Antiferromagnetic order and phase transitions in GdS as studied with X-ray resonance-exchange scattering

We report on X-ray resonance exchange and neutron scattering of metallic GdS. At the L_II and L _III absorption edges of Gd, resonance enhancements of more than two orders of magnitude over the non-resonant magnetic scattering are observed. Polarisation analysis proves that these enhancements are due to dipolar transitions from the 2p to the 5d states. The branching ratio between the L_II and L _III edges of 2.5 suggests a polarisation of the 5d electrons in the ground state. The antiferromagnetic order is of type II in the fcc lattice. Single crystal diffraction of hot neutrons suggests that the spin direction lies within the (111) planes with a value for the sublattice magnetisation of 6.51(3) . The critical exponent for the sublattice magnetisation has a value of β = 0.38(2) in agreement with a pure Heisenberg model. Above T _N, a sharp component persists in the critical diffuse scattering. Lattice distortions give indications for two additional low-temperature phase transitions at about 49 K and 32 K. We argue that these transitions are not connected to spin reorientations and discuss the possible influence of fourth-order exchange interactions. Received 19 November 1999 and Received in final form 12 December 2000     

Electronic splitting in the excited states of DNA base homodimers and -trimers: an evaluation of short-range and Coulombic interactions

The nature of the electronic interactions of the stacked nucleic-acid bases (adenine, thymine, cytosine, and uracil) in homodimer and -trimer complexes in their electronically excited states was investigated and analysed in terms of orbital-overlap and Coulombic interactions. The mutual orientations of the adjacent bases were selected so as to correspond to the A- and B-DNA conformations. The extent of the electronic interaction is larger for the former conformation. It is shown that the orbital-overlap interactions at the distance of two bases relevant to the DNA structure do not contribute significantly to the overall electronic coupling. Only the states which are caused by the (pi --> pi*) transitions manifest an electronic coupling.     

Role of alkali cations for the excited state dynamics of liquid water near the surface

Time-resolved liquid jet photoelectron spectroscopy was used to explore the excited state dynamics at the liquid water surface in the presence of alkali cations. The data were evaluated with the help of ab initio calculations on alkali-water clusters and an extension of these results on the basis of the dielectric continuum model: 160 nm, sub-20 fs vacuum ultraviolet pulses excite water molecules in the solvent shell of Na(+) or K(+) cations and evolve into a transient hydrated complex of alkali-ion and electron. The vertical ionization energy of this transient is about 2.5 eV, significantly smaller than that of the solvated electron.     

A novel online approach to the determination of isotopic ratios for organically bound chlorine, bromine and sulphur

A novel approach for the measurement of (37)Cl, (81)Br and (34)S in organic compounds containing chlorine, bromine, and sulphur is presented to overcome some of the major drawbacks of existing methods. Contemporary methods either require reference materials with the exact molecular compositions of the substances to be tested, or necessitate several laborious offline procedures prior to isotope analysis. In our online setup, organic compounds are separated by gas chromatography (GC) coupled to a high-temperature reactor. Using hydrogen as a makeup gas, the reactor achieves quantitative conversion of chlorinated, brominated and sulphurated organic compounds into gaseous hydrogen chloride (HCl), hydrogen bromide (HBr), and hydrogen sulphide (H(2)S), respectively. In this study, the GC interface was coupled to a quadrupole mass spectrometer operated in single-ion mode. The ion traces of either H(35)Cl (m/z 36) and H(37)Cl (m/z 38), H(79)Br (m/z 80) and H(81)Br (m/z 82), or H(2)(32)S (m/z 34) and H(2)(34)S (m/z 36), were recorded to determine the isotopic ratios of chlorine, bromine, and sulphur isotopes. The conversion interface presented here provides a basis for a novel method for compound-specific isotope analysis of halogenated and sulphur-containing compounds. Rapid online measurements of organic chlorine-, bromine- and sulphur-containing mixtures will facilitate the isotopic analysis of compounds containing these elements, and broaden their usage in fields of environmental forensics employing isotopic concepts.     

The detection of UV-induced membrane damages by a combination of two biosensor techniques

Ketoprofen is an important anti-inflammatory drug, but its dermal application is limited because of the photosensitizing properties causing phototoxic reactions of the skin when exposed to UV light. We have recently investigated the peroxide formation of ketoprofen in solutions of linoleic acid during UV irradiation. To continue these studies and focus on UV-induced changes in membrane integrity and barrier function we established an in vitro model using two biosensor techniques simultaneously. Support-fixed bilayers were irradiated with different doses of UV-B up to damaging intensities with or without ketoprofen. Cyclic voltammetry was carried out to detect alterations in membrane permeability; quartz crystal microbalance (QCM) measurements were helpful in analyzing whether a permeability increase was caused by depletion of membrane components. In absence of ketoprofen, increasing UV-B doses induce membrane permeabilities of both unsaturated and saturated bilayers. QCM measurements could not reveal a significant loss of membrane components as a reason for the permeability. In contrast, 0.3 mM ketoprofen induced a dose-dependent increase in membrane permeability. QCM results indicated a mass loss. Although this model does not explain all molecular mechanisms of membrane damage by ketoprofen, the combined application of both QCM and CV is a novel and powerful tool to investigate functional mechanisms of UV-induced membrane damages.     

Photoluminescence enhancement in organic microcavity structures

We have studied the photoluminescence properties of the metal chelates aluminum-tris(8-hydroxychinoline) (Alq₃) and tris(4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono)-1,10-phenanthroline europium(III) (Eu(TTFA)₃Phen) sandwiched between a planar Fabry-Perot microcavity structure. A strong influence of the position of the emission thin film on the luminescence spectra has been observed. The emission intensity normal to the substrate surface is drastically enhanced by an order of a magnitude if the active layer is placed at the antinode of the standing wave in the cavity. In comparison to non-resonant Alq₃ structures, the FWHM of the spectrum is reduced from 100 nm to about 20 nm. We also show that the Eu(TTFA)₃Phen luminescence is spacially directed due to the inherent linewidth of only Sum.