Positron annihilation study of ordering processes in rare-earth cobalt permanent magnets

The ordering process in 1/5 and in 2/17 Sm-Co permanent magnets was followed by positron annihilation lifetime and Doppler broadening measurements. No correlation between magnetic hardness and positron annihilation parameters was found.     

Darstellung und eigenschaften cyclischer silylamide des berylliums. Kristall- und molekülstruktur eines dimeren diaza-disila-berylla-cyclohexans

Four novel cyclic beryllium silylamides (I–IV; R = CH₃) were obtained by reaction of BeMe₂ with the appropriate aminosilanes. In solution and in the solid state they form dimers and probably higher associated oligomers, which,

according to their ¹H NMR spectra, interconvert depending on temperature and solvent. Crystallic II dimerizes via a centrosymmetric BeNBe′N′ four-membered ring, the BeN and BeN′ distances being 1.714(3) and 1.683(3) Å. The CN and SiN bonds of the tetracoordinate N atom (1.502(3) and 1.752(3) Å) are longer than those formed by the tricoordinate N atom (1.473(3) and 1.722(2) Å); the BeN (tricoordinate N) distance is 1.550(4) Å.     

meso-tetraphenylporphyrin dimer derivatives as potential photosensitizers in photodynamic therapy. Part 2

Studies on the synthesis, singlet oxygen and fluorescence yields and pharmacokinetic properties of three different dimeric porphyrins with an amide linkage (D2-D4) are described and compared with the results recently reported for a dimeric porphyrin (D1). The pharmacokinetic behavior of all dimers were examined in Balb/c mice bearing MS-2 fibrosarcomas. The maximal efficiency and selectivity of photosensitizer accumulation in each tumor tissue takes place at 24 h after drug administration of 1.0 mg kg-1 into DL-alpha-dipalmitoylphosphatidylcholine liposomes by intravenous injection. Since the dimeric porphyrins exhibit high quantum yields of singlet oxygen generation, long triplet lifetimes and high photostability, the results obtained suggest that the evaluated dimeric structures may be promising candidates for further use in PDT experiments. The results also allow the possibility to establish a correlation between the chemical structure of the dyes and the efficiency/selectivity of the tumor accumulation and can be used for building up optimal photosensitizing agents for tumors.     

Comparative characterization of differently grown ZnO single crystals by positron annihilation and Hall effect

High-quality ZnO single crystals of dimensions 10×10×0.3 mm³, grown either using a pressurized melt or a hydrothermal growth approach, have been investigated in their as-received state and are compared regarding their properties revealed by positron annihilation and Hall effect measurements. By positron annihilation performed at room temperature it is found that the pressurized melt grown crystals contain a certain amount of Zn+O divacancies, but no Zn vacancies are detected, whereas the hydrothermally grown crystals contain a dominating defect yet unknown in its structure but possibly connected to the Zn vacancy. Furthermore, the influence of an additional refined chemical–mechanical polishing of the crystal surface by a special procedure on the depth distribution of vacancy-type defects is demonstrated. Hall measurements, performed in the temperature range 20–325 K, showed that the crystal growth method has a strong influence on the carrier mobility, and the estimated acceptor densities also differ significantly in both types of crystal.     

Positron lifetimes in V3Si

From positron annihilation lifetime measurements on V₃Si single crystals we estimated that about 17.2% of the positrons annihilate with core electrons.     

Decay and polarization of luminescence pulses in the 336 nm emission band of KI-Tl

Model predictions for the impurity luminescence center in KI-Tl type crystals are found to agree with experimental results obtained by excitation of KI-Tl at 12K with polarized light pulses in the A and C impurity absorption band. On excitation in the C absorption band, as compared to the A band excitation, a noticeable initial population of the metastable M state has been observed.     

Temperature dependence of the penetration depth in Sr2RuO4: evidence for nodes in the gap function

We report measurements of the magnetic penetration depth in single crystals of Sr2RuO4 down to 0.04 K using a tunnel-diode based, self-inductive technique. We observe a power law temperature dependence below 0.8 K, with no sign of a second phase transition nor of a crossover predicted for a multiband superconductor. A power law dependence suggests that the gap function has nodes, inconsistent with candidate p-wave states. We argue that nonlocal effects, rather than impurity scattering, can explain the observed T2 dependence instead of the T-linear behavior expected for line nodes.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.