Intermediate Electron Research Articles

(e, 2e) triple differential cross sections of alkali and alkali earth atoms: Na, K and Mg, Ca

Recently low-energy measurements have been reported for alkali targets Na and K and alkali earth targets Mg and Ca in coplanar symmetric geometry. We report the results of our calculation of triple differential cross section (TDCS) for electron impact single ionization (i.e. (e, 2e) processes) of alkali atoms Na, K and alkali earth atoms Mg, Ca in coplanar symmetric geometry. We have performed the present calculations using the distorted-wave Born approximation (DWBA) formalism at intermediate incident electron energies used in the recently performed experiments. Ionization takes place from the valence shell for all the targets investigated and the outgoing electrons share the excess energy equally. We have also considered the effect of target polarization in our DWBA calculations which may be an important quantity at incident electron energies used in the present investigation. We find that the DWBA formalism is able to reproduce most of the trend of experimental data and may provide a future direction for further investigation of ionization process on alkali and alkali earth metals. It is also observed that the second-order effects are more important to understand the collision dynamics of (e, 2e) processes on alkali earth targets

Two photon absorption in quantum dot-in-a-well infrared photodetectors

Two photon absorption processes in InAs∕In01.5Ga0.85As∕GaAs quantum dot-in-a-well photodetectors are studied using free electron laser excitation. Two photon induced, normal incidence photocurrent, observed in the range of 20–30μm, arises from sequential near-resonant two-step transitions involving electron ground to first excited states in the dot, to quantum well final states. We find a two photon absorption coefficient of β∼1×107cm∕GW at 26.5μm (47meV) and 0.8V applied bias. Second-order autocorrelation measurements exhibit two characteristic time constants of ∼3 and ∼40ps. The latter is associated with the intermediate state electron lifetime, whereas the short decay is explained by the involvement of acoustic phonon assisted transitions.

In Situ Radiation Damage Studies of Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12

AbstractGarnets, A3B2C3O12, are considered to be potential host phases for the immobilization of high-level nuclear waste as they can accommodate a number of elements of interest, including Zr, Ti and Fe. The naturally occurring garnet, kimzeyite, Ca3(Zr,Ti)2(Si,Al,Fe)3O12, can contain ˜30wt% Zr. An understanding of the radiation tolerance of these materials is crucial to their potential use in nuclear waste immobilization. In this study two synthetic analogues of kimzeyite of composition Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12 were monitored in situ during irradiation with 1.0 MeV Kr ions using the intermediate voltage electron microscope-Tandem User Facility (IVEM) at Argonne National Laboratory. The structure of these materials was previously determined by neutron diffraction and 57Fe Mössbauer spectroscopy. Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12 have very similar structural properties with cubic Ia3d symmetry, the only significant difference being the presence of Zr and Hf, respectively, on the 6 coordinated B sites.

Flavivirus-like particles in the cardiac myocyte of a patient with arrhythmogenic right ventricular cardiomyopathy

Flavivirus-like particles in the cardiac myocyte of a patient with arrhythmogenic right ventricular cardiomyopathy

Strong variation of the phase lag in the vicinity of autoionizing resonances

We have observed the phase lag in the sinusoidal variation of the asymmetric photoelectron angular distributions into two ionization channels of atomic barium using an $\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ excitation scheme. We excite atomic barium from the $6s6p\text{ }{^{1}P}_{1}$ intermediate state electron to an energy between the $5{d}_{3∕2}ϵ$ and the $5{d}_{5∕2}ϵ$ thresholds with concurrent one-photon and two-photon interactions. The left-right asymmetry of the photoelectron angular distributions for the two energy-resolved product states, $6{s}_{1∕2}ϵ$ and $5{d}_{3∕2}ϵ$, show sinusoidal modulation as a function of the relative phase of the $\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ fields. The phase difference between these signals, or phase lag, varies rapidly with the laser frequency when the interaction is nearly resonant with autoionizing resonances in the continuum. An exceptionally large phase lag variation of over $2\ensuremath{\pi}$ was measured.

FSCB, a Novel Protein Kinase A-phosphorylated Calcium-binding Protein, Is a CABYR-binding Partner Involved in Late Steps of Fibrous Sheath Biogenesis

We report characterization of a novel testis- and sperm-specific protein, FSCB (fibrous sheath CABYR binding), that is expressed post-meiotically and localized in mouse sperm flagella. FSCB was identified as a binding partner of CABYR, a calcium-binding protein that is tyrosine-phosphorylated during capacitation. Orthologous genes of FSCB are present in other mammals, including rat and human, and conserved motifs in FSCB include PXXP, proline-rich and extensin-like regions. FSCB is phosphorylated by protein kinase A as shown by in vitro phosphorylation assay and also by determining phosphorylation sites in native FSCB from mouse sperm. Calcium overlay assay showed that FSCB is a calcium-binding protein from sperm. FSCB is a post meiotic protein first expressed at step 11 of mouse spermatogenesis in the elongating spermatids, and it subsequently incorporates into the flagellar principal piece of the sperm. Ultrastructurally, FSCB localized to a cortical layer of intermediate electron density at the surface of the ribs and longitudinal columns of the fibrous sheath. Due to its temporal appearance during spermiogenesis and location at the cortex of the fibrous sheath, FSCB is postulated to be involved in the later stages of fibrous sheath assembly.

Kondo effect of a quantum dot coupling indirectly to the conduction electrons

When a quantum dot in the Kondo regime couples to two leads (the conduction electron reservoirs) indirectly through intermediate electron levels, two features are noteworthy concerning the Kondo effect. First, the Kondo peak in the spectrum of local density of states becomes narrower as the coupling to the leads is much larger than the interdot coupling, which is just opposite to the case of direct dot-lead coupling. Secondly, the increment of the coupling to the leads and the deviation of the intermediate levels from the Fermi level can effectively facilitate the formation of the negative differential conductance.

Organometallic Catalysis in Aqueous Solution. The Hydrolytic Activity of a Water-Soluble ansa-Molybdocene Catalyst

The synthesis, characterization, and reactivity of the new water-soluble ansa-molybdocene catalyst [{C2Me4(C5H4)2}Mo(OH)(OH2)][OTs] (3) and the related hydroxo-bridged dimer [{C2Me4(C5H4)2}Mo(μ-OH)]2[OTs]2 (5) are described. The effect of the ethylene bridge on the metallocene structure was evaluated by comparing the crystal structures of {C2Me4(C5H4)2}MoH2 (2) and 5 to those of the non-ansa analogues. The ethylene bridge changed the bite angles of the metallocene fragment by only a few degrees in both ansa structures. To probe the electronic consequences of the tetramethylethylene bridge, the {C2Me4(C5H4)2}Mo(CO)H (4) complex was prepared. On the basis of the ν(C⋮O) stretching frequencies, the ansa ligand C2Me4(C5H4)2 was found to be electron-withdrawing relative to two η5-C5H5 ligands. The reactivity of 3 in nitrile hydration, phosphate ester hydrolysis, and carboxylic acid ester hydrolysis was explored, and the rate constants for these transformations were compared to rate constants obtained using the Cp2Mo(OH)(OH2)+ and Cp‘2Mo(OH)(OH2)+ catalysts. In all cases, the Cp2Mo(OH)(OH2)+ catalyst, having intermediate electron density, had the largest rate constants. The reactivity trends for the three catalysts are explained by the relative electrophilicities of the Mo centers. If electron-donating cyclopentadienyl ligands are employed, the reactivity of the bound substrate is decreased relative to Cp and the rate is decreased. Conversely, if electron-withdrawing Cp cyclopentadienyl ligands are employed, the reactivity of the bound hydroxo nucleophile is decreased and the rate is decreased. In the case of the Cp2Mo(OH)(OH2)+ complex, these two opposing trends converge, and optimal activity is observed.

Grain Growth in Nanocrystalline Metal Thin Films under In Situ Ion-Beam Irradiation

Abstract In-situ observations in a transmission electron microscope (TEM) were used to study the microstructure evolution in metal Zr, Pt, Cu, and Au nanocrystalline thin films under ion-beam irradiation. Free-standing films were prepared by sputter deposition. Samples were irradiated in-situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with Ar and Kr ions to fluences in excess of 1016 ion/cm2. As a result of irradiation, grain growth was observed in all samples using Bright Field (BF) imaging in the TEM. The average grain size increased monotonically with ion fluence until it reached a saturation value. Similarly to thermal grain growth, the ion-irradiation induced grain growth curves could be best fitted with curves of the type: Dn-D0n=KΦ. The irradiations were done at temperatures ranging from 20 to 773 K. The results suggest the existence of three regimes with respect to irradiating temperature: (i) a purely thermal regime, which appears to start above the bulk coarse-grained recrystallization temperature, (ii) a thermally assisted regime where thermal diffusion and irradiation effects combine to increase the rate of grain growth relative to that resulting from either of these mechanisms alone, and (iii) an athermal regime (low-temperature regime) where irradiation can by itself cause grain growth. The transition temperature between the athermal regime and the thermally assisted regime depends on the material, but is in the range 0.14–0.22 times the melting point. The influence of the ion type was also investigated on Zr-Fe irradiated with 600 keV Kr ions versus 600 keV Ar ions.

A Simple Pore Water Hydrogen Diffusion Syringe Sampler

Molecular hydrogen (H(2)) is an important intermediate product and electron donor in microbial metabolism. Concentrations of dissolved H(2) are often diagnostic of the predominant terminal electron-accepting processes in ground water systems or aquatic sediments. H(2) concentrations are routinely measured in ground water monitoring wells but are rarely measured in saturated aquatic sediments due to a lack of simple and practical sampling methods. This report describes the design and development (including laboratory and field testing) of a simple, syringe-based H(2) sampler in (1) saturated, riparian sediments, (2) surface water bed sediments, and (3) packed intervals of a fractured bedrock borehole that are inaccessible by standard pumped methods.

Surface nanocrystal hydrogen-bonded complex for photochemical water splitting

Photocatalytic splitting of water in the heterogeneous system consisting of photosensitizer, intermediate electron carrier and catalyst for hydrogen evolution was studied. Molecules and nanocrystals of Mg phthalocyanine were used as photosensitizer; p-benzoquinone served as intermediate electron carrier, and finely dispersed platinum as catalyst for hydrogen evolution. With finely dispersed platinum in the system, the quantum efficiency by the hydrogen formation in the presence of molecules and nanocrystals of the MgPc sensitizer increased by an order of magnitude. It was found that the new adsorption centers are formed on the platinum-modified surface of silicon dioxide (SiO 2). Adsorption of sensitizing molecules of Mg phthalocyanine on them increases the lifetime of the excited states of the latter. The luminescence characteristics of the adsorbates of Mg phthalocyanine nanocrystals on SiO 2 were studied.

Interaction of heme and heme–hemopexin with an extracellular oxidant system used to measure cell growth-associated plasma membrane electron transport

Since redox active metals are often transported across membranes into cells in the reduced state, we have investigated whether exogenous ferri-heme or heme bound to hemopexin (HPX), which delivers heme to cells via receptor-mediated endocytosis, interact with a cell growth-associated plasma membrane electron transport (PMET) pathway. PMET reduces the cell-impermeable tetrazolium salt, WST-1, in the presence of the mandatory low potential intermediate electron acceptor, mPMS. In human promyelocytic (HL60) cells, protoheme (iron protoporphyrin IX; 2,4-vinyl), mesoheme (2,4-ethyl) and deuteroheme (2,4-H) inhibited reduction of WST-1/ mPMS in a saturable manner supporting interaction with a finite number of high affinity acceptor sites (Kd 221 nM for naturally occurring protoheme). A requirement for the redox-active iron was shown using gallium-protoporphyrin IX (PPIX) and tin-PPIX. Heme–hemopexin, but not apo-hemopexin, also inhibited WST-1 reduction, and copper was required. Importantly, since neither heme nor heme–hemopexin replace mPMS as an intermediate electron acceptor and since inhibition of WST-1/mPMS reduction requires living cells, the experimental evidence supports the view that heme and heme–hemopexin interact with electrons from PMET. We therefore propose that heme and heme–hemopexin are natural substrates for this growth-associated electron transfer across the plasma membrane.

Carrier-density effects in many-polaron systems

Many-polaron systems with finite charge-carrier density are often encounteredexperimentally. However, until recently, no satisfactory theoretical description of thesesystems has been available even in the framework of simple models such as theone-dimensional spinless Holstein model considered here. In this work, previous resultsobtained using numerical as well as analytical approaches are reviewed from a unifiedperspective, focusing on spectral properties which reveal the nature of the quasiparticles inthe system. In the adiabatic regime and for intermediate electron–phonon coupling, acarrier-density driven crossover from a polaronic to a rather metallic system takes place.Further insight into the effects due to changes in density is gained by calculating thephonon spectral function, and the fermion–fermion and fermion–lattice correlationfunctions. Finally, we provide strong evidence against the possibility of phase separation.

Precursor non-superconducting pairing and novel superconductivity in underdoped and optimally doped cuprates

The precursor non-superconducting pairing of carriers and the novel superconductivity in high- T c cuprates are studied within the large (bi)polaron model and the new BCS-like fermion and boson mean-field approximations by treating a two-component charge carrier picture. It is argued that pre-formed pairs (large bipolarons) reside in underdoped cuprates between the CuO 2 planes and remain localized in the z-direction. With lowering temperature, these large bipolarons condense into a superfluid quasi-one-dimensional Bose-liquid state. In the intermediate electron–phonon coupling regime, the Cooper-pair formation and condensation in the CuO 2 layers occur also at different temperatures. Large bipolarons and polaron Cooper pairs in optimally doped cuprates condense into a superfluid Bose-liquid state almost at the same temperature due to the doping-induced dimensional crossover.

Total electron scattering cross sections of alkanes at intermediate energies

The total electron scattering cross sections (TCS) are reported for C2H6 (ethane), C3H8 (propane) and C4H10 (butane) for 300–4000eV electrons by measurement of the electron-beam intensity attenuation through a gas cell. These experimental TCS are employed to deduce a simple analytical formula to express the TCS of alkanes at intermediate electron energies in terms of the number of carbon atoms, hydrogen atoms and the electron energy. The present experimental TCS are compared with the previous experimental measurements and the predictions of those by theoretical models.

Coulomb Effect of Positron for (e,e') Reactions in Quasielastic Region

The static Coulomb field of medium and heavy nuclei has been one of persistent problems in medium energy electron scattering. In particular, in the early 1990’s, Ohio group for the ðe; e0Þ reactions treated exactly the Coulomb distortion using partial wave expansion of the electron wave function, referred to the full distorted wave Born approximation (DWBA) in quasielastic region. This calculation is able to compare various nuclear models and furnish an invaluable check for several approximate methods of the Coulomb distortion. However, it is numerically challenging and computation time increases rapidly with higher incident electron energy. In particular, it is not possible to separate the cross section into the longitudinal and the transverse structure functions. In order to avoid the difficult problems related to the DWBA calculations, there are two approaches to treat approximately the electron Coulomb distortion. The first approach is known as the effective momentum approximation (EMA). This is that the electron momenta are modified by the value of the Coulomb potential at the center of target nucleus. This approximation describes the Coulomb distortion for light nuclei and for high incident electron energies but does not for heavy nuclei and for intermediate incident electron energies. Recently, there were experimental data measured from Saclay for C and Pb with positron beam. They improved the EMA calculation with the effective momentum p0 1⁄4 p ð2=3ÞVð0Þ, where Vð0Þ is the Coulomb potential at the origin. With this method, they claimed that if one uses this new EMA the total structure functions induced by positrons have roughly the same magnitude as those by electrons as a function of the energy transfer. As the other approach, from the middle of 1990’s, Kim and Wright have developed an approximate treatment of the electron Coulomb distortion. This approximation leads to an r-dependent momentum which includes the Coulomb distortion arising from the four potential A . The advantage of this approximation is that the separation of the cross section into a longitudinal term and a transverse term is possible. This approximation of the Coulomb distortion agreed with the full DWBA results (about 1–2%) near the peaks of the cross sections for heavy nucleus like Pb on ðe; e0Þ and ðe; e0pÞ reactions with the momentum transfer greater than approximately 300MeV/c. More recently, Kim and Wright improved the above method of the r-dependent momentum for the electron (or positron) current by averaging the phase shift of the leptons, referred to ad hoc approximation (see ref. 8 in detail). They compared the ad hoc approximation, the full DWBA, and the EMA calculations for the Coulomb distortion with a simple harmonic oscillation model. They showed that the total structure functions did not have the same magnitude because of the phase shift of the leptons even though they used the same kinematics as the Saclay experiments. In this report, we examine the Coulomb distortion effect of the positron for the cross section with the ad hoc approximation. In addition, we also investigate the positron Coulomb distortion effect at high incident positron energy of 2.02 GeV for several nuclei, C, Fe, and Au, which is the same kinematics as the SLAC data for the electron scattering. We use a relativistic single particle model for the bound state based on the –! model and apply the same potential for the outgoing nucleon as the bound state coupled with a free relativistic current operator. Since only electrons are observed in the ðe; e0Þ reaction, we do not take into account whether or not the ejected nucleons are ‘‘lost’’, which is no nucleon flux loss. This assumption guarantees current conservation and contains the final state interaction. In the plane wave Born approximation (PWBA) in which the electrons or the positrons are described as plane waves, the cross section for the inclusive ðe; e0Þ scattering can be written as d d d! 1⁄4 M q q4 RLðq; !Þ þ tan 2 q 2q2 ! RTðq; !Þ # ; ð1Þ

Hysteresis of pulsed characteristics of field emission from nanocarbon materials

AbstractCharacteristics of pulsed‐field electron emission from nanocarbon films were investigated in experiments with a wide (1.5–5 mm) quasiplanar test gap. Parameters of the applied voltage (amplitude, shape and duration of pulsed component and d.c. bias) were varied; the maximum peak voltage value amounted to 40–45 kV. Dynamic I–V plots measured in 10–100 µs pulses combined inherent features of field emission (linearity in Fowler–Nordheim (FN) coordinates) with strong hysteresis. For the explanation of the latter, a model of emission via intermediate electron energy states is proposed. The simple quantitative model excellently agrees with the measured characteristics for different emitter samples and over a broad range of voltage parameters, but fails to simulate the phenomenon of emission efficiency boosting by fast voltage drops, also observed in the experiment. A possible mechanism of this dynamic effect may consist in the redistribution of electrons among the different field focuses at the nonuniform emitter surface under the effect of a strong nonstationary electric field. Copyright © 2007 John Wiley & Sons, Ltd.

Evaluation of Laser-Assisted Bremsstrahlung with Dirac-Volkov Propagators

We study spontaneous bremsstrahlung emission in a highly intense laser field. In this regime the interaction with the laser field has to be treated nonperturbatively by using the relativistic formalism including Dirac-Volkov propagators, while the interaction with the Coulomb field and the bremsstrahlung radiation can be treated in first-order perturbation theory. For the intermediate electron state a fully laser-dressed propagator is used, including radiative corrections to avoid singularities on the mass shell. We find that the use of the Dirac-Volkov propagator is crucial to obtain correct numerical results. The cross section of the process is evaluated for laser intensities of order 10(20) W/cm(2) and relativistic energies of the initial electron.

Effect of two intermediate electron donors, NADPH and FADH2, on Spirulina Δ6-desaturase co-expressed with two different immediate electron donors, cytochrome b 5 and ferredoxin, in Escherichia coli

When the gene desD encoding Spirulina Delta(6)-desaturase was heterologously expressed in E. coli, the enzyme was expressed without the ability to function. However, when this enzyme was co-expressed with an immediate electron donor, i.e. the cytochrome b (5) domain from Mucor rouxii, the results showed the production of GLA (gamma-linolenic acid), the product of the reaction catalyzed by Delta(6)-desaturase. The results revealed that in E. coli cells, where cytochrome b (5) is absent and ferredoxin, a natural electron donor of Delta(6)-desaturase, is present at a very low level, the cytochrome b (5) domain can complement for the function of ferredoxin in the host cells. In the present study, the Spirulina-ferredoxin gene was cloned and co-expressed with the Delta(6)-desaturase in E. coli. In comparison to the co-expression of cytochrome b ( 5 ) with the Delta(6)-desaturase, the co-expression with ferredoxin did not cause any differences in the GLA level. Moreover, the cultures containing the Delta(6)-desaturase co-expressed with cytochrome b (5) and ferredoxin were exogenously supplied with the intermediate electron donors, NADPH (nicotinamide adenine dinucleotide phosphate, reduced form) and FADH(2) (flavin adenine dinucleotide, reduced form), respectively. The GLA level in these host cells increased drastically, by approximately 50%, compared to the cells without the intermediate electron donors. The data indicated that besides the level of immediate electron donors, the level of intermediate electron donors is also critical for GLA production. Therefore, if the pools of the immediate and intermediate electron donors in the cells are manipulated, the GLA production in the heterologous host will be affected.

Different regimes of electronic coupling and their influence on exciton recombination in vertically stacked InAs/InP quantum wires

In the present work we study the influence of stacking self-assembled InAs quantum wires (QWRs) on the emission wavelength and the excitonic recombination dynamics. The reduction in the InP spacer layer thickness, d(InP), produces both a size filtering effect towards large wire ensembles and an increase in the vertical coupling for electrons and holes along the stack direction. The different vertical coupling for electrons and holes induces a different behaviour in the exciton recombination dynamics, depending on the InP spacer layer thickness: weak electron coupling and negligible hole coupling for d(InP) > 10 nm, intermediate electron coupling and weak hole coupling for 5 nm ⩽ d(InP) ⩽ 10 nm and strong electron coupling and moderate hole coupling for d(InP) < 5 nm. Such exciton dynamics have been established by comparing the experimental time decay results with a multi-quantum well model accounting for the vertical carrier coupling.