Farbzentren rhombischer und monokliner Symmetrien in NaF

A number of zero-phonon absorption lines ofF aggregate color centers is studied inx-irradiated NaF crystals under uniaxial stress. The color centers associated with the lines are found to exhibit rhombic (D_2h, D₂, C_2v; rotation axes along 〈110〉 and 〈100〉) and monoclinic (C_2h, C₂, C_s; rotation axis along 〈110〉) symmetries. The transitions of the rhombic color centers correspond to 〈100〉 and 〈110〉 dipoles, those of the monoclinic centers to 〈112〉 and 〈110〉 dipoles. The most prominent line at 5803 Å is due to a 〈112〉 dipole transition within a monoclinic color center. Several models of centers are discussed.     

The influence of high uniaxial stress on the ΓI,C conduction band effective mass of gallium arsenide

From a study of the Zeeman splitting of the shallow donor states, the variation of the effective mass in the Γ_IC conduction band minimum of gallium arsenide has been measured as a function of applied uniaxial stress. The effective mass is found to be proportional to the stress, the percentage increase being (0.23 ± 0.06), (0.115 ± 0.015) and (0.075 ± 0.015) kbar^?1 for 〈100〉, 〈110〉 and 〈111〉 stress respectively. The precision inherent in the far-infrared technique employed is greater than that of previous experimental methods.     

Phononenfreie Absorptions- und Emissionsübergänge in strahlungsverfärbten NaF-Kristallen

Inγ-irradiated NaF crystals at 5456, 5754, 5837, 6070 and 6146 å zero-phonon lines are observed in absorption and emission. These resonance lines are accompanied by multiphonon absorption and emission bands with their spectral positions nearly mirror symmetric with respect to the zero-phonon line position. By excitation spectra higher excited states are detected. Uniaxial stress splitting of the lines 5754 and 6070 å is studied in absorption and emission. The color centers associated with the lines are found to exhibit rhombic symmetry with rotation axes along 〈110〉 and 〈100〉 directions and monoclinic symmetry with a rotation axis along 〈110〉 direction. The transitions in both centers correspond to 〈110〉 dipoles. In the monoclinic center the transition between the ground and the second excited state seems to correspond to a 〈112〉 dipole.     

The β-moments for the second-forbidden β-decay from137Cs,the CVC theory,and the residual interactions

Theβ-moments of the 7/2⁺(1.176 MeVβ ^?)3/2⁺ transition in the decay of¹³⁷Cs have been extracted from the available experimental data. We have obtained the following values: 〈R _ij 〉/R ²=0.035±0.020, 〈iT _ij 〉/R ²=0.067±0.036, and ?0.085≦〈S _ijk ～>/R ²≦0.085. These results show that theβ-moments are reduced with respect to those evaluated by means of the conventional pairing model. These quenching effects are similar to those previously found in the analyses of first-forbiddenβ-decays and are attributed to residual charge-exchange interactions. The results forΛ(=?〈iA _ij 〉/ξ〈R _ij 〉) are discussed in the light of the predicitons of the CVC theory.     

A trigonal color center in NaF

Uniaxial stress splitting experiments suggest the 5456 Å zero-phonon line in NaF to be due to a transition of the F ₃ ⁺ center. The line and a number of phonon-assisted peaks are observed in absorption and emission.     

Optical investigation of HoFeO3

The absorption spectra of HoFeO₃ were investigated in the near infrared spectral region at temperatures of 1.2, 4.2, 20 and 77 °K respectively. At every temperatureT≦20 °K all the absorption lines show the same splitting which is attributed to the groundstate; this splitting is (7.2±0.5) cm^?1 at 20 °K and decreases to (4.9±0.8) cm^?1 extrapolated to 0 °K. From these splittings the holmium-iron and the holmiumholmium interactions can be deduced. Measurements with an external magnetic field yield a magnetic moment ofμ=(7.6±0.7)μ _B per holmium ion. The moments are directed at angles of ?28° and ?152° with respect to theb-axis.     

Isomer shifts and changes of the nuclear charge radii in99Ru and101Ru

The recoilless nuclear gamma resonance of the 127 keV γ-rays of¹⁰¹Ru was observed in ruthenium metal, RuO₂ and [Ru(NH₃)₄(HSO₃)₂]. By comparison of the isomer shifts observed in these materials for the 127 keV absorption line with the corresponding shifts of the 90keV γ-rays of⁹⁹Ru one obtains δ〈r ²〉 [127 keV]/ δ〈r ²〉 [90 keV]=1.78±0.26 for the ratio of the changes of the mean square nuclear charge radii between the first excited and the ground states in these nuclei. An estimate of electron density differences based on free-ion relativistic self-consistent field calculations yields δ〈r ²〉[90keV]≈+1.4·10^?3 for⁹⁹Ru and δ〈r ²〉/〈r ²〉 [127 keV]≈+2.4·10^?3 for the¹⁰¹Ru case. These results are discussed in terms of the core excitation model.     

硅中金受主能级在单轴应力下瞬态电容的研究

用恒定温度下瞬态电容法研究了硅中金受主能级在沿〈100〉,〈110〉,〈111〉晶向单轴应力作用下的能级移动。考虑到硅的导带在单轴应力下的分裂,导出了单轴应力下深中心中电子发射率的公式。根据该式和发射率的实验数据以及切变畸变势常数Ξ_u,求出了金受主能级激活能随应力的改变。在实验应力范围内(0—9kbar),激活能与应力成线性关系。当应力平行于〈110〉〈111〉晶向时,激活能随应力改变的斜率分别为α_<110>=-3.2±0.6meV/kbar,α_<111>=-0.3±0.6meV/kbar;当应力平行于〈100〉晶向,若取Ξ_u=9.2eV,α_<100>=-5.8±0.8meV/kbar,若取Ξ_u=11.4eV,α_<100>=-5.3±0.8meV/kbar,α表现出强烈的各向异性。进一步确定了金受主能级相对于零压力下导带底的绝对移动的压力系数。若取Ξ_u=9.2eV,这些系数分别为S_<100>=-1.3±0.8meV/kbar,S_<110>=0.7±0.6meV/kbar,S_<111>=-0.7±0.6meV/kbar。如取Ξ=11.4eV,则S_<100>=-3.5±0.8meV/kbar,S_<110>=0.0±0.6meV/kbar,S_<111>=-1.0±0.6meV/kbar。同一组的三个绝对移动值之间的偏离都超过了实验误差这一事实,说明了金中心具有立方对称性,同时中性金与带负电的金的基态都是非简并的可能性很小。 关键词：     

Hydrogen maser atomic beam resonances

Au^? ions at anionic places are formed in gold doped crystals by a reducing treatment withF centers. The ultraviolet absorption consists of 4 bands, which are namedA, B, C, andD in analogy to the isoelectronic centers of the s² type, like Tl⁺. TheB band oszillator strength strongly increases with temperature in accordance with a phonon allowed transition. The ratio of the dipole strength of theC band to that of theA band as a function of the relative position of theB band is compared with Suganos prediction. Zero phonon lines are found at helium temperatures for theA band in NaCl (2,985 Å), KCl (3,068 Å), and KBr (3,145 Å) and for theC band in KCl (2,329 Å). In KCl the Huang-Rhys factor isg=3.4 for theA band. The vibronic structure comes from the relatively large radius 6s ² state of the negative ion. Uniaxial stress splits the zero phonon line. The results definitely agree with the stress splitting behaviour of a degenerateΓ ₁→Γ ₄ transition. Inversion symmetry of the center is confirmed by the absence of a linear Stark effect.     

Paraelastische Defektelektronen-Zentren in Alkalihalogeniden

We have investigated the stress-induced alignment of Cl₂ ^? and Br₂ ^? molecular ions substituting for Cl^? and Br^? in KCl and KBr, respectively (so calledH-centers). By means of the dichroism induced in the optical absorption (H-band) by uniaxial stress we determined the static coupling to the lattice and the kinetics of the reorientation. Contrary to the O₂ ^?-center, which also substitutes for an anion, the reorientation-process of theH-center can be described by an Arrhenius-relation. TheH-centers freeze in between 10 and 13°K. In KBr a new center was found which is closely related to theH-center, but differs in the kinetics of the reorientation. It is presumably anH-center associated with another lattice defect. Further we have studied the paraelastic properties of the defects which in KCl and KCl:NaCl give rise to theV ₁-band. We found that depending upon the doping of the crystal different defects produce theV ₁-band.     

Theoretical integrated intensities for the 2ν2 and 2ν2-ν2 bands of HCN and DCN

Dipole moment functions, both perpendicular and parallel to the molecular axis, are calculated from the SCF and MRD-CI results of a previous study for the normal ν₂ bending vibrations of HCN and DCN. Vibrationally averaged dipole moments and the infrared transition matrix elements are then obtained from the dipole moment functions and vibrational wave functions. MRD-CI results, with known experimental values in parentheses, for HCN are 〈0|μ|0〉 = ?2.954(?2.985) D, 〈1|μ|1〉 = ?2.915(±2.942) D, 〈0|μ|1〉 = 0.148(0.147) D, 〈0|μ|2〉 = ?0.027 D, 〈1|μ|2〉 = 0.210 D. Calculated absolute intensities at 1 atm and 0°C for the (02⁰0) ← (000), (02⁰0) ← (010), and (02²0) ← (010) bands of HCN are 25 (40 ± 10 as estimated from spectra), 8.5, and 17.0 atm^?1 cm^?2, respectively. Results for DCN are also reported.     

Optical investigation of ErFeO3

The absorption spectrum of single crystals of ErFeO₃ has been investigated in the red and near infrared spectral region in the temperature range between 1.2 °K and 4.2 °K and at 20 °K and 77 °K. Between 77 °K and 4.2 °K a constant splitting of the absorption lines is observed. Below the Néel-temperature of the erbium sublattice at 4.5 °K the splitting of the absorption lines increases; the saturation value extrapolated to 0 °K of the splitting of the lowest crystal field level of the⁴ I _15/2 groundterm is (6.08±0.30) cm^?1. By measuring the Zeeman effect the groundstate magnetic moment is determined asμ=(6.6±0.2)μ _B. The measured temperature dependence of the splitting of the lowest crystalfield level of the⁴I_15/2 groundterm is compared with that calculated by a Monte Carlo method.     

Die Isotopieverschiebung von151Eu,152Eu,153Eu,154Eu und die Kernmomente der radioaktiven Isotope152Eu und154Eu

The isotope shift of the stable¹⁵¹Eu,¹⁵³Eu, and the radioactive¹⁵²Eu,¹⁵⁴Eu isotopes and the hyperfine splitting of the¹⁵²Eu isotope was investigated using a digital recording Fabry-Perot-spectrometer. From isotope shift measurements on the line λ 5 765 Å (4f ⁷ 6s 6p z⁶P_7/2-4f ⁷ 6s² a⁸S_7/2) the relative isotope shift was derived:¹⁵¹Eu:0;¹⁵²Eu: 0.923(8);¹⁵³Eu: 1;¹⁵⁴Eu: 1.197(8). The results show that there is a strong increase in the change of the mean square nuclear charge radius δ〈r²〉 when only one neutron is added to the 88 neutrons of the¹⁵¹Eu nucleus, whereas the change of δ〈r²〉 between¹⁵²Eu and¹⁵³Eu is of the same order of magnitude as that between¹⁵³Eu and¹⁵⁴Eu. From the hyperfine splitting of the radioactive isotope¹⁵²Eu in the line δ 6865 Å (4f ⁷ 6s 6p z ¹⁰ P _9/2-4f ⁷ 6s² a⁸S_7/2) the sign of the magnetic dipole moment μ_I(¹⁵²Eu) was found to be negative, and with this result and earlier experimental data the signs of the nuclear quadrupole momentsQ(¹⁵²Eu) andQ (¹⁵⁴Eu) could be determined to be positive.     

Magnetoreflectance of the Γ6 – Γ8 exciton ground state in cubic ZnSe

Magnetoreflectance measurements on the ground state of the Γ₆ – Γ₈ free exciton in cubic ZnSe in magnetic fields up to 18 T are reported. The splitting between the |1, ±1〉 states was derived from the measured difference spectrum between σ⁺ and σ^--polarized reflectance in Faraday configuration. The splitting between the two states corresponding to |2, 0〉 and |1, 0〉 at B = 0 was determined by means of a lineshape analysis. We derive an electron g-factor g = 1.48 ± 0.25, in reasonable agreement with existing k · p calculations, and obtain an effective hole g-value $\text{K}\text{?}\text{= -0.26±0.06}$. In addition, we find an upper limit for the short range electron-hole spin exchange energy Δ ? 0.1 meV, which is considerably smaller than values, which is considerably smaller than values reported in the literature, but agrees with recent results on ZnTe obtained by uniaxial stress and also magnetoreflectance measurements.     

Magnetic-field dependent phonon states in paramagnetic CeF3

Raman scattering experiments in paramagnetic uniaxial CeF₃ at helium temperature demonstrate a splitting of some degenerate (E_g)-phonon states in an external magnetic field parallel to the crystal axis. A linear splitting is observed in low fields, followed by a field independent (saturation) splitting in high fields. In addition, changes in the Raman scattering efficiencies and a reduction of the line width of phonon transitions are observed with increasing magnetic fields. No such effects are observed for magnetic fields perpendicular to the crystal axis. The splittings of degenerate phonon modes are related to the paramagnetic saturation 〈S_z〉.     

The cross section of14N(n,p)14C at stellar energies and its role as a neutron poison forS-process nucleosynthesis

The absolute average cross section 〈σ〉 of the¹⁴N(n, p)¹⁴C reaction has been measured using neutron spectra that closely resemble Maxwell-Boltzmann distributions with thermal energies of kT=25.0 and 52.4 keV: 〈σ〉=0.81±0.05 and 0.52±0.06 mb, respectively. The resulting reaction rates are nearly the same at T₉=0.29 and 0.61, and their average, N_A〈συ〉=(1.3±0.1)×105 cm³ s^?1 mol^?1, is about a factor of three smaller than the previously adopted values obtained by extrapolation between thermal and higher-energy data. Thus, the¹⁴N(n, p)¹⁴C reaction plays a correspondingly smaller role as a neutron poison fors-process nucleosynthesis.     

Spins,moments and charge radii in the isotopic series181Hg-191Hg

The hyperfine structure splitting and the isotope shift in the (6 s^{2 1}S₀ - 6s 6p³P₁,λ=2,537 Å) line of very neutron deficient Hg isotopes were determined by the β radiation detected optical pumping method (β-RADOP). In addition, nuclear magnetic resonance was observed in the atomic ground state. The results are Mean-square nuclear charge radii are calculated. Interpreting the sudden change of nuclear radius between¹⁸⁷Hg and¹⁸⁵Hg δ〈r²〉^187,185=0.42(5)fm² as oblate-prolate shape transition, one obtains δ〈β²〉 =0.054(5).     

Characteristics of π−-meson multiplicity distributions in central collisions of12C and16O with nuclei at a momentum of 4.5 GeV/c per incident nucleon

Multiplicities of π^?-mesons from central collisions of¹²C and¹⁶O(4.5 GeV/c/n momentum) with several target nuclei were studied in a streamer chamber experiment. The parameter η=(〈n _? ² 〉?〈n_?〉²)/〈n_?〉 was determined for several samples of events characterized by different values of Θ _ch — the “vetoangle” for emission of the projectile charged fragments. The value of η is shown to decrease when the projectile mass number increases from 12 to 16 and to decrease significantly when Θ _ch increases from 0° to 4°, whereas it remains nearly constant when Θ _ch increases further to 14°. The results are compared with predictions of some theoretical models.     

Anionische Silberzentren („B-Zentren“) in Alkalihalogeniden

It is proposed that negatively charged silver ions on anion sites are responsible for theB bands found in silver doped alkalihalides. Experimental investigations confirming this model of theB center are presented. Optical absorption and emission ofB centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of theB center. Thermal or optical excitation causes theB centers to dissociate into neutral silver atoms andF centers. The Ag⁰ centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. TetragonalB _A centers were formed during optical bleaching of theB band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those ofF _A centers.B _A fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of theB absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration5s² for the Ag^? ion. The results are compared with those for the isoelectronic centers In⁺ and Sn⁺⁺. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu^? and Tl^? centers.     

Non-perturbative QCD vacuum frome +e−→I=1 hadron data

We estimate the gluon vacuum condensate α _s 〈F ²〉 from thee ⁺e^?→I=1 hadron cross-section known below 2 GeV using moment sum rules ratios. We obtain α _s 〈F ²〉= (3.9±1.0)10^?2GeV⁴. We also re-evaluate the contribution of the dimension-six vacuum condensates to the above sum rule and test the factorization hypothesis of the four-quark operator. Useful rules for the evaluation of the dimension-six vacuum condensates contributions are given.