Elektrisches Kernquadrupolmoment von Cäsium 134

The hyperfine structure splitting of the 7² P _3/2 state of Cs¹³⁴ has been measured by optical double resonance spectroscopy in zero magnetic field. The following interaction constants have been obtained: Magnetic hfs constantA(7² P _3/2, Cs¹³⁴)=16.851 (16) MHz. Quadrupole coupling constantB(7² P _3/2, Cs¹³⁴)=18.07 (12) MHz. Then the electric quadrupole moment of Cs¹³⁴ can be calculatedQ _hfs(Cs¹³⁴)=+0.436 (3) barn without Sternheimer correction, andQ(Cs¹³⁴)=+0.356(2) barn with Sternheimer correction. A method for the production of alkali resonance cells with quantities of less than 10¹⁴ atoms of the radioactive isotope is described.     

On theg-value shift and intersystem crossing in photo-excited radical-triplet systems

The electron paramagnetic resonance (EPR) signals of photoexcited quartet (Q₁) states for zinc(II) tetra-tert-butyl-phthalocyanine (ZnPc) ligated by 3- and 4-(N-nitronyl-nitroxide) pyridine radicals (3-NOPy, 4-NOPy) were observed in toluene solution at room temperature by means of X-band (9.4 GHz) time-resolved EPR (TREPR) spectroscopy. Theg values of Q₁ in the ZnPc-3-NOPy and ZnPc-4-NOPy complexes were found to beg=2.0025 andg=2.0036, respectively. The obtainedg value (2.0036) for ZnPc-4-NOPy is in good agreement with the value (g=2.0037) of the Q₁ state calculated under the strong-exchange limit. Theg value (2.0025) is just an average of the Q₁ and D₁ (g=2.0013) states for ZnPc-3-NOPy. Theg value of Q₁ for zinc(II) meso-tetraphenylporphine (ZnTPP) ligated by 3-NOPy showed a slight shift (g=2.0027) at X-band and no shift (g=2.0031) at W-band from the calculatedg value (g=2.0031) (J. Fujisawa, Y. Iwasaki, Y. Ohba, S. Yamauchi, K. Koga, S. Karasawa, M. Fuhs, K. Möbius, S. Weber, Appl. Magn. Reson. 21, 483–493, 2001). These changes in theg value were found to originate from an averaging of the TREPR spectra over the Q₁ and photoexcited doublet (D₁) states via a fast intersystem crossing (ISC) process. The ISC rates between these two states were estimated by means of numerical calculations with the modified Bloch equations as 1.2·10⁸ and 6·10⁷ s^?1 for the ZnTPP-3-NOPy complex at the X- and W-bands, respectively. The lower limit of the ISC rate was obtained as 10⁹s^?1 for the ZnPc-3-NOPy complex and the higher limit was found to be 3.1·10⁸ s^?1 for the ZnPc-4-NOPy complex.     

Theg B*Bϱ andg D*Dϱ coupling constants in QCD

Theg _B*B? andg _D*D? coupling constants are calculated at the intermediate momentum transfer interval 0.5 GeV²≤Q ²≤1.5 GeV², in the context of QCD sum rules. Our predictions are in good agreement with those of the Vector Dominance Model     

Spin exchange polarization and measurement of the hyperfine splitting of short-lived8Li

Free⁸Li (T _1/2=0.84 sec) atoms were polarized by spin exchange with optically pumped rubidium vapour. The⁸Li nuclei were produced by the reaction⁷Li(d, p)⁸Li in a thin LiF target. They recoiled into helium which was used as a buffer gas for optical pumping as well. Nuclear polarization up to 2% was detected by β-decay asymmetry. Rf transitions between hfs levels of the atomic ground state yielded the hfs separation ΔW=382.543(7) MHz. The nuclear spin assignmentI=2 was confirmed.     

Zeemaneffektuntersuchungen im Er I

Zeeman effect investigations in Er I were made in the 3000 to 5000 Å range at field strengths of about 26 kOe. A total of 74g _J-values were determined. Theg _J-values for 11 levels of the configuration 4f ¹¹ 5d 6s ² and for 4 levels of 4f ¹² 6s ² are compared with theoretical values calculated by other authors.     

Jahn-Teller-type phase transitions in oxide ceramics and other solids with Cu(II) — from the viewpoint of a solid state chemist

After a short outline of the vibronic features of the local Jahn-Teller effect (E _g ? ε _g coupling) the cooperative aspects — in particular the various order patterns of the elongated octahedra in Cu(II) solids — are reviewed. While the elastic interactions can be described by energy terms of Q_ε symmetry, the local Jahn-Teller effect has Q_θ symmetry (? = 0). Phase transitions in dependence on the Cu(II) concentration are analysed in detail for oxidic ordered perovskites Ba(Sr)₂Zn(Ni)_1-x Cu_xW(Te)O₆, which exhibit ferrodistortive order patterns connected with first order phase transitions at high x-values. At lower x-values at the short-range order scale elastic Q_ε-type strains prevail as in the cases of antiferrodistortive order, leading to continuous phase transitions. Here, the cluster distribution has been successfully analysed by EPR spectroscopy.     

Zur Rumpfpolarisation des Rb-Atoms: Hyperfeinstruktur des 72 P 1/2-Terms von Rb85

The hyperfine structure splitting of the 7² P _1/2 state of Rb⁸⁵ has been investigated with optical double resonance spectroscopy. The results are:v _F=3?F=2=52.95(6) Mc/s,A _J=1/2=17.65(2) Mc/s,g _J (7² P _1/2, Rb)=0.6668(1). From the hfs interaction constants of the 7² P multiplett a 12% core polarization contribution to the magnetic hfs of the 7² P _3/2 state can be deduced. Comparison is made between the values of 〈r ^?3〉 calculated from either the magnetic hfs or the² P fine structure separation.     

Sum rules in QCD for g ηΛΛ and g πΛΣ

New relations between the Borel QCD sum rules for the strong couplings of the Σ⁰ and Λ hyperons are derived. It is shown that, using the sum rules for g _MΣΣ, M = π ⁰, η, as a starting point, one can readily obtain the corresponding sum rules for the coupling constants g _ηΛΛ and g _πΛΣ ⁰. The values of these constants are calculated in a specific parameter region.     

Calculation of the Hyperfinestructure and gJ-Values of the 3d 4s 4p-Configuration of Scandium

Wave functions for the 3d 4s 4p, 3d² 4p and 4s² 4p configurations of ScI are calculated, taking into account departures from SL-coupling and configuration interaction and on fitting the radial integrals to the experimental fine structure energies. Using these wave functions g J -values are derived. The intermediate coupled hfs matrix elements of the 3d 4s 4p configuration are reduced to the unknown electron coupling constants a_s, a_p and a_d and calculated on estimating these constants from the spin-orbit coupling constants and fitting them to some experimental A-values. By this way the absolute phases of the experimental A((¹P)²D)- and A((¹P)²F)-values are obtained. Good agreement between calculated and most experimental data has been achieved.     

Determination of isotope shift crossed-second-order-effects and hyperfine-structure parameters in the Eu I configuration 4f 7 6s7s

In the Eu I configuration 4f ⁷(⁸ S)6s7s the isotope shift (IS) and hyperfine-structure (hfs) of the levelse ⁸ S _7/2 andf ⁸ S _7/2 were determined from the transitions 684.5 nm, 733.7 nm and 821.0 nm to 4f ⁷6s6p. Together with experimental results of our previous measurements a theoretical analysis of the IS and hfs for the complete configuration 4f ⁷ 6s7s can now be carried out. From the IS of the four 6s7s-levels we evaluated the two crossed-second-order-parametersg ₃(4f,6s)= ?l.l(l)mK andg ₃(4f, 7s)= ?0.1(l)mK. The ratiog ₃/G ₃ is determined for various Eu configurations and found to be equal to 5.6(3)·10^?6 in complete agreement with a theoretical value following from Hartree-Fock calculations. The single electron hfs splitting constantsa ¹⁰(4f)= ?1.9 (3) mK,a ¹⁰(6s)=396(3)mK, anda ¹⁰(7s)=65(3)mK are also determined and compared with those of other Eu configurations.     

Doppler-free multiphotonic spectroscopy in hydrogen applications to Rydberg’s constant

We recall the principle and the general features of Doppler-free multiphotonic absorption. We discuss problems of lineshape and light shift, which are determinant for very high resolution measurements. We describe the recent application of the method on hydrogen atoms or deuterium atoms in an atomic beam experiment. The high precision obtained, of the order of 10⁻⁹, permits to confirm preceding measurements of the ratioM _p /M _e of the masses of proton and electron, and to deduce a new value of the Rydberg’s constant. Improvements of the experiment are in progress for the precision 10⁻¹⁰.     

2 S 1/2-states of Pb3+-ions: Correlations betweeng-values and hyperfine splitting constantsA in the EPR-spectra

The EPR-spectra of the² S _1/2-state of Pb³⁺-ions in different host lattices are characterized by a pronounced hyperfine splitting constantA, a smallg-shift from the free spin valueg _e and an appreciable amount of superhyperfine interaction. The unpaired 6s-electron of the Pb³⁺-ion is predominantly found in molecular orbitals built from the central 6s-function and orbitals of the sorrounding ligand ions. This assumption leads to correct estimates of theg-shift (Watanabe theory) and the reduction ofA due to covalent bonding effects. Theg- andA-values are shown to be correlated.     

Beta-decay energies and nuclear masses of very neutron-rich Rb and Cs isotopes

Theβ-endpoint energies of very neutron-rich Rb and Cs isotopes with mass numbers 94≦A≦98 and 142≦A≦146, respectively, have been measured with a plastic scintillatorβ-telescope at the on-line mass separator OSTIS. From these,Q _β-values and mass excesses are calculated and compared with the results obtained in direct mass determinations.     

Spin exchange polarization and hfs anomaly measurement of β-active37K

Rf-spectroscopic measurements in the atomic ground state of the 1.2 sec-isotope³⁷K yielded independent values of magnetic moment μ _I (³⁷K)=0.20320 (6) nm (diamagnetically corrected), hfs separation ΔW(³⁷K)=240.2672 (7) MHz and the resulting hfs anomaly³⁷Δ³⁹=(0.249±0.035) · 10^?2. The³⁷K was polarized by spin exchange scattering with optically pumped⁸⁷Rb; as a detector for rf transitions the asymmetry of the β-decay of the polarized³⁷K nuclei was used. A detailed analysis of the spin exchange mechanism under the conditions of experiments of this type was performed. This led to an explanation for the observed strong enhancement of the resonance signal heights by the spin bath. Furthermore the dependence of the nuclear polarization and relaxation time constants on the strength of the hfs coupling could be demonstrated.     

Development of metal borohydrides for hydrogen storage

A metal borohydride M(BH₄)_n is a potential candidate for hydrogen storage materials because of its high gravimetric hydrogen density. The important research issues for M(BH₄)_n are to control the thermodynamic stability and to achieve the faster reaction kinetics. To clarify the thermodynamic stability, M(BH₄)_n (M=Mg, Ca∼Mn, Zn, Al, Y, Zr and Hf; n=2-4) were synthesized by mechanical milling and its thermal desorption properties were investigated. The hydrogen desorption temperature T_d of M(BH₄)_n decreases with increasing Pauling's electronegativities χ_P of M. Because Mn, Zn, and Al borohydrides (χ_P?1.5) desorb borane, they are too unstable for hydrogen storage applications. The enthalpy changes of desorption reaction ΔH_des can be estimated by using our predicted heat of formation of M(BH₄)_n ΔH_boro and reported data for decomposed products ΔH_prod, which are useful indicators for searching M(BH₄)_n with appropriate stability for hydrogen storage material. In the latter case, microwave processing was adopted for achieving fast reaction kinetics. Among metal borohydrides, LiBH₄ was rapidly heated above 380 K by microwave irradiation, 13.7 mass% of hydrogen was desorbed by microwave irradiation. The composites of LiBH₄ with B or C desorbed hydrogen within 3 min. Microwave heating aids in realizing faster kinetics of the hydrogen desorption reaction.     

Tailored Nitroxide Radicals and Biradical Containing <Superscript>13</Superscript>C Enriched Acetylene Groups: ENDOR and DFT Investigation

A specially synthesized nitroxide biradical R₅-C≡¹³C-(p-C₆H₄)₂-¹³C≡C-R₅ (B4) and two radicals, R₅-C≡¹³CH (RCC) and R₅-C≡¹³C-C₆H₅ (RCCPh), where R₅ is 1-oxyl-2,2,5,5-tetramethyl-pyrroline group, have been studied by X- and W-band electron paramagnetic resonance (EPR) spectroscopy, and by W-band electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constants on ¹³C atoms were experimentally determined and also calculated using ORCA 3.0.3 program package. The biradical and radicals geometries were optimized on UKS/B3LYP/cc-pVDZ level. Hfs constants were calculated using density functional theory (DFT) with PBE0 functional and N07D, and were compared with the experimental value of the hfs constant on ¹³C atoms, measured from ENDOR spectra. It is concluded that at small values of the exchange integral as J ≤ a/2 ≈ 7–8 G, the current quantum chemical approaches do not allow determining precise values of the hfs constants on the ¹³C atoms in the bridge connecting two paramagnetic nitroxide rings of the biradical.     

Excited quartet and doublet states in the complex of tetraphenylporphine zinc (II) and a nitroxide radical in solution: X- and W-band time-resolved EPR studies

The photoexcited quartet (Q₁) and doublet (D₁) states of the complex of tetra-phenyl-porphine zinc (II) and a nitroxide radical have been studied in toluene solution by X-(9.4 GHz) and W-band (94 GHz) time-resolved electron paramagnetic resonance spectroscopy. The spin-polarized signals of the Q₁ and D₁ states are observed and assigned by analysis of theg-values. Line broadening and spin polarization mechanisms in this system are discussed.     

Activation energy of hydrogen desorption from high-performance titanium oxide carrier-selective contacts with silicon oxide interlayers

The impact of hydrogen desorption on the electrical properties of TiO_x on crystalline silicon (c-Si) with SiO_y interlayers is studied for the development of high-performance TiO_x carrier-selective contacts. Compared with the TiO_x/c-Si heterocontacts, a lower surface recombination velocity of 9.6 cm/s and lower contact resistivity of 7.1 mΩ cm² are obtained by using SiO_y interlayers formed by mixture (often called SC2). The hydrogen desorption peaks arising from silicon dihydride (α1) and silicon monohydride (α2) on the c-Si surface of the as-deposited samples are observed. The α1 peak pressure of as-deposited heterocontacts with SiO_x interlayers is lower than that of heterocontacts without a SiO_y interlayer. Furthermore, the hydrogen desorption energies are found to be 1.76 and 2.13 eV for the TiO_x/c-Si and TiO_x/SC2-SiO_y/c-Si heterocontacts, respectively. Therefore, the excellent passivation of the TiO_x/SC2-SiO_y/c-Si heterocontacts is ascribed to the relatively high rupture energy of bonding between Si and H atoms.     

Linear-chain ferromagnetism in the one-dimensional compounds CoCl2(pyrazole)2 and CoCl2(indazole)2

Two linear-chain compounds of formula CoCl₂L₂, with L = pyrazole (pz) and indazole (indz), have been investigated with the aid of IR, far-IR, ligand field and ESR spectra. Magnetic susceptibility data have been collected in the temperature region 4.2–80 K. Analysis of the susceptibility data in terms of the Ising model yielded the values of the ferromagnetic intrachain exchange constants J = 7.2(6) and 7.4(9) cm^-1 for the pz and indz compound, with corresponding g-values of g∥ = 7.9(7), g⊥ = 4.6(9) and g∥ = 10.8(9), g⊥ = 3(1), respectively. The results are discussed and compared with the similar compound CoCl₂(pyridine)₂. The influence of the bridge geometry and of the π-bonding properties of the organic ligands on the exchange constant are considered for the explanation of different J-values. It is suggested that the π-bonding properties of the organic ligand influence the magnitude of the superexchange constant to a great extent.     

A high-field EPR tour of radicals in photosystems I and II

High magnetic-field electron paramagnetic resonance (HFEPR) has been extensively applied to the study of photosynthetic reaction centers. HFEPR experiments have provided accurate measurements ofg-values of radicals in photosystems I and II. Theg-values not only reflect the structure of the radical, but also its immediate environment. Hence, valuable information about radicalprotein interactions can be obtained fromg-values. Data on tyrosine-, quinone-, pheophytin- and chlorophyll-based radicals are reviewed. Experiments dealing with spin pairs in photosystem I and II are examined. The spectrometer with which numerous experiments on photosystems have been carried out is described, as well as the 10 T magnet around which the spectrometer is based.