Showing papers by "Moscow Institute of Physics and Technology published in 1999"

AC Stark shift of atomic energy levels

Abstract: Calculated and experimental data on the AC Stark shift of atomic levels in an external, subatomic-strength variable field are considered. Theoretical predictions concerning the disturbance of atomic spectra by fields of atomic and superatomic strength are discussed. The limiting value of the atomic AC Stark shift in a light-frequency radiation field is estimated.

Fabrication of Microarray of Gel-Immobilized Compounds on a Chip by Copolymerization

Abstract: The manufacturing of microchips containing oligonucleotides and proteins immobilized within gel pads, ranging in size from 10 × 10 to 100 × 100 μm, is described. The microchips are produced by phot...

Ionization processes in spark discharge plasmas

Abstract: The dominant mechanisms of electron generation and loss during different phases of spark discharge are considered. The focus is on spark formation in long non-uniform-field air gaps through leader and streamer processes. The streamer development in dry air and the effects of humidity, temperature and density on the properties of a long positive streamer in air are discussed. The ionization kinetics in the leader channel in long air gaps is described. The ionization mechanisms which lead to the re-breakdown within a post-arc air channel and to the streamer breakdown in undisturbed air and in Ar are presented.

Ion acceleration by superintense laser pulses in plasmas

Abstract: Ion acceleration by petawatt laser radiation in underdense and overdense plasmas is studied with 2D3V-PIC (Particle in Cell) numerical simulations. These simulations show that the laser pulse drills a channel through the plasma slab, and electrons and ions expand in vacuum. Fast electrons escape first from the electron-ion cloud. Later, ions gain a high energy on account of the Coulomb explosion of the cloud and the inductive electric field which appears due to fast change of the magnetic field generated by the laser pulse. Similarly, when a superintense laser pulse interacts with a thin slab of overdense plasma, its ponderomotive pressure blows all the electrons away from a finite-diameter spot on the slab. Then, due to the Coulomb explosion, ions gain an energy as high as 1 GeV.

Pickup Ion Acceleration at Low- β p Perpendicular Shocks

Abstract: Multiscale hybrid kinetic simulations of low-{beta}{sub p} supercritical shocks demonstrate that pickup ions may be strongly accelerated by shock surfing, also known as multiply reflected ion acceleration. {copyright} {ital 1999} {ital The American Physical Society}

Two-loop renormalization group restrictions on the standard model and the fourth chiral family

Abstract: In the framework of the two-loop renormalization group, the restrictions on the Higgs mass from the electroweak vacuum stability and from the absence of the strong coupling are refined, while the more precise value of the top mass is taken into account. When the SM cutoff is equal to the Planck scale, the Higgs mass must be $M_{\mathrm H} = (161.3 \pm 20.6)^{+4}_{-10}$ GeV and $M_{\mathrm H}\ge 140.7^{+10}_{-10}$ GeV, where the $M_{\mathrm H}$ corridor is the theoretical one and the errors are due to the top-mass uncertainty. The SM two-loop $\beta$ functions are generalized to the case with massive neutrinos from extra families. The requirement of self-consistency of the perturbative SM as an underlying theory up to the Planck scale excludes a fourth chiral family. Under the precision-experiment restriction $M_{\mathrm H}\leq 215$ GeV, the fourth chiral family, if alone, is excluded even when the SM is regarded as an effective theory. Nevertheless a pair of chiral families constituting a vector-like one could exist.

Quasiclassical Theory and Molecular Dynamics of Two‐Component Nonideal Plasmas

Abstract: A simple model of two-component plasmas based on pseudopotentials and quasi-classical dynamics is derived from Planck's model In the framework of this model bound states are treated quantum-mechanically and free pair states are treated in a quasi-classical way The statistical thermodynamics, structure factors and conductance of the model are investigated by molecular dynamics simulations

Finding local structural similarities among families of unrelated protein structures: a generic non-linear alignment algorithm.

Abstract: We have developed a generic tool for the automatic identification of regions of local structural similarity in unrelated proteins having different folds, as well as for defining more global similarities that result from homologous protein structures The computer program GENFIT has evolved from the genetic algorithm-based three-dimensional protein structure comparison program GA_FIT1, 2 GENFIT, however, can locate and superimpose regions of local structural homology regardless of their position in a pair of structures, the fold topology, or the chain direction Furthermore, it is possible to restrict the search to a volume centered about a region of interest (eg, catalytic site, ligand-binding site) in two protein structures We present a number of examples to illustrate the function of the program, which is a parallel processing implementation designed for distribution to multiple machines over a local network or to run on a single multiprocessor computer Proteins 1999;34:341–355 © 1999 Wiley-Liss, Inc

Spin-gap behavior and charge ordering in α ′ − NaV 2 O 5 probed by light scattering

Abstract: We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ for the temperature range $5 \mathrm{K}l~Tl~300 \mathrm{K}.$ This system undergoes a phase transition into a singlet ground state at $T=34 \mathrm{K}$ accompanied by the formation of a superstructure. For $Tl~34 \mathrm{K}$ several modes were detected. Three of these modes are identified as magnetic bound states. Experimental evidence for charge ordering on the V sites is detected as an anomalous shift and splitting of a V-O vibration at $422 {\mathrm{cm}}^{\ensuremath{-}1}$ for temperatures above 34 K. The smooth and crossoverlike onset of this ordering at ${T}_{\mathrm{CO}}=80 \mathrm{K}$ is accompanied by pretransitional fluctuations both in magnetic and phononic Raman scattering. It resembles the effect of stripe order on the super structure intensities in ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4+\ensuremath{\delta}}.$

Fragmentation of Na 2 ! dimer ions in kilo-electron-volt collisions with He: A coupled wave-packet study

Abstract: The dynamics of the fragmentation of Na2 dimer ions in keV collisions with He is investigated theoretically in the framework of the semiclassical coupled wave-packet method. The fast collisional motion is treated classically, whereas both the electronic motion and the vibrational or dissociative motion of the dimer are treated quantally. The frozen vibrational and rotational approximations are used to lighten the computational effort. The method is able to describe both the impulsive and electronic fragmentation mechanisms. The calculations are undertaken in a basis of 14 electronic states of the Na2-He system. Aside from the entrance #g3s channel, the main contributions to the Na2 fragmentation are those of the #u3s , #g3p , and $u3p channels. For each electronic state dissociation probabilities as functions of the impact parameter are presented for various dimer orientations to extract physical insight in the fragmentation mechanism. This analysis shows in particular that the contribution of the electronic mechanism is due to collisions where the He atom passes between the two Na nuclei. Doubly differential cross sections for dissociation are calculated for the v"0 initial state of the dimer and for the distribution of vibrational states. Good agreement with experiment is obtained assuming a small amount of initial vibrational excitation of the dimer. %S1050-2947!99"03101-7&

Influence of compressibility on propagation of curved flames

Abstract: The nonlinear problem of propagation of a curved stationary flame in a compressible flow is studied by means of two-dimensional numerical simulations of the complete set of equations of flame dynamics including chemical kinetics, thermal conduction, and fuel diffusion. It is shown that curved shape of a flame in a compressible fuel mixture leads to stronger increase of the flame velocity than in the case of isobaric flames. It is found that acceleration of a flame due to the development of the curved shape generates a shock of noticeable intensity ahead of the flame front. The shock compresses the fresh fuel and, in turn, changes parameters of the flame. As a result, strongly curved flames in compressible flows can be observed in tubes much narrower than in the case of isobaric flows. Additional amplification of the flame velocity is obtained due to the development of asymmetrical structures at the flame fronts in wide tubes.

Superconductivity-induced phonon anomalies in high- T c superconductors: A Raman intensity study

Abstract: We have studied an intensity increase of a number of Raman-active phonons below the superconducting transition temperature in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x},$ ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x},$ and ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6+x}$ crystals. It is shown that the effect depends on polarization, doping, and resonance conditions. Different scattering mechanisms that can be responsible for the effect are briefly discussed.

A non-thermal mechanism of spark breakdown in Ar

Abstract: Experiments are described in which streamer breakdown, producing short circuiting, is observed in long non-uniform Ar gaps under standard conditions. The breakdown is developed for a few microseconds and the average breakdown field is around 650 V cm-1. These results differ substantially from the previous observations in long air gaps. A 1.5-dimensional simulation is carried out which supports the general observed properties of the streamer propagation in long Ar gaps and the main features of the streamer breakdown. There are two distinct phases of the breakdown. The first phase consists of an ionizing wave traversing the gap a few times. The second phase consists of a uniform increase in electron density along the whole channel length. The breakdown is of a non-thermal nature and is due to two-step electron-impact ionization through excited states of Ar atoms and due to the effect of electron-electron collisions which drive the non-equilibrium electron energy distribution towards a Maxwellian distribution.

Semiclassical Dynamics and Time Correlations in Two‐Component Plasmas

Abstract: The semiclassical dynamics of a charged particle moving in a two-component plasma is considered using a corrected Kelbg pseudopotential. We employ the classical Nevanlinna-type theory of frequency moments to determine the velocity and force autocorrelation functions. The constructed expressions preserve the exact short and long-time behavior of the autocor-relators. The short-time behavior is characterized by two parameters which are expressable through the plasma static correlation functions. The long-time behavior is determined by the self-diffusion coefficient. The theoretical predictions are compared with the results of semiclassical molecular dynamics simulation.

Nonlinear equation for curved nonstationary flames and flame stability

Abstract: A time-dependent nonlinear equation for a nonstationary curved flame front of an arbitrary expansion coefficient is derived under the assumptions of a small but finite flame thickness and weak nonlinearity. On the basis of the derived equation, stability of two-dimensional curved stationary flames propagating in tubes with ideally adiabatic and slip walls is studied. The stability analysis shows that curved stationary flames become unstable for sufficiently wide tubes. The obtained stability limits are in a good agreement with the results of numerical simulations of flame dynamics and with semiqualitative stability analysis of curved stationary flames. Possible outcomes of the obtained instability at the nonlinear stage are discussed. The instability may result in extra wrinkles at a flame front close to the stability limits and in self-turbulization of the flame far from the limits. The self-turbulization can also be interpreted as a fractal structure. The fractal dimension of a flame front and velocity of a self-turbulized flame are evaluated.

Shallow acceptor centres in silicon studied by means of spin rotation of negative muons

Abstract: The residual polarization of negative muons has been studied for phosphorus-doped and antimony-doped silicon crystals. The measurements were carried out in a transverse magnetic field of 0.1 T over the temperature region 4 K-300 K. The ionized and neutral states of the pseudo-acceptor were observed in antimony-doped silicon for the first time. The rate of transition from the neutral to the ionized state of the acceptor was found to be equal to over the temperature range 4 K-12 K. The estimated rates of relaxation of the magnetic moment of the acceptor-centre electron shell are and in phosphorus-doped silicon and and in antimony-doped silicon at 4 K and 15 K respectively. The experimental results obtained are interpreted in terms of spin-lattice relaxation of the acceptor magnetic moment and of the acceptor-donor pair formation.

Heat-induced changes in optical properties of human whole blood in vitro

Abstract: The effect of anomalous optical behavior of biological tissue at high-intensity laser irradiation can be caused by heat- induced changes in optical properties of consisting components, mainly muscle tissue and blood. We registered the spectral transmission of fresh human whole blood and serum samples in the wavelength range of 300 - 700 nm at the heating of samples in the temperature range of 35 - 65 degrees Celsius. The results showed an increase of 10 - 15% in the transmission of blood serum at the temperature rising up to 50 - 60 degrees Celsius. In the case of diluted whole blood a sharply enhanced transmission was observed at the temperature of 56 - 60 degrees Celsius, while further heating resulted in a decreased transmission down to the initial level. The significant changes (of a three orders of magnitude) in the transmission of whole blood at the wavelength of Nd:YAG laser (1064 nm) were observed. The obtained results can be considered as one of the possible explanations of the anomalous light distribution in certain tissues.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electron Distribution Function in a Nonideal Plasma in Strong Electric and Laser Fields

Abstract: Using a two-term approximation to the kinetic equation, the generalized electron energy and momentum distribution which governs the electron transport and rate properties in nonideal plasma is analyzed. A class of general solutions of the equation is found in the model case which is widely used in semiconductor plasma. An equation that generalizes the well-known Zeldovich-Raizer quantum kinetic equation in the case of nonideal plasma is deduced. Explicit nonequilibrium solutions of this equation for the electron energy and momentum distribution function in a strong laser field are constructed.

Role of quantum interference in thermodynamic equilibrium

Abstract: The influence of quantum interference between different decay channels of three-level atoms on various characteristics of matter and radiation is considered for the case of thermodynamic equilibrium of atoms in a photon gas. It is shown that for rare gases when collisions between atoms are not important, the direct consequence of such an interference is a spontaneously induced atomic coherence between two upper levels. Therefore, for an accurate description of such atoms and radiation in thermodynamic equilibrium with each other it is not sufficient to specify only Boltzmann exponents, which correspond to the diagonal elements of the density matrix. The reason for this is that the conventional Boltzmann description is applied to infinitely sharp eigenstates of the unperturbed Hamiltonian. As we show, it becomes essential to take into account nonzero width of the levels if the levels are relatively close to each other. This produces nonzero nondiagonal elements of the density matrix that modify the emissivity of the equilibrium medium and lead to a zero point at a certain frequency as well as to an enhanced intensity in the red wing at the corresponding temperature. It is shown that the occupation number of the photons is not changed and obeysmore » the equilibrium Planck distribution. {copyright} {ital 1999} {ital The American Physical Society}« less

Thermodynamic Properties of Dense Plasma of Shock—Compressed Metals

Abstract: Quasi-chemical representation (“Chemical Model”) was extrapolated for description of thermodynamic prop erties of plasma of shodc compressed extra-foam nickel and iron samples. Modified Coulomb correction [High Temperature, 18 (1980) 4471, was generalised for the case of multiply ionised partially degenerated hot dense plasmas. Intensive short-range repulsion of heavy particles (atoms and ions) and electron degeneracy was taken into account in combination with simplified form of “Cold Additive”. Satisfactory agreement of theory and experiment was obtained for nickel and iron Hugoniots. This agreement proved to be valid when the approximation was extrapolated into the region of extra-dense plasma of highly compressed solid iron

Computer modelling of structure and dynamics of C-50 n-paraffin crystal, hexagonal phase of C-50 crystal and ethylene/propylene statistical copolymers

Abstract: 3D crystal of n-paraffin C-50 was created by computer molecular dynamics method. The crystal consists of 48 chain molecules containing 48 -CH 2 -groups and two -CH 3 end groups each. Constant temperature-constant pressure calculations, spatially periodic boundary conditions, integration step 0.001 ps were used. Chemical bonds were treated as absolutely rigid, bond angles bending were treated trough U(θ)= K θ (θ-θ 0 ) 2 function and torsion rotations trough U φ =C φ /2[1+Z φ (3cos 3 (φ)-2cos(φ))] function. Non-bonded interactions were represented as truncated and shifted L-D 6-12 potential function. All atoms were introduced explicitly. At 200K and 36 ps of relaxation we got computer C-50 crystal with orthorhombic symmetry (o-phase) and cell parameters a=7,416A, b=4,723A, c=2,625A and density ρ=1,014 g/cm 3 which are close to known from literature. Heating of the crystal up to 450K have shown thermal expansion along all crystallographic axes well coinciding with known experiment. At 400K the system posses transition to hexagonal phase (h-phase). At 450K the crystal melts, but high orientation of chain axes still exists in the system. Analysis of rotational and translation mobility of h-phase at 400K have shown that all chains undergo fast rotation around chain axes for angles about 200-360° during 15 ps. Quenching of the h-phase to 200K decrease the rotation amplitude up to ±12° corresponding to vibrations. The same type vibration motions (around chain axes) exists in o-phase at 200K. Replacing of some H-atoms in chains of initial crystal by -CH 3 groups gave us the possibility to mimic Ethylene/Propylene copolymers. In this work we have created only random E/P copolymers with propylene (PP) units content up to 20%. At constant temperature (200K) increase of PP units in the copolymer produce a disordering.

Experimental determination of the factor representing spontaneous emission into a mode of a semiconductor laser operating on a leaky wave

Abstract: The factor representing spontaneous emission into a semiconductor laser mode was determined directly for the first time. The values found (1 × 10-6 — 2 × 10-6 ) are in satisfactory agreement with calculated data.

Three-body ion–molecule processes

Abstract: Theoretical models used in the studies of three-body ion–molecule reactions are considered and the experimental methods for the study of these processes are described. The results of experimental studies are presented, including those for gas phase reactions at ultra-low temperatures and for the kinetic isotope effect induced by the symmetry of the species. The bibliography includes 130 references.

Attenuation of an optical wave propagating in a waveguide, formed by layers of a semiconductor heterostructure, owing to scattering on inhomogeneities

Abstract: The scattering of an optical wave, propagating in a waveguide made up of layers of a semiconductor heterostructure, is analysed. The attenuation coefficient of the wave is found both for quasi-homogeneous single-crystal layers of a semiconductor solid solution and for layers containing quantum dots.

Dynamics of electron packets and photocounts

Abstract: An alternative approach to the theory of photocounts is discussed. Mechanisms of sharpening of electronic distribution at the expansion of many-electron packets is investigated. Arising of such inhomogeneities initiates a disintegration of many-electron system on smeared on- electron formation. It is shown that interelectronic Coulomb interaction results in the sharpening and localization of such one-electron formations. The motion of an electronic wave packet in the uniform field in the interelectrode space of the vacuum photodetector is investigated. It is shown that the dimensions of such a packet must be of an order or more than one micron. It is shown that such localized charges can give rise to abrupt pulses of the current in the external circuit of the photodetector. The determination of parameters of such a packet by powerful laser pulse scattered is discussed. The motion of one-electron wave packet in the nonuniform field of a negative charged spherical electrode is numerically investigated. It is shown that there is the possibility to unsqueeze electron packets in the transverse direction to macroscopic size by scattering on such electrodes. An experiment of observation of such macroscopic packet on the luminophore screen is discussed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.