Polymer translocation in a double-force arrangement

Using Langevin dynamics simulations, we investigate the translocation dynamics of an externally driven polymer chain through a nanopore, where a pulling force F is exerted on the first monomer whilst there is an opposing force F _E < F within the pore. Such a double-force arrangement has been proposed recently to allow better dynamical control of the translocation process in order to sequence biopolymers. We find that in the double-force arrangement translocation becomes slower as compared to the case under a single monomer pulling force of magnitude F - F _E , but scaling of the translocation time as a function of the chain length ∼ N ² does not change. The waiting time (m) for monomer m to exit the pore is found to be a monotonically increasing function of the bead number almost until m N , which indicates relatively well-defined slowing down and control of the chain velocity during translocation. We also study the waiting time distributions for the beads in the chain, and characterize in detail fluctuations in the bead positions and their transverse position coordinates during translocation. These data should be useful in estimating position-dependent sequencing errors in double-force experiments.     

Theory of gel point in real polymer solutions

We develop a new theory of gelation which takes into account (i) delay of the gel point and (ii) change of functionality due to ring formation. We show that the problem of finding the gel point in real polymer solutions reduces to the problem of calculating the total ring concentration and the extent of reaction of intermolecular reaction at the gel point. In this paper, we solve a special case of this problem, on the basis of the independence assumption between intermolecular reaction and cyclization which takes into account only (i) the delay of the gel point: making use of the asymptotic equality of the total ring concentration, we acquire an approximate solution for the gel point D_c as a function of the inverse concentration , the relative frequency of cyclization and dimension d. Applying the observed values of in linear polyesters, the theoretical result reproduces well the Wile and the Gordon-Scantlebury observations, showing the existence of a critical dilution beyond which gelation can not occur, and an asymptote . As the classical gel point is approached, the present theory reduces to the linear equation, which makes one-to-one correspondence with the real slope , suggesting the inequality which is just what polymer chemists have quested for so far, with the physical meaning having remained unknown. Receveid: 30 December 1997 / Revised: 28 May 1998 / Accepted: 12 June 1998     

Effect of atmosphere on reductions in the glass transition of thin polystyrene films

We have used nulling ellipsometry to measure the glass transition temperature, T _g , of thin films of polystyrene in ambient, dry nitrogen, and vacuum environments. For all environments, the measured T _g values decrease with decreasing film thickness in a way that is quantitatively similar to previously reported studies in ambient conditions. These results provide strong reinforcement of previous conclusions that such reduced T _g values are an intrinsic property of the confined material. Furthermore, the results are in contrast to recent reports which suggest that the T _g reductions measured by many researchers are the results of artifacts (i.e. degradation of the polymer due to annealing in ambient conditions, or moisture content).     

Molecular dynamics simulation of semiflexible polyampholyte brushes--The effect of charged monomers sequence

Planar brushes formed by end-grafted semiflexible polyampholyte chains, each chain containing an equal number of positively and negatively charged monomers, are studied using molecular dynamics simulations. Keeping the length of the chains fixed, the dependences of the average brush thickness and equilibrium statistics of the brush conformations on the grafting density and the salt concentration are obtained with various sequences of charged monomers. When similarly charged monomers of the chains are arranged in longer blocks, the average brush thickness is smaller and the dependence of brush properties on the grafting density and the salt concentration is stronger. With such long blocks of similarly charged monomers, the anchored chains bond to each other in the vicinity of the grafting surface at low grafting densities and buckle toward the grafting surface at high grafting densities.     

A study of KCL lattice defects by thermoluminescence experiments

The thermoluminescent emission of X-irradiated potassium chloride is recorded simultaneouslyvs. temperature and wavelength. Samples of different origin and prepared through different processes, including thermal treatments, are examined. Most records show essentially two glow peaks, the wavelength of the one at higher temperature being slightly shifted toward the red. On the whole, experimental results suggest that luminescent centres are originated by potassium ion vacancies lying at a variable distance from interstitial potassium ions. The observed red shift is ascribed to the Coulomb energy of the pairs of these point defects of opposite charges.     

On the ground state of ferromagnetic Hamiltonians

It is generally believed that the ground state of the ferromagnetic Heisenberg-Dirac-Van Vleck Hamiltonians acting ons=1/2 spins of a lattice withN sites has the maximum possible value of the total spinS=N/2 and isN+1 times degenerate. We present a rigorous proof of this statement, independent of the lattice dimension and topology. A member of the Doppler Institute of Mathematical Physics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Prague.     

Origin of the defect states at ZnS/Si interfaces

Electrical characterisation of silicon surfaces contaminated by a zinc-sulphide overlayer has been carried out by forming Schottky diodes on the silicon after the ZnS has been etched off. The techniques include current-voltage, capacitance-voltage, and deep-level transieni spectroscopy. The Schottky diodes show clear memory of the presence of the ZnS overlayer and the electrical characteristics are far from ideal. Five deep levels in the sub-surface region of the silicon are detected, corresponding to the Zn⁺, Zn⁺⁺, S^–, S^–– states and probably to a Zn–B complex (p-type). Diffusion of the zinc and sulphur into the silicon is therefore confirmed and this diffusion is thought to create a compensated layer at the interface. These impurity states control the electrical characteristics of the surface in these diodes.     

Characterization of the structure and the size distribution of branched polymers formed by co-polymerization of MMA and EGDMA

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl methacrylate (EGDMA) in solution leads to the formation of polydisperse branched PMMA which grows in size until the system gels. The structure and the size distribution of the PMMA aggregates were characterized at infinite dilution using static and dynamic light scattering and size exclusion chromatography (SEC). The reaction extent was measured using SEC and Raman spectroscopy. The results show that the structure and size distribution of PMMA aggregates formed close to the gel point are compatible with those of percolating clusters. The structure factor of semi-dilute solutions of PMMA aggregates is the same as that of dilute solutions at distance scales much smaller than the correlation length of the concentration fluctuations (). However, the cut-off function of the pair correlation function at for semi-dilute solutions is more gradual than the cut-off function at for dilute solutions. Received 11 May 1998 and Received in final form 22 October 1998     

Boerdijk-Coxeter helix and biological helices

Helices and dense packing of spherical objects are two closely related problems. For instance, the Boerdijk-Coxeter helix, which is obtained as a linear packing of regular tetrahedra, is a very efficient solution to some close-packing problems. The shapes of biological helices result from various kinds of interaction forces, including steric repulsion. Thus, the search for a maximum density can lead to structures related to the Boerdijk-Coxeter helix. Examples are presented for the -helix structure in proteins and for the structure of the protein collagen, but there are other examples of helical packings at different scales in biology. Models based on packing efficiency related to the Boerdijk-Coxeter helix, explain, mainly from topological arguments, why the number of amino acids per turn is close to 3.6 in -helices and 2.7 in collagen. Received 26 November 1998 and Received in final form 12 April 1999     

Measurement of the stopping power of Au and MgF2 for slow muons

The spectral flux densityn(T) of ^– emerging from a Au or MgF₂ moderator has been measured at low energiesT using time-of-flight. Fromn(T) the stopping powerS(T) of Au was determined for 2 keVT22 keV, and of MgF₂ for 2 keVT12 keV. For Au,S(T) is smaller than calculated values obtained from proton atomic data practically in the wholeT range (Barkas effect); at lowT S(T) approaches the calculated values. For MgF₂,S(T) agrees fairly with the calculated values above 5 keV and then drops below these values. We ascribe this dropping to the large energy gap of the MgF₂ insulator.We wish to thank H. Angerer, H. Plendl, G. Schmidt, and C.A. Schug for help with the data taking, J. Homolka for computational help, H. Hagn, and H. Weiss for technical assistance and P. Maier-Komor and R. Scherrer for manufacturing the windows and targets. The hospitality of PSI and financial support by the German Bundesministerium für Forschung und Technologie are acknowledged.     

Refined interpretation of Christiansen-filter experiments and neutron scattering lengths of the lead-isotopes

A refined interpretation of Christiansen filter experiments is described, which allows for the effects of inhomogeneities in the powder column of the filter. Using this procedure the evaluation of experiments on enriched samples of lead isotopes provided the neutron coherent scattering lengths (in fm) for the separated isotopes:b (204) =10.6 ± 2.0;b (206)=9.23 ± 0.05;b (207)=9.28 ± 0.04 andb (208)=9.50 ± 0.06. The corresponding potential scattering radius R was obtained by taking account of resonance contributions as earlier used in the determination of the neutron's electric polarizability. The found R=9.74 ± 0.07 fm is in good argreement with the literature. This confirms the correctness of the used resonance contributions.Work partially supported by the Bundesministerium für Forschung und Technologie     

On the competiting role of mean-field and residual interactions in flow observables

Theoretical analyses of heavy-ion reactions are performed in the framework of the semi-classical Landau-Vlasov approach. The incident energies are investigated in the range from intermediate to low energy regimes, where transverse collective motion has been experimentally evidenced. The influence of the equation of state (E.O.S.) parameters on various collective observables is studied in relation with the action of the residual interactions. From the sensitivity to both aspects, and taking into account the experimental biases limitations, our investigation indicates that E.O.S. signatures should be more expected at energies below 100 MeV per nucleon.     

Momentum distributions of projectile fragments produced in the cold and hot fragmentation of relativistic136Xe and197Au projectiles

The longitudinal-momentum distributions of projectile fragments from 0.8 A GeV¹³⁶Xe and 1 A GeV¹⁹⁷Au projectiles impinging on targets of beryllium and aluminium, respectively, have been measured using the projectile-fragment separator FRS at GSI. Different momentum distributions have been found for two different classes of fragmentation processes: the abundant hot fragmentation with several nucleons evaporated from the prefragments, and the rare cold fragmentation with only protons removed from the projectile, but no nucleons evaporated. The data are compared to model calculations.This article comprises part of the Ph.D. thesis of B. Voss     

Influence of the pump threshold on the single-frequency output power of singly resonant optical parametric oscillators

We demonstrate that for a given pump source, there is an optimum pump threshold to achieve the maximum single-frequency output power in singly resonant optical parametric oscillators. Therefore, cavity losses and parametric amplification have to be adjusted. In particular, continuous-wave output powers of 1.5 W were achieved with a 2.5 cm lithium niobate crystal in comparison with 0.5 W by a 5 cm long crystal within the same cavity design. This counter-intuitive result of weaker amplification leading to larger powers can be explained using a model from L.B. Kreuzer (Proc. Joint Conf. Lasers and Opt.-Elect., p. 52, 1969). Kreuzer also states that single-mode operation is possible only up to pump powers which are 4.6 times the threshold value. Additionally, implementing an outcoupling mirror to increase losses, single-frequency waves with powers of 3 W at 3.2 μm and 7 W at 1.5 μm could be generated simultaneously.     

Energy dependence of the isotopic effect in the (n,p) reaction on the germanium isotopes

Cross sections for^70,72,73,74Ge(n, p)^70,72,73,74Ga,⁷⁰Ge(n, 2n)⁶⁹Ge,⁷²Ge (n,)⁶⁹Zn ^m and⁷⁴Ge(n, )⁷¹Zn ^m reactions are measured in the energy range from 13.0 to 16.6 MeV by the activation method using Ge(Li) detector-ray spectroscopy and compared with predictions of the reaction model incorporating preequilibrium and equilibrium emission mechanisms to interpret the energy dependence of the isotopic effect occuring in the (n, p) reaction. The fitted single-particle state-density parametersg, determined here for the germaniums are discussed together with theg-values found previously for the Se, Zr and Pd isotopic chains. A validity of the consistency condition between the precompound and compound models, which relatesg to the experimental level-density parametera viaa= ² g/6 is demonstrated.     

The influence of state dependent short range correlations on the depletion of the nuclear Fermi sea

The inlfuence of state dependent short range correlations on the occupation numbers of the single particle shell model orbits of the doubly closed shell nuclei¹⁶O and⁴⁰Ca is examined. The study shows that the effect of the state dependence of the short range correlations is rather small. The total depletion of the nuclear Fermi sea changes slightly compared with the one calculated by considering state independent short range correlations.     

Rotational excitation of CH<Subscript>4</Subscript> by He atoms

Quantum close-coupling and coupled-state approximation scattering calculations for rotational energy transfer of rotationally excited CH₄ due to collisions with He are presented for collision energies between 10⁻⁷ and 3000 cm⁻¹ using the MP4 potential of Calderoni et al. [J. Chem. Phys. 121, 8261 (2004)]. State-to-state cross sections and rate coefficients from selected initial rotational states of CH₄ in symmetries A, E, and F are studied from the ultra-cold to the thermal regime. Comparison of the cross sections with available theoretical results and experimental data show good agreement. Applications to astrophysics and cold laboratory environments are briefly addressed.     

Investigation of the fusion of heavy nearly symmetric systems

Excitation functions in the vicinity of the Coulomb barrier have been measured for the formation of evaporation residues in¹⁰⁰Mo-induced fusion reactions with^{90, 92, 96}Zr,^{92, 96, 98, 100}Mo,¹⁰⁴Ru and¹¹⁰Pd as well as for the system⁹⁶Zr+⁹⁶Zr. From these data the fusion probability in central collisions was extracted covering a range of 4 orders of magnitude. At the fusion barriers expected from systematics we find that the fusion probability is suppressed by one to three orders of magnitude. It is rising very gradually at higher energies and reaches for the heaviest systems saturation only at energies as high as 30 MeV above the barrier. The observed hindrance of the fusion process increases roughly with the growing Coulomb repulsion between the collision partners, but there is also a distinct influence of their individual nuclear structure. The data are compared to the extra-push model, the surface-friction model and the diabatic fusion model. A parameterisation of the extra-push energy and its fluctuation in terms of a macroscopic quantity like the Coulomb repulsion combined with a microscopic quantity characterizing the nuclear structure is proposed. As a byproduct of this work a new alpha emitter,¹⁹¹Po, could be identified. Its half-life is (15.5 _–2.5 ⁺⁶ ) ms, the alpha energy is (7314±20) keV.     

Methods for controlling of the laser-induced absorption in a BTO crystal by using of cw-laser radiation

The iron-atom concentration distribution as well as the gas-phase temperature was measured via laser-induced fluorescence (LIF) during iron-oxide nanoparticle synthesis in a low-pressure hydrogen/oxygen/argon flame reactor using ironpentacarbonyl (Fe(CO)₅) as precursor. Temperature measurements based on multi-line NO-LIF imaging are used to correct for temperature-dependent ground-state populations. The concentration measurement is calibrated based on line-of-sight absorption measurements. The influence of the precursor on the flame is observed at precursor concentrations larger than 70 ppm as the flame front moves closer to the burner surface with increasing Fe(CO)₅ concentration.