A Highly Triflated Rare‐Earth Ion in [Eu(O3SCF3)8]5−

The reaction of Eu₂O₃ with fuming nitric acid, trifluormethanesulfonic acid, and its anhydride in torch‐sealed glass ampoules at 120 °C gave the europium compound (NO)₅[Eu(O₃SCF₃)₈] (orthorhombic, Fddd, Z=16, a=1932.69(4), b=2878.44(7), c=2955.12(7) pm, V=16439.7(7) ų). The compound exhibits the [Eu(O₃SCF₃)₈]^5? anion showing for the first time a lanthanide ion that is exclusively coordinated by eight triflate anions. The anion has been further investigated by DFT calculations, which also allowed clear assignment of the vibrational spectra. Moreover, magnetochemical and luminescence measurements gave additional insight into the properties of this complex. The luminescence spectra revealed that the Eu³⁺ ions are in a pseudo D_4d symmetric environment.     

Gas-chromatographische Simultanbestimmung von O2, N2, CO,CO2, N2O,SO2, CH4, C2H4 und C2H6 im ppm-Bereich. I. Mitteilung

Zusammenfassung Die vorliegende Arbeit beschreibt ein neues Verfahren zur gas-chromatographischen Simultananalyse von N₂, O₂, CO, CO₂, N₂O, SO₂, CH₄, C₂H₄ und C₂H₆ im Konzentrations-bereich von 10% bis 10 ppm ohne Voranreicherung. Die temperaturprogrammierte Trennung der Einzelkomponenten erfolgt nach Vorsäulensplitting auf zwei parallel geschalteten Säulen. Zur Emittlung der Retentionszeiten und der Peakflächen werden zwei voneinander unabhängige Ultraschalldetektoren verwendet, deren Analogsignale nach Digitalisierung in einem Mikrocomputer verarbeitet werden. Instrumentierung und chromatographische Einzelheiten werden beschrieben und diskutiert.

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Simultaneous gas chromatographic determination of N₂, O₂, CO, CO₂, N₂O, SO₂, CH₄, C₂H₄ and C₂H₆ at the ppm-level. Part I

Summary A new procedure for the simultaneous determination of N₂, O₂, CO, CO₂, N₂O, SO₂, CH₄, C₂H₄ and C₂H₆ by gas chromatography is described. Concentrations from 10% down to 10 ppm can be determined without preconcentration. After a pre-column splitting the individual compounds of the sample are separated by a uniform temperature program on two different columns in parallel. Detection of the effluents is achieved by two individual ultrasonic detectors, the data from which are processed in a micro-computer. Instrumentation and gas chromatographic details are described and discussed.

     

Oscillations due to two-electron exchange during He-Ne collisions

Ne (-)(1s(2)2s(2)2p(5)3s(2)) and He (-)(1s2s(2)) autodetachment peaks in electron spectra arising from He-Ne collisions at energies ranging from 350 eV to 6 keV have been measured. The areas of these two peaks are found to oscillate approximately in antiphase, with a constant wavelength, on a (velocity)(-1) scale. It is shown that this behavior can be explained by the "quasiresonance" situation for the separating particles in the channels Ne (-)-He (+) and He (-)-Ne (+). This explanation implies that the oscillations can be understood as being due to the simultaneous "hopping" of two electrons.     

Photoassociation spectroscopy of cold He(2 3S) atoms

We observe vibrational states by photoassociation spectroscopy of cold He(2 ^{3}S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 ^{3}S_{1}-2 ^{3}P_{2} transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.     

Reactions of excited atoms and molecules with atoms and molecules

Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH⁺, RH ₂ ⁺ , and H ₂ ⁺ by collisions of excited rare gas atoms R^* with H₂. With HD as the target molecule the isotope effect for the production of RD⁺ and RH⁺ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.     

The decomposition of C4H8 complexes at controlled internal energies

A coincidence technique is used to study the influence of the internal energy of the reactant ion on the cross section of the ion-molecule reactions in the C₂H₄⁺ + C₂H₄ system. The experiment is performed at thermal collision energies. In the ion-molecule reactions of C₂H₄⁺ + C₂H₄ our measurements indicate a barrier between the initially formed collision complex (C₂H₄)₂⁺* and a tight complex (C₄H₈⁺)*. Using an extension of our earlier developed statistical model, now including a potential barrier between the initially formed loose complex (C₂H₄)₂⁺* and the tight complex (C₄H₈⁺)*, our experimental data can be reproduced. For comparison also the internal energy dependence of the unimolecular decomposition of photoionised 1-C₄H₈⁺ is measured. Assuming that the photoionised 1-C₄H₈⁺ is identical with the tight (C₄H₈⁺)* complex, the model applied to the ion-molecule reactions describes also the unimolecular decay of 1-C₄H₈⁺ correctly, using the same set of parameters.     

The variation of photoelectron angular distributions with energy

Photoelectron angular distributions for the ionization of the outerp-shell of the rare gases Ar, Kr, and Xe, and of the outerS- andd shell of Hg, are measured at various photon energies obtained from rare gas resonance radiation. The distributions are found to agree with the theoretically predicted form. Values of the anisotropy parameter β are determined, and their dependence on electron energy is compared with available numerical results, as well as with model calculations performed in this work. In the case of Hg the possible influence of spin-orbit coupling on the β-values is discussed.     

Optimal surface functionalization of silicon quantum dots

Surface functionalization is a critical step for Si nanocrystals being used as biological probes and sensors. Using density-functional tight-binding calculations, we systematically investigate the optical properties of silicon quantum dots (SiQDs) with various termination groups, including H, CH(3), NH(2), SH, and OH. Our calculations reveal that capping SiQDs with alkyl group (-Si-C-) induces minimal changes in the optical spectra, while covering the surface with NH(2), SH, and OH results in evident changes compared to hydrogenated SiQDs. The structural deformations and electronic property changes due to surface passivation were shown to be responsible for the above-described features. Interestingly, we find that the optical properties of SiQDs can be controlled by varying the S coverage on the surface. This tuning effect may have important implications in device fabrications.     

Photoproduction of positive pions from hydrogen at large angles in the energy range 0.3–2.0 GeV

The differential cross-section for the reaction γp↦π ⁺ n was measured using the bremsstrahlung beam of the Bonn 2.5 GeV electron synchrotron. The pions were detected and momentum analysed in a multichannel magnet spectrometer. Data reduction resulted in 1278 measured cross-sections which are presented as energy distributions at six laboratory angles between 180° and 95°. The range of laboratory photon energies extended from 0.3 to 2.1 GeV. The statistical accuracy is better than 3 percent, the systematic error is about 5 percent. The data are compared with other experimental results and predictions of a phenomenological analysis. These cross-sections are the result of a measurement program which was started in the seventies. Further results will be reported in forthcoming publications. Received: 2 February 2000 / Accepted: 14 August 2001