Pair Conversion Research Articles

Driving current and temperature dependent magnetic-field modulated electroluminescence in Alq 3-based organic light emitting diode

Driving current and temperature dependences of magnetic-field modulated electroluminescence (EL) in tris(8-hydroxyquinoline) aluminum (Alq 3)-based OLEDs have been thoroughly investigated. At low temperatures, the applied magnetic-field induces a sharp increase of the EL in low field regime ( B ⩽ 35 mT) and a slow but apparent decrease at high fields (35 ⩽ B ⩽ 500 mT). The low-field increase in EL (LFE) survives at all working temperatures while the high-field decrease (HFE) gradually disappears as temperature is increased. At a given temperature, the higher the current level, the smaller LFE and stronger HFE are observed. To explain the observed MFEs a composite model based on magnetic-field dependent singlet-to-triplet conversion of electron-hole pairs and magnetic-field mediated triplet–triplet annihilation process is proposed in this paper.

Real-Time Observation of Formation and Relaxation Dynamics of NH4 in (CH3OH)m(NH3)n Clusters

The formation and relaxation dynamics of NH4(CH3OH)m(NH3)n clusters produced by photolysis of ammonia-methanol mixed clusters has been observed by a time-resolved pump-probe method with femtosecond pulse lasers. From the detailed analysis of the time evolutions of the protonated cluster ions, NH4(+)(CH3OH)m(NH3)n, the kinetic model has been constructed, which consists of sequential three-step reaction: ultrafast hydrogen-atom transfer producing the radical pair (NH4-NH2)*, the relaxation process of radical-pair clusters, and dissociation of the solvated NH4 clusters. The initial hydrogen transfer hardly occurs between ammonia and methanol, implying the unfavorable formation of radical pair, (CH3OH2-NH2)*. The remarkable dependence of the time constants in each step on the number and composition of solvents has been explained by the following factors: hydrogen delocalization within the clusters, the internal conversion of the excited-state radical pair, and the stabilization of NH4 by solvation. The dependence of the time profiles on the probe wavelength is attributed to the different ionization efficiency of the NH4(CH3OH)m(NH3)n clusters.

Magnetic field effects on the yields of dyes in sensitized transformation of diphenylamine in micellar systems

A strong influence (an increase in the rate of formation of the diphenylbenzidine dication by up to three times) of an external magnetic field on the yields of colored products was observed when micellar solutions of benzophenone and diphenylamine derivatives were UV irradiated in the presence of acids. A mechanism of the phenomenon was suggested. The mechanism by based on the concept of the simultaneous occurrence of two processes, (1) geminate recombination of ketyl and aminyl radicals with the formation of triarylmethane dye and (2) bulk recombination of two radical cations produced in the protonation of aminyl radicals with the formation of the diphenylbenzidine dication. Magnetic field is a factor determining the rate of the triplet-singlet conversion of radical pairs formed in the photoreduction of the ketone by amine and, therefore, the ratio between the stationary concentrations of aminyl radicals and radical cations. The influence of the structure of ketones, the ratio between the reagents and surfactant, and the pH value on the dye yield and the scale of magnetic-spin effects were analyzed.

Role of particle interactions in the Feshbach conversion of fermionic atoms to bosonic molecules

We investigate the Feshbach conversion of fermionic atom pairs to condensed bosonic molecules with a microscopic model that accounts for the repulsive interactions among all the particles involved. We find that the conversion efficiency is enhanced by the interaction between bosonic molecules, while it is suppressed by the interactions between fermionic atoms and between atoms and molecules. In the adiabatic limit, the combined effect of these interactions can lead to a ceiling of less than 100% on the conversion efficiency for a narrow Feshbach resonance. Our theory agrees with the recent Rice experiment on 6Li.

Electrodynamics of Magnetars. III. Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere

We first examine the QED processes that create electron-positron pairs in magnetic fields approaching and exceeding 1014 G. The formation of free and bound pairs is addressed, and the importance of positronium dissociation by thermal X-rays is noted. We calculate the collision cross section between an X-ray and a gamma ray, and point out a resonance in the cross section when the gamma ray is close to the threshold for pair conversion. We then show how the pair creation rate in the open-field circuit and the outer magnetosphere can be strongly enhanced by current-driven instabilities near the light cylinder. A cascade develops when the current has a strong fluctuating component. We examine the mechanism of particle heating and show that a high rate of pair creation can be sustained close to the star, but only if the spin period is shorter than several seconds. The dissipation rate in this turbulent state can easily accommodate the observed radio output of the transient radio-emitting magnetars and even their infrared emission. Finally, we outline how a very high rate of pair creation on the open magnetic field lines can help to stabilize a static twist in the closed magnetosphere and to regulate the loss of magnetic helicity by reconnection at the light cylinder.

Evaluation of theoretical conversion coefficients using BrIcc

A new internal conversion coefficient database, BrIcc has been developed which integrates a number of tabulations on internal conversion electron (ICC) and electron–positron pair conversion coefficients (IPC), as well as Ω(E0) electronic factors. A critical review of general formulae and procedures to evaluate theoretical ICC and IPC values are presented, including the treatment of uncertainties in transition energy and mixing ratio in accordance with the Evaluated Nuclear Structure Data File. The default ICC table, based on the Dirac–Fock calculations using the so called “Frozen Orbital” approximation, takes into account the effect of atomic vacancies created in the conversion process. The table has been calculated for all atomic shells and to cover transition energies of 1–6000keV and atomic numbers of Z=5–110. The software tools presented here are well suited for basic nuclear structure research and for a range of applications.

Picosecond Kinetic Study of the Photoinduced Homolysis of Benzhydryl Acetates: The Nature of the Conversion of Radical Pairs into Ion Pairs

The conversion of benzhydryl acetate geminate radical pairs to contact ion pairs following photoinduced homolysis in solution is studied using picosecond pump-probe spectroscopy. The dynamics for the decay of the geminate radical pairs into contact ion pairs is modeled within a Marcus-like theory for nonadiabatic electron transfer. A second decay channel for the geminate radical pairs is diffusional separation to free radicals. The kinetics of this latter process reveals an energy of interaction between the two radicals in the geminate pair.

First Results from GLAST-LAT Beam Test at CERN-PS and SPS

The GLAST Large Area Telescope (LAT) is a high-energy gamma-ray observatory sensitive to gamma rays in the energy range between 20 MeV and 300 GeV. The LAT consists of an array of tracking tower modules (TKR), composed of planes of silicon-strip detectors (SSDs) interleaved with converter tungsten layers, followed by a calorimeter (CAL) and surrounded by an anticoincidence system (ACD). The photon detection is based on the pair conversion process and silicon-strip detectors will reconstruct tracks of electrons and positrons. During the summer, a beam test campaign at CERN PS and SPS has been made in order to study the performance of a Calibration Unit (CU), consisting of two TKR modules and tree CAL modules. Experimental results on the performance of the tracker towers will be discussed.

A Novel Transcoding Technique between EVRC and G.729A for Mobile Multimedia Devices

In this paper, we propose a novel transcoding technique between enhanced variable rate codec (EVRC) and G.729A for mobile multimedia devices. The simplest way to communicate between heterogeneous speech networks is the cascade connection of two different codecs, called tandem coding. However, tandem coding not only has high computational loads, but also makes long delay. These problems can be solved by the effective transcoding algorithm using line spectral pair (LSP) and pitch delay conversion in speech codec. The proposed algorithm consists of LSP conversion, pitch delay conversion and algorithm for delay reduction. Experimental results show the proposed algorithm has lower computational complexity, shorter algorithm delay, and similar speech quality in comparison of the tandem algorithm.

Specific features of two-photon optical nutation in a system of biexcitons in semiconductors

Specific features of two-photon nutation in a system of coherent biexcitons in CuCl-type semiconductors are studied. It is shown that, depending on the parameters of the system, nutation represents a process of periodic conversion of photon pairs into biexcitons and vice versa. The possibility of phase control of optical nutation is predicted.

Effect of Mg content on structural, electrical, and optical properties of Li-doped Zn1−xMgxO thin films

The authors have grown Li-doped p-type ZnMgO films on glass substrates with different Mg contents (11–28at.%) by pulsed laser deposition. Hall measurements suggest that the resistivity increases with Mg concentration. Acceptor levels related to LiZn located at about 150 and 174meV above the valence band maximum were discriminated in photoluminescence spectra for Li-doped Zn0.89Mg0.11O and Zn0.72Mg0.28O films, respectively. The conversion of donor-acceptor pair to a free-to-neutral-acceptor (e,A0) transition was also observed in Zn0.89Mg0.11O:Li film. The optical band gap and the acceptor binding energy increase with increase of Mg content in the films, which leads to reduction in the hole concentration and increase in the resistivity.

Self-induced decoherence in dense neutrino gases

Dense neutrino gases exhibit collective oscillations where ``self-maintained coherence'' is a characteristic feature, i.e., neutrinos of different energies oscillate with the same frequency. In a nonisotropic gas, however, the flux term of the neutrino-neutrino interaction has the opposite effect of causing kinematical decoherence of neutrinos propagating in different directions, an effect that is at the origin of the ``multiangle behavior'' of neutrinos streaming off a supernova core. We cast the equations of motion in a form where the role of the flux term is manifest. We study in detail the symmetric case of equal neutrino and antineutrino densities where the evolution consists of collective pair conversions (``bipolar oscillations''). A gas of this sort is unstable in that an infinitesimal anisotropy is enough to trigger a runaway towards flavor equipartition. The ``self-maintained coherence'' of a perfectly isotropic gas gives way to ``self-induced decoherence.''

Space instrumentation environmental verification: Dynamic tests on GLAST LAT tracker trays

The GLAST satellite is a mission for the detection of gamma rays of both galactic and extra-galactic origin in the energy range 20 keV–300 GeV. The satellite is planned to be launched in 2007 by NASA. Gamma rays detection is based on the pair conversion mechanism achieved by a multilayer silicon strip detector interleaved with a tungsten target for photon conversion and e + –e − pairs tracking. The GLAST program did foresee a wide use of vibration tests both in development and in qualification phases. The acceptance phase in aerospace environment is usually demanding in terms of vibration tests. Furthermore in this program the trays’ design and manufacturing procedure might have been highly vibration sensitive, and experimental validation was considered highly recommendable. This paper aims to give evidence of the performed tests. A critical analysis of the tests is proposed and some results will be also presented.

Effect of Ionic Solutes on the Hydrogen Bond Network Dynamics of Water: Power Spectral Analysis of Aqueous NaCl Solutions

To understand the modifications of the hydrogen bond network of water by ionic solutes, power spectra as well as static distributions of the potential energies of tagged solvent molecules and solute ions have been computed from molecular dynamics simulations of aqueous NaCl solutions. The key power spectral features of interest are the presence of high-frequency peaks due to localized vibrational modes, the existence of a multiple time scale or 1/falpha frequency regime characteristic of networked liquids, and the frequency of crossover from 1/falpha type behavior to white noise. Hydrophilic solutes, such as the sodium cation and the chloride anion, are shown to mirror the multiple time scale behavior of the hydrogen bond network fluctuations, unlike hydrophobic solutes which display essentially white noise spectra. While the power spectra associated with tagged H2O molecules are not very sensitive to concentration in the intermediate frequency 1/falpha regime, the crossover to white noise is shifted to lower frequencies on going from pure solvent to aqueous alkali halide solutions. This suggests that new and relatively slow time scales enter the picture, possibly associated with processes such as migration of water molecules from the hydration shell to the bulk or conversion of contact ion pairs into solvent-separated ion pairs which translate into variations in equilibrium transport properties of salt solutions with concentration. For anions, cations, and solvent molecules, the trends in the alpha exponents of the multiple time scale region and the self-diffusivities are found to be strongly correlated.

Charge Transfer between a Superconductor and a Hopping Insulator

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron-Cooper pair conversion process time-reversal reflection, where electrons tunnel into a superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. A specific feature of this problem is the interplay between the time-reversal reflection at the interface and transport through the percolation cluster. To allow for this interplay, we have generalized the connectivity criterion of the percolation theory to include surface effects. We show that the time-reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.

Novel Insights into the Mechanism of the Ortho/Para Spin Conversion of Hydrogen Pairs: Implications for Catalysis and Interstellar Water

The phenomenon of exchange coupling is taken into account in the description of the magnetic nuclear spin conversion between bound ortho- and para-dihydrogen. This conversion occurs without bond breaking, in contrast to the chemical spin conversion. It is shown that the exchange coupling needs to be reduced so that the corresponding exchange barrier can increase and the given magnetic interaction can effectively induce a spin conversion. The implications for related molecules such as water are discussed. For ice, a dipolar magnetic conversion and for liquid water a chemical conversion are predicted to occur within the millisecond timescale. It follows that a separation of water into its spin isomers, as proposed by Tikhonov and Volkov (Science 2002, 296, 2363), is not feasible. Nuclear spin temperatures of water vapor in comets, which are smaller than the gas-phase equilibrium temperatures, are proposed to be diagnostic for the temperature of the ice or the dust surface from which the water was released.

False positive results in chimeraplasty for von Willebrand Disease

Chimeraplasty or the use of chimeric RNA/DNA oligonucleotides (RDOs) to correct single-base mutations emerged in the field of gene therapy with reported base pair conversions of up to 40%. We investigated the applicability of chimeraplasty to correct a point mutation in the von Willebrand Factor (VWF) gene resulting in a von Willebrand Disease (VWD) type 3 phenotype. Although we have access to VWD type 3 dogs, we used wild type endothelial cells for in vitro studies, as isolation of endothelial cells from VWD type 3 dogs is not straightforward due to the bleeding diathesis. RDOs to convert the wild type VWF gene into VWD type 3 gDNA were constructed and used in various transfection conditions. However, no gene conversion could be detected either in the RNA or in the DNA isolated from transfected cells, not even with the sensitive colony hybridisation technique, despite the presence of RDOs in the cell nucleus. On the other hand, sequence analysis of isolated DNA of transfected cells did reveal the presence of VWF type 3 DNA. However, this apparent conversion is very likely not the result of RDO-mediated nucleotide conversion as the same VWF type 3 DNA sequence was also detected in negative control experiments where no RDO was used. Our negative results are in line with the emerging reports of chimeraplasty failure and can contribute to the call for an international ‘chimeraplasty consortium’ with free exchange of results to clarify the controversy about the applicability of the RDO-mediated base conversion.

Cherenkov radiation frome+e−pairs and its effect onνeinduced showers

We calculate the Cherenkov radiation from an e{sup +}e{sup -} pair at small separations, as occurs shortly after a pair conversion. The radiation is reduced (compared to that from two independent particles) when the pair separation is smaller than the wavelength of the emitted light. We estimate the reduction in light in large electromagnetic showers, and discuss the implications for detectors that observe Cherenkov radiation from showers in the Earth's atmosphere, as well as in oceans and Antarctic ice.

GLAST LAT tracker signal simulation and trigger timing study

The GLAST Large Area Telescope (LAT) is a project for a high-energy gamma-ray observatory. The LAT tracker consists of an array of tower modules, composed of planes of silicon-strip detectors (SSDs) interleaved with converter sheets. The photon detection is based on the pair conversion principle and silicon-strip detectors are used to precisely track the electron and positron produced by the initial gamma-ray. The tracker digital signal read-out has been reproduced by means of a full simulation code describing all the physical process that take place in a SSD. The simulation has been applied to a trigger timing study and the LAT tracker hit capture efficiency has been evaluated. The results obtained will be shown.

Electron to selectron pair conversion in a supersymmetric bubble with jet production by Bose enhancement

In the standard model, energy release in dense stars is severely restricted by the Pauli exclusion principle. However, if, in regions of space of high fermion degeneracy, there is a phase transition to a state of exact supersymmetry (SUSY), fermion to sfermion pair conversion followed by radiative transitions to the Bose ground state could lead to a highly collimated gamma ray burst. We calculate the cross section for $ee\ensuremath{\rightarrow}\stackrel{\texttildelow{}}{e}\stackrel{\texttildelow{}}{e}$ in a SUSY bubble and construct a monte carlo for the resulting sfermion amplification by stimulated emission.