Showing papers by "University of Mons published in 1996"

Tuning and test of fragmentation models based on identified particles and precision event shape data

Abstract: Event shape and charged particle inclusive distributions are measured using 750000 decays of the Z to hadrons from the DELPHI detector at LEP. These precise data allow a decisive confrontation with models of the hadronization process. Improved tunings of the JETSET, ARIADNE and HERWIG parton shower models and the JETSET matrix element model are obtained by fitting the models to these DELPHI data as well as to identified particle distributions from all LEP experiments. The description of the data distributions by the models is critically reviewed with special importance attributed to identified particles.

Spatial extent of the singlet and triplet excitons in transition metal‐containing poly‐ynes

Abstract: We present a joint experimental and theoretical investigation of the electronic excitations in transition metal‐containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that π conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain‐length dependence of the singlet–singlet, S0→S1, singlet–triplet, S0→T1, and triplet–triplet, T1→Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems.

Conjugated Polymer Surfaces and Interfaces: Electronic and Chemical Structure of Interfaces for Polymer Light Emitting Devices

Abstract: In this book, we attempt to bring together in one place the results of a relatively large number of basic studies of conjugated polymer surfaces, as well as the 'early stages of metal-polymer inter ...

Spermiotoxicity and embryotoxicity of heavy metals in the echinoid Paracentrotus lividus

Abstract: Spermio- and embryotoxicities of Cu, Ag, Cd, and Hg were investigated in Paracentrotus lividus, the dominant echinoid species of the Mediterranean. Spermiotoxicity was studied by assessing the effects of sperm exposure on fertilization rate (FR) as well as on the induction of transmissible damages to the offspring. Embryotoxicity was studied by assessing developmental defects in larvae exposed to the tested metals throughout their development. Sperm exposures resulted in significant decreases of FR, depending on both metal concentration and duration of the exposure. Lowest spermiotoxic concentrations recorded when sperm were exposed for 75 min to the metals were 10−7 M Hg(II), 10−6 M Ag(I), 10−5 M Cu(II), and 10−5 M Cd(II). Tested metals did not exert any transmissible damage to spermatozoa that could result in larval malformations in the offspring, even for concentrations that dramatically reduced FR. Single-element exposures of embryos for 72 h resulted in developmental defects whose occurrence and severity showed a steep dose dependence, indicating that once a threshold is reached, any further increase in toxicant concentration rapidly enhances the impairment of target function(s). Those observations suggest the involvement of a saturable protective mechanism. Lowest observed embryotoxic concentrations of the metals were 10−7 M Hg(II), 2.5 × 10−7 M Ag(I), 5 × 10−7 M Cu(II), and 10−5 M Cd(II) and are in the range of concentrations reported in heavily polluted marine environments. Thus, the possibility of impairment of echinoid development actually exists in metal-contaminated marine environments, possibly threatening echinoid populations in those environments.

Symmetrized density-matrix renormalization-group method for excited states of Hubbard models.

Abstract: We extend the density-matrix renormalization-group (DMRG) method to exploit parity, ${\mathit{C}}_{2}$ (rotation by \ensuremath{\pi}), and electron-hole symmetries of model Hamiltonians. We demonstrate the power of this method by obtaining the lowest-energy states in all eight symmetry subspaces of Hubbard chains with up to 50 sites. The ground-state energy, optical gap, and spin gap of regular U=4t and U=6t Hubbard chains agree very well with exact results. This development extends the scope of the DMRG method and allows future applications to study of optical properties of low-dimensional conjugated polymeric systems. \textcopyright{} 1996 The American Physical Society.

Energy dependence of the differences between the quark and gluon jet fragmentation

Abstract: Three jet events arising from decays of the Z boson, collected by the DELPHI detector, were used to measure differences in quark and gluon fragmentation. Gluon jets were anti-tagged by identifying b quark jets. Unbiased quark jets came from events with two jets plus one photon. Quark and gluon jet properties in different energy ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same energy in symmetric three jet event topologies were also compared. Using three independent methods, the average value of the ratio of the mean charged multiplicities of gluon and quark jets is = 1.241 ± 0.015 (stat.) ± 0.025 (syst.). Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent energy. The string effect has been observed in fully symmetric three jet events. The measured ratio R_{gamma} of the charged particle flow in the qoverline{q} inter-jet region of the qar{q}g and qar{q}gamma samples agrees with the perturbative QCD expectation. The dependence of the mean charged multiplicity on the hadronic center-of-mass energy was analysed in photon plus n-jet events. The value for alpha_s(M_Z) determined from these data using a QCD prediction with corrections at leading and next-to-leading order is αs(MZ)= 0.116 ± 0.003 (stat.) ± 0.009 (syst.).

In situ demonstration of germinal cell apoptosis during diethylstilbestrol-induced testis regression in adult male Syrian hamsters.

Abstract: Testis regression was induced in male Syrian hamsters by chronic exposure to diethylstilbestrol (DES), and estradiol-17 beta agonist. Experimental groups (n = 4-5) were killed at increasing time intervals over a period of 6 mo after initiation of treatment. Apoptosis in testes was demonstrated by in situ analysis of DNA fragmentation. Cell proliferation was monitored by immunostaining nuclei of S-phase cells after pulse labeling with 5-bromo-2'-deoxyuridine. Levels of FSH and testosterone, measured by RIA fell rapidly in DES-treated hamsters. In parallel, testis weight and seminiferous tubule area underwent an 80% decrease during the first 2 wk of DES administration. The composition of seminiferous epithelium was also drastically affected by DES, since it became progressively confined to Sertoli cells, spermatogonia, and spermatocytes. Testis regression was associated with an important increase of apoptosis, which started 3 days after the beginning of DES administration. Apoptosis was still 10- to 50-fold higher than in control testes by the end of treatment; it affected primarily spermatocytes and, to a much lesser extent, spermatogonia. Cell proliferation was not inhibited by chronic DES administration. In conclusion, these data indicate that apoptosis can by itself account for estrogen-induced testis regression.

Terraced spreading mechanisms for chain molecules.

Abstract: The behavior of monomolecular layers of fluid spreading on a solid substrate is examined using molecular dynamics simulations of polymerlike liquids. We consider drops of chain molecules of lengths 8 and 16, composed of Lennard-Jones atoms bound by confining potentials, spreading on an atomic solid substrate. Different strengths of the solid-fluid interaction are studied, in cases where a spreading drop forms distinct layers. We emphasize the conformational properties of the molecules while spreading and the dynamics of the individual layers. In particular, the questions of interlayer mass transfer and permeability are examined in the light of theoretical models. \textcopyright{} 1996 The American Physical Society.

Efficient green light‐emitting diodes from a phenylated derivative of poly(p‐phenylene–vinylene)

Abstract: We have used polyvinylcarbazole, PVK, in combination with poly(p‐phenylene vinylene), PPV and a green‐emitting, soluble derivative, poly(4,4′‐diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple‐layer light‐emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn‐on voltages of ∼8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%–0.8% for Al and Ca cathodes; at least ten times higher than for single‐layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single‐layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents.

Low-Lying Electronic Excitations and Nonlinear Optic Properties of Polymers via Symmetrized Density Matrix Renormalization Group Method

Abstract: A symmetrized Density Matrix Renormalization Group procedure together with the correction vector approach is shown to be highly accurate for obtaining dynamic linear and third order polarizabilities of one-dimensional Hubbard and $U-V$ models. The $U-V$ model is seen to show characteristically different third harmonic generation response in the CDW and SDW phases. This can be rationalized from the excitation spectrum of the systems.

Aging effect on inductive capacity of human demineralized bone matrix

Abstract: Demineralized bone powder (DBP) prepared from human cortical bone was implanted into subcutaneous pouches of athymic Nu/Nu mice for 28 days. The osteoinductive capacity was evaluated by histomorphometry of the induced cartilage and bone, and by alkaline phosphatase activity in the implant. Very small amounts of new bone and cartilage were found at histological analysis, confirming that human DBP is much less osteoinductive than that from other species. Whereas the morphometric data of the implants from the young and aged donors were not significantly different, the alkaline phosphatase activity was significantly lower in the implants from the old donors than from the younger ones. This difference between the morphometric and biochemical results could reflect the fact that the enzymatic activity is already present in the osteoprogenitor cells. At 28 days, the osteoblastic activity in contact with DBP from the aged group is characterized by a decrease in the enzymatic amount which is not yet visible at the tissue level. This tendency to a decrease in the osteoinductive capacity of bone matrix is an additional aspect of the age-related alterations which occur in bone tissue and could be attributed to modifications of different proteins of the bone matrix, including bone morphogenetic protein.

Intercomparison of instruments for tropospheric measurements using differential optical absorption spectroscopy

Abstract: The results of an intercomparison campaign of eight different long path UV-visible DOAS instruments measuring NO2, O3 and SO2 concentrations in a moderately polluted urban site are presented. For effective optical path lengths of 230 and 780 m the overall spread of these measurements (±1σ) are 5×1010, 6×1010 and 1×1010 molec·cm-3 (2.0, 2.4, and 0.4 ppb) for these molecules respectively when all instruments used a common set of absorption cross sections. The remaining differences are not completely random and the systematic differences are attributed to the different retrieval methods used for each instrument.

Wall tension and heterogeneous substrate.

Abstract: The wall tension of a heterogeneous wall is considered within a semi-infinite planar Ising model. Using Monte Carlo simulation techniques, we have obtained the microscopic validity of Cassie's law describing the wall tension of a heterogeneous substrate as the average of the wall tensions corresponding to the pure components.

Cerebellar hypermetria associated with a selective decrease in the rate of rise of antagonist activity.

Abstract: Classically, cerebellar hypermetria observed during fast and accurate movements is ascribed to a delayed onset of the electromyographic activity of the antagonist muscle. We describe here 3 patients presenting a late-onset cerebellar degeneration and exhibiting a hypermetria during their fast and accurate movements in spite of a normal onset latency of the antagonist activity. Hypermetria was found to be due to a slower rate of rise of the antagonist activity.

Large enhancement of deuteron polarization with frequency modulated microwaves

Abstract: We report a large enhancement of 1.7 in deuteron polarization up to values of 0.6 due to frequency modulation of the polarizing microwaves in a two liter polarized target using the method of dynamic nuclear polarization. This target was used during a deep inelastic polarized muon-deuteron scattering experiment at CERN. Measurements of the electron paramagnetic resonance absorption spectra show that frequency modulation gives rise to additional microwave absorption in the spectral wings. Although these results are not understood theoretically, they may provide a useful testing ground for the deeper understanding of dynamic nuclear polarization.