Analytische Probleme der Darstellung radioaktiven Schwefelwasserstoffs mit hoher spezifischer Aktivit?t

Zusammenfassung Es wird ein Verfahren im Mikromaßstab zur Darstellung und Analyse von höchstreinem Schwefelwasserstoff von hoher bis sehr hoher spezifischer Aktivität beschrieben. Durch Anwendung der Adsorptions-Chromatographie und isothermer Destillation konnte bei hohen Ausbeuten der Gehalt an Verunreinigungen unter 10^–2 Molprozent gesenkt werden. Die massenspektrometrische H₂S-Analyse und ihre Problematik wurden ausführlich behandelt. Durch geeignete Versuche und ihre theoretische Interpretation konnte eindeutig nachgewiesen werden, daß im Massenspektrometer durch Nebenreaktionen des H₂S sowohl CS₂ als auch H₂O sekundär gebildet werden, wodurch eine Verfälschung des Analysenergebnisses eintritt.Als Kontrollmethode für die Reinheit des H₂S während des Ablaufes der Sulfidierungsreaktionen im Innern der geschlossenen Apparatur erwies sich die Messung des Tripelpunktdruckes als besonders geeignet. Sie wurde dafür zu einer Mikromethode für nur wenige Milliliter radioaktiven Gases modifiziert, die noch ca. 7 ppm Verunreinigungen zu erfassen erlaubt.

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Analytical problems of the preparation of radioactive hydrogen sulphide of high specific activity

A procedure in micro scale is described for the preparation and analysis of hydrogen sulphide of maximum purity with high to very high specific activity. High yield and impurity contents of less than 10^–2 Mol-% could be achieved by the use of adsorption chromatography and isothermal distillation.The mass-spectrometric H₂S analysis and its problems are discussed in detail. By suitable experiments and their theoretical interpretation it could clearly be shown that CS₂ and H₂O are formed secondarily by side-reactions of hydrogen sulphide in the mass spectrometer, thus causing erroneous results. Measurement of the triple-point pressure proved to be particularly suitable for checking the purity of H₂S during the course of the sulphurizing reactions within the closed apparatus. It has been modified to a micro method for only a few ml of radioactive gases, permitting the determination of about 7 ppm of impurities.

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Der Deutschen Forschungsgemeinschaft und dem Fonds der Chem. Industrie gebührt Dank für die Förderung dieser Untersuchungen.

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Herrn Prof. Dr. E. Asmus zum 60. Geburtstag gewidmet.     

Über das «pK» elektronisch angeregter Azulenium-Kationen

It is shown that azulenium cations in the first electronically excited state S₁ are stronger acids than in the ground state S₀. Their apparent pK″*, obtained from the H₀-dependence of the quenching of the azulenium cation S₁ → S₀ fluorescence does not correspond to a true acid-base equilibrium in the electronically excited state S₁. The pK″* values are kinetically controlled, the rate of reprotonation of azulene in the S₁ state being too low to compete with the internal conversion to S₀.     

Magnetic correlations in Eu x Sr1−x S and the ferromagnet-spin glass transition

Neutron (Bragg and small angle) scattering and susceptibility measurements are used to study magnetic ordering in Eu _x Sr_1–x S with ferromagnetic nearest neighbor exchangeJ ₁ and antiferromagnetic next-nearest neighbor exchangeJ ₂. We present data for 0.50x0.70 which cannot be analyzed within the merely geometrical treatments of percolation theory. Breakdown of ferromagnetism occurs atx _c =0.51, far above the percolation thresholdx _p =0.136, and a spin-glass phase is observed in the intermediate concentration regime. Close tox _c , the ferromagnetic state is also displaced by the spinglass phase at lower temperatures. Both properties are a general characteristic of diluted systems with competing interactions. An effective decoupling of finite magnetic clusters from the ferromagnetic net arises from frustration, which enhances the ground-state entropy. Anomalous properties below the Curie temperatureT _c as well as atT _c support this microscopic picture.     

Defect structures of ion-implanted α-tin

Single crystalline and polycrystalline -tin has been implanted at room temperature with 80-keV ions of radioactive^119m Sn,¹¹⁹Sb, and^119m Te. The radioactive nuclei decay to the Mössbauer level of¹¹⁹Sn. Mössbauer spectra of the emitted 24-keV radiation have been measured for different source temperatures by resonance counting techniques. Five individual lines in the spectra are characterized mainly by their isomer shifts and Debye temperatures. From these parameters the radiogenic¹¹⁹Sn atoms are concluded to be located in regular substitutional and interstitial lattice sites and in defect complexes. Simple models for the defects are proposed: A Sn-vacancy pair consists of Sn atoms on (nearly) substitutional sites with a dangling bond into an adjacent vacancy. In a complex oxygen-containing defect the Sn atoms have approximately a 5s ² configuration withp-bonds to two nearest neighbour atoms. Sn atoms, having an atomic 5s ² 5p ² configuration and large vibrational amplitudes, are concluded to be in non-bonding regular interstitial sites. For special implantation conditions minor fractions of SnO₂ molecules are formed in the bulk. The interstitial¹¹⁹Sn and the¹¹⁹Sn-vacancy pairs are proposed to represent elementary point defects in -tin. Conclusions are also drawn concerning the lattice location and the defects created in the implantation process of the implanted parent isotopes.     

Mössbauer spectroscopy with the 93 keV-resonance in67Zn

The procedures and results of high resolution Mössbauer experiments performed with the 93.3 keV resonance in absorbers of⁶⁷ZnO,⁶⁷ZnS (both wurtzite and sphalerite structures),⁶⁷ZnSe,⁶⁷ZnTe and⁶⁷ZnF₂ are reported. An essentially linear dependence between the isomer shift and the Pauling electronegativity of the ligands was found. A value of r ²=+11×10^–3 fm² is estimated. The Mössbauer parameters for both ZnS structures are equal within present limits of error. In particular, no quadrupole interaction was observed in the wurtzite modification. This indicates a nearly identical local symmetry at the Zn site in the two ZnS structures.Work supported by the Bundesministerium für Forschung und Technologie, Federal Republic of Germany and the Kernfor-schungszentrum Karlsruhe     

Effect of elastic strain and stress on the superconducting transition temperature of quench-condensed films

The volume dependence of the superconducting transition temperatureT _c of amorphous and highly disordered films is investigated by bending the film substrate. The volume coefficientd InT _c /d InV of annealed Sn and Pb films agrees with the volume coefficient known from hydrostatic pressure experiments. This bending technique is applied to superconducting modifications which can be produced only by quench condensation onto a substrate at liquid helium temperature. Amorphous films such as Bi, Ga, Sn₉₀Cu₁₀ and Pb_xBi_1–x, as well as highly disordered Sn and Pb films, are investigated. The relations found between the volume coefficient, the electron-phonon coupling constant and the Grüneisen parameter _G are in reasonable agreement with the volume dependence ofT _c for non-transition metals, obtained on the basis of the McMillan equation. In particular, the results represent an expansion of this kind of investigation to very strong-coupling superconductors.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthdayPaper based in part on a Habilitationsschrift submitted to the Fakultät für Physik, Universität Karlsruhe (TH)     

On the short-range order of amorphous Al−Au films

Al_100–x Au _x films (20x71) are produced by quench condensation. The analysis of electron diffraction patterns as well as resistivity measurements reveal the liquid-like structure of these films. In contrast to amorphous Al–Cu films the amorphous Al–Au filsm exhibit prepeaks in their interference functions. Atomic distances as large as the smallest Au–Au separations existing in the compound Al₂Au are responsible for these prepeaks. The CaF₂-type structure of this particular compound is due to an ionic bonding contribution in Al₂Au. This bonding contribution, probably caused by the large electronegativity difference between Al and Au, seems to be responsible for the pronounced chemical short-range order in amorphous Al–Au films.     

The magnetic moment of103Ru

The nuclear magnetic moment of¹⁰³Ru has been determined using the technique of low-temperature nuclear orientation on¹⁰³Ru as a dilute impurity in Fe. The Fe lattice was cooled to 1.7 mK using a PrNi₅ enhanced nuclear demagnetisation stage precooled with a³He–⁴He dilution refrigerator. The¹⁰³RuFe system was observed to have a long nuclear spin-lattice relaxation time, but analysis of the relaxation indicates that the¹⁰³Ru nuclei are close to thermal equilibrium with the Fe lattice at times>200 min after demagnetising. From the near thermal equilibrium anisotropy of the 497 keV -ray in¹⁰³Rh we have deduced the nuclear magnetic moment as |(¹⁰³Ru)|=0.18(2)_N. This value is almost independent of the choice of spinI=3/2 or 5/2 for the nucleus, but is only consistent with systematics ifI(¹⁰³Ru)=3/2 and <0. UsingI=3/2 we obtain the E2/M1 mixing ratio of the 497 keV -ray as =–0.42(4). The sign of the anisotropy of the 610 keV -ray from the 650 keV level in¹⁰³Rh is only consistent with an allowed -decay to the state ifI(650 keV,¹⁰³Rh)=5/2. The E2/M1 mixing ratio of the 610 keV -ray is then =+0.15(3) or +60( _–40 ⁺ ), <–80. From the initial rise in -ray anisotropy following demagnetisation we determine the Korringa constant for¹⁰³RuFe to beC=5.7(5) K·s.     

Magnon spectrum and curie temperature of doped ferromagnetic semiconductors

The influence of a partially filled conduction band on the magnetic properties of ferromagnetic semiconductors is studied within the framework of thes-f model. Allowing for magnon scattering to arbitrary order in the form of virtual electron- holecreation the magnon spectral density is derived, from which one gets magnon energies which are substantially renormalized by the presence of conduction electrons. In particular it is shown how the quasiparticle structure of the electronic excitation energies [4, 5] leads to scattering corrections in the magnon spectrum. These corrections are always negative and reduce the positive mean field part, which is proportional to the electron densityn. The calculated magnon spectrum is then used to determine the strikingn-dependence of the Curie-temperatureT _c of Gd-doped EuO and EuS.