the hyperfine fields at the¹⁸¹Ta-nuclei in W a Knight-shift of 2(1)% and in Ta of 1.7(8)% was deduced. For the spectrum of the magnetically splitted Ta-metal absorber a reduced dispersion parameter of 2ξ=20(2) was measured, while the linewidth showed only a slight narrowing.     

Electron spin resonance measurements on single crystals of the organic superconductor κ-(BEDT-TTF)2I3

Angularly and temperature-dependent electron spin resonance (ESR) measurements on single crystals of the organic superconductor κ-(bis(ethylenedithiolo)-tetrathiafulvalene)₂I₃ are reported. The results on the anisotropy of theg-value, the temperature dependence of the susceptibility as well as of the ESR linewidth are discussed.     

Quantum transport in lattices of coupled electronic billiards

A new system with dynamic chaos—2D lattice of single Sinai billiards coupled through quantum dots—is studied experimentally. Localization in such a system was found to be substantially suppressed, because the characteristic size of the billiard for g≤1 (g is conductance measured in e ₂/h units) is the localization length rather than the de Broglie wavelength of an electron, as in the usual 2D electron system. Lattice ballistic effects (commensurate peaks in the magnetoresistance) for g?1, as well as extremely large magnetoresistance caused by the interference in chaotic electron trajectories, were found. Thus, this system is shown to be characterized by simultaneous existence of effects that are inherent in order (commensurate peaks of magnetoresistance), disorder (percolation charge transport), and chaos (weak localization in chaotic electron trajectories).     

Mode-crossing signals of high order on the 4d 95s 2 2 D 5/2 state of Cd II isotopes

In the linearly polarized radiation field of a 442 nm He-Cd laser containing a natural isotope mixture, saturation-induced mode-crossing signals of the 4d ⁹5s ^{2 2} _5/2 state of Cd II are observed due to the even as well as the odd isotopes. The signal width of about 10^?4 T yields high resolution. Thus the signal splitting respective to the magnetic quantum number can be resolved. Theg _J - andg _F -factors of the² D _5/2 state are determined as follows:g _J=1.1980±0.0036,g _F(F=2)=1.397±0.008,g _F(F=3)=1.002±0.009.