Intensity Of External Field Research Articles

Simulating trends in reaction path geometry as a function of external fields. A generalized electronic diabatic model for two‐dimensional energy surfaces

AbstractWe introduce a protocol to represent quantum states as a linear superposition of model electronic diabatic basis states coupled in an external (static) electric field. By considering an entire family of these models, we uncover trends in reaction‐path geometry and the topology of potential‐energy surfaces, including all those that can be realized in a two‐dimensional configurational space. Our approach can be used as a tool to model the key parameters (e.g., diabatic basis states, external field intensity) that yield desired geometrical characteristics in an actual potential energy surface. In this work, external agents such as laser fields, or a group of neighboring charges, are regarded as essential requirements to prompt, or trigger, the occurrence of a chemical process. In these cases, reaction path geometry can be modulated externally so as to yield processes that would appear to occur far from gas‐phase geometries. This phenomenology is intrinsically nonadiabatic. Our present approach accounts for the possibility of such features, i.e., the occurrence of quantum states whose electronic structures resemble products, while at geometries that are more similar to those of reactants. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Optical transitions in semiconductor nanospherical layer under the presence of perturbating electrical field

The optical transitions in spherical nanolayer are studied in the “strong” quantization regime. The dependences of threshold frequencies and corresponding intensities of interband and inband–intersubband dipole transitions from the geometrical sizes of the system are obtained. The confinement Stark effect in the layer is considered. Correspondingly, the energy shifts of the radial and orbital motions of charge carriers in the layer and corresponding perturbated envelope single-electron wave functions are calculated under the external homogeneous electrical field. The influence of quantum-confined Stark effect on the interband and intersubband optical transitions in a spherical nanoheterolayer are studied as well. The explicit dependence of the shifts of threshold frequencies from the intensity of external field and from the geometrical sizes of the sample are revealed. The calculations are carried out separately for both cases of perturbation of the radial and orbital motions of charge carriers in the layer. The comparison analysis of corresponding optical absorption shapes in the cases when external field is absent and when it is present is carried out. The quantitative estimations for concrete CdS/HgS/CdS structure are given as well.

High-order harmonic generation in a two-electron quantum dot molecule

We present results on the occurrence of high-order harmonic generation in a two-electron quantum dot molecule. The two-electron quantum dot molecule consists of two interacting electrons in a double quantum dot system. We consider coherent interaction of the quantum dot molecule with a low-frequency and high-intensity driving field. We calculate the emission spectrum numerically for material parameters corresponding to GaAs and show that high-order harmonic generation is possible in this system. Our findings are in very good agreement with a model system based on level crossing theory. The model system is used for the derivation of a relation for the cutoff frequency of the harmonics. This cutoff frequency is shown to be crucially dependent on the external field frequency and intensity.

The action of intense nonresonance laser radiation on the phase equilibrium of systems with coupled states*

This paper analyzes the state of a bound two-electron formation (a bipolaron) in an electromagnetic radiation field that is not considered weak. By using the Kramers-Henneberger unitary transformation, the external, rapidly oscillating action is transferred to the argument of the potential energy. It is shown that increasing the external field intensity produces a monotonic decrease of the dissociation energy of a bound two-electron formation and reduces the number of its vibrational states. It is established that the external field produces attenuation of the electron-phonon interaction. The degree of dissociation of the bipolaron is computed as a function of the external field intensity, the temperature, and the concentration. The transition between the bound two-particle state and the single-particle state is treated as a smeared phase transition. © 2004 Optical Society of America

Confinement Stark effect and electroabsorption in semiconductor cylindrical layer

The influence of transversal homogeneous electrical field on one-electron states in quantized cylindrical layer is considered. The explicit dependence of Stark shift magnitude on external field intensity and geometrical sizes of system is obtained. The electroabsorption coefficient for interband and inband (“intersubband”) transitions in a layer in dipole approximation is calculated. The corresponding selection rules for these transitions are obtained.

Generalized Taylor dispersion in suspensions of gyrotactic swimming micro-organisms

We calculate the average swimming velocity and dispersion rate characterizing the transport of swimming gyrotactic micro-organisms suspended in homogeneous (simple) shear. These are requisite effective phenomenological coefficients for the macroscale continuum modelling of bioconvection and related collective-dynamics phenomena. The swimming cells are modelled as rigid axisymmetric dipolar particles subject to stochastic Brownian rotations. Calculations are effected via application of the generalized Taylor dispersion scheme. Attention is focused on finite (as opposed to weak) shear. Results indicate that the largest transverse average swimming velocities (essential to gyrotactic focusing) appear shortly after transition from the ‘tumbling’ mode of motion to cells swimming in the equilibrium direction. At sufficiently large shear rates, dispersivity is not monotonically decreasing with external-field intensity. Exceptional dispersion rates which are unique to non-spherical cells appear in the ‘intermediate domain’ of external fields. These are rationalized in terms of the corresponding deterministic problem (i.e. in the absence of diffusion) when cell rotary motion is governed by the simultaneous coexistence of multiple stable attractors.

On the behavior of charged particles in the near wall region of a channel flow

Direct Numerical Simulation (DNS) is used to compute a horizontal gas channel flow at Re b=2800 (based on bulk velocity and half channel width), and to investigate the motion and dispersion of solid particles under external influences such as gravity and/or electrostatic field. Other forces such as non-linear drag corrected for wall proximity and shear-induced lift are also implemented. Emphasis is put on the time correlations of the fluid velocity along the discrete particle path in order to investigate the integral time scales of the fluid seen by the particles in the near wall region. The effect of parameters such as wall distance or external field intensity is highlighted.

Conformational changes coupled with the isotropic–anisotropic transition Part 2. Effect of the external orientational field

Under certain conditions, polypeptide liquid crystal may exhibit an isotropic–anisotropic transition coupled with the intramolecular coil–helix changes. Based on the theoretical model that describes this phenomenon well [Lin J, et al. Polymer 2000; Lin J. Polymer 1997, 1998], the influence of external orientational field on such conformationally coupled transition was studied by introducing an energy term related to the quadrupole field. The calculations carried out show that the coupled isotropic–anisotropic and coil–helix transitions are accelerated when an external orientational field is applied. The effect of the external field intensity and molecular parameters of polypeptide has also been studied. The information obtained through the investigation may find important applications, such as envisaging new routes for processing the rigid–flexible block copolymer with better mechanical properties and constructing molecular devices for information storage and processing.

The near-field effect in a system of two-level dipole particles

We have solved a self-consistent problem on interaction of two dipole atoms located at an arbitrary distance from each other with the field of a quasiresonance light wave, whose intensity is sufficient for the system to manifest nonlinear properties. The atoms are considered as two-level systems described by means of Bloch optical equations, while the field inside of the system includes both Coulomb and retarded parts. We consider a situation where atoms are identical and the distance between them is much smaller than the length of an outer light wave. The distribution of an electric field both inside of a small object and outside of it is found numerically. It is shown that the amplitude of the electric field in a wave zone depends substantially on the frequency of the external field and interatomic distance, while the field distribution differs from the field pattern of an electric dipole. At definite values of the external field intensity an optical multistability is a feature of the system under investigation. We have elucidated the conditions under which the multistability is manifested in the present system. The results obtained are considered as the near-field effect in the optics of small objects, which makes it possible to investigate the structure of small objects by means of optical radiation.

Field- and flow-dependent trapping of red blood cells on polycarbonate accumulation wall in sedimentation field-flow fractionation

Sedimentation field-flow fractionation (SdFFF) instrumentation is now mature. Methodological procedure and particle separation development rules are well established even in the case of biological species. However, in some biological applications, retention properties of samples not predicted by any field-flow fractionation (FFF) elution models are observed. It is demonstrated that the trapping of cellular material in the separation system is not related to geometrical instrumentation features but to channel wall characteristics. The physicochemical particle–wall attractive interactions are different depending on the flow-rate and field intensity applied. Separation power in SdFFF for biological species is therefore limited by the intensity of these interactions. In terms of separation, a balance is to be found between external field and flow intensity to limit particle–wall interactions.

Dynamics of an M-level equidistant radiator in the presence of a thermal electromagnetic field

We study the spontaneous emission of a multilevel equidistant excited quantum oscillator (nuclei, atoms, molecules) in the case in which the symmetry of the dynamical transitions between the levels is submitted to the SU(1,1) symmetry group and compare our results with those well known for the SU(2) symmetry. It is shown that the equidistant radiator in the process of spontaneous decay in the case of SU(1,1) symmetry exhibits either only acceleration or only a decrease in spontaneous emission rates, while in the case of SU(2) symmetry the radiation pulse is similar to that for the Dicke super-radiance. The exact steady-state solution for the atomic density operator for an equidistant radiator driven by a totally incoherent broad-band (chaotic) field for a SU(1,1) symmetry group is obtained using the Fokker-Planck equation. It is found that for a strong-field limit in the case of SU(1,1) symmetry the fluctuations in the atomic population increase with increasing intensity of the external field and do not achieve their maximal value as in the case of SU(2) symmetry. Moreover, in the case of SU(1,1) transition symmetry we find that there is a superbunching effect for certain values of group parameter I, while in the case of SU(2) symmetry only bunching and antibunching occur. It is found that for vanishing values of the external field intensity the second-order intensity correlation function achieves negative values.

Rheology of dilute suspensions of Brownian dipolar axisymmetric particles

We study the respective effects of shear rate and of external field intensity and direction on the contribution to the bulk stress of Brownian dipolar axisymmetric particles suspended in a steady macroscopically homogeneous shear flow of an incompressible Newtonian fluid. Towards this end we obtain the steady orientational distribution and make use of existing general dynamic theories of dilute suspensions. The calculation focuses on the limit of weak rotary diffusion. Thus, unlike previous analyses, the present contribution is not restricted to weak shear effects.Explicit results are presented for the bulk stress when the external field acts in the plane of the simple shear flow. In cases when the deterministic rotary motion possesses a single sufficiently stable node a simple unified description of the respective effects of both the intensity and azimuthal direction of the external field is provided by the boundary-layer approximation. This approximation enables a qualitative explanation of existing numerical results as well as furnishing quantitatively accurate analytical results at relatively moderate values of the rotary Peclét number and the Langevin parameter (∼10). Furthermore, at still larger values of these parameters use of the present asymptotic approximation is clearly preferable since the numerical schemes rapidly deteriorate when steep orientational gradients appear.Singularities of the bulk stress are rationalized in terms of the corresponding deterministic rotary motion. This is particularly interesting because some of these singular phenomena (e.g. those associated with an ‘intermediate regime’ of the field intensity and direction, for which more than one stable attractor exists in the deterministic problem) have no counterparts in suspensions of dipolar spheres or torque-free axisymmetric particles.Finally, the present results obtained for the orientational distribution are also applicable to the study of other aspects of the macroscale description of suspensions of dipolar axisymmetric particles. In this context we mention the extension of continuum modelling of suspensions of swimming micro-organisms so as to enable the analysis of fully developed bioconvection.

Associated production of Higgs bosons with Z bosons by charged leptons in strong external fields

The probabilities of the associated production of a Higgs boson with a Z boson by a charged lepton in the field of a plane electromagnetic wave of arbitrary intensity and in a constant crossed field are obtained. The behavior of the cross section of the process as a function of the particle energies and the external field intensity is investigated for various values of the Higgs boson mass. It is shown that there is a logarithmic increase in the photoproduction cross section at superhigh energies up to a value significantly exceeding the cross section of the reaction e++e−→Z+H, which is presently regarded as the most probable channel for the production of Higgs bosons.

Numerical coupled Liouville approach: Quantum dynamics of linear molecular aggregates under intense electric fields

We develop a numerical calculation scheme of a dynamics of the quantum network for linear molecular aggregates under time-dependent electric fields. Each molecule is assumed to be an electric dipole arranged linearly with an arbitrary angle from the longitudinal axis. This approximation is considered to be appropriate for the aggregates with large intermolecular distances and allows us to treat intermediate- and large-size aggregates without enormous direct calculations of the Coulomb interactions. The molecular interactions are taken into account by adding the radiations from these dipoles to the external electric fields. The dynamics is performed by solving the coupled Liouville equation constructed from the Liouville equation for each dipole. The effects of the retarded electric fields are evaluated with numerically exact precision by using the sixth-order Runge–Kutta scheme. As a simple example, we examine the linear aggregates involving two dipoles composed of two-state molecules under the continuous laser fields. The effects of the intensity of external fields, the intermolecular distances, and the angles between the dipole and the longitudinal axis on the population differences are investigated. The linear polarizability spectra are calculated by using the definition of nonperturbative polarizability. An abrupt change like the phase-transition behavior in the variation in the population differences for the applied field intensities is observed for the dimer models. Based on these results, we anticipate the population differences for larger (intermediate)-size aggregates. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 77–87, 1998

Numerical coupled Liouville approach: Quantum dynamics of linear molecular aggregates under intense electric fields

We develop a numerical calculation scheme of a dynamics of the quantum network for linear molecular aggregates under time-dependent electric fields. Each molecule is assumed to be an electric dipole arranged linearly with an arbitrary angle from the longitudinal axis. This approximation is considered to be appropriate for the aggregates with large intermolecular distances and allows us to treat intermediate- and large-size aggregates without enormous direct calculations of the Coulomb interactions. The molecular interactions are taken into account by adding the radiations from these dipoles to the external electric fields. The dynamics is performed by solving the coupled Liouville equation constructed from the Liouville equation for each dipole. The effects of the retarded electric fields are evaluated with numerically exact precision by using the sixth-order Runge–Kutta scheme. As a simple example, we examine the linear aggregates involving two dipoles composed of two-state molecules under the continuous laser fields. The effects of the intensity of external fields, the intermolecular distances, and the angles between the dipole and the longitudinal axis on the population differences are investigated. The linear polarizability spectra are calculated by using the definition of nonperturbative polarizability. An abrupt change like the phase-transition behavior in the variation in the population differences for the applied field intensities is observed for the dimer models. Based on these results, we anticipate the population differences for larger (intermediate)-size aggregates. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 77–87, 1998

Resonance fluorescence near a photonic band edge: Dressed-state Monte Carlo wave-function approach

We introduce a dressed-state Monte Carlo wave-function technique to describe resonance fluorescence in a broad class of non-Markovian reservoirs with strong atom-reservoir interaction. The method recaptures photon localization effects which are beyond the Born and Markovian approximations, and describes the influence of the driving field on the atom-reservoir interaction. Using this approach, we predict a number of fundamentally new features in resonance fluorescence near the edge of a photonic band gap. In particular, the atomic population exhibits inversion for moderate applied field intensity. For a low external field intensity, the atomic system retains a long-time memory of its initial state.

On magnetic turbulence in interstellar clouds

view Abstract Citations (73) References (86) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Magnetic Turbulence in Interstellar Clouds Myers, P. C. ; Khersonsky, V. K. Abstract Observations of diffuse, dark, and giant molecular clouds and their cores are analyzed to determine properties of their turbulent motions. Estimates of characteristic cloud internal density, external extinction, and external radiation field intensity are used to deduce the electron fraction chie due to both photoionization and cosmic rays. This ionization fraction exceeds that due to cosmic rays alone, by factors approximately 5 for dark cloud cores to approximately 4000 for giant molecular clouds with embedded OB stars. Estimates of characteristic cloud size, density, velocity dispersion, ionization fraction, and magnetic field strength then indicate that four diagnostic numbers exceed unity by a significant factor: the Reynolds number, the magnetic Reynolds number, the Hartmann number, and the 'wave coupling number,' or ratio of cloud size to minimum hydromagnetic wavelength. These results indicate that virtually all observed interstellar clouds have strong coupling between the magnetic field and the neutral gas, through ion-neutral collisions, even if the field is weaker than its equipartition value. This coupling allows energetically significant magnetohydrodynamic (MHD) waves to propagate above cutoff, so that MHD waves, chaotic motions, and clumpy density structure are probably more pervasive in interstellar clouds than would be expected from cosmic-ray ionization alone. This strong coupling implies that the timescale for ambipolar diffusion is at least approximately 107 yr for low-mass cores, and is at least approximately 108 yr for the gas around cores. These timescales may be too long for all of the mass in a low-mass core to condense via ambipolar diffusion. The observed velocity dispersion is strongly correlated with the estimated electron fraction, according to the power law nu approximately chie p, with p approximately = 0.3. This trend, and those already known among velocity dispersion, size, and density, suggest that increasing extinction may influence the structure of cloud density and velocity dispersion by driving a cycle of decreasing ionization, decreasing MHD wave activity, decreasing velocity dispersion, and increasing density. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175434 Bibcode: 1995ApJ...442..186M Keywords: Interstellar Magnetic Fields; Ionization; Magnetic Flux; Magnetohydrodynamic Waves; Molecular Clouds; Turbulence; Abundance; Ambipolar Diffusion; Density Distribution; Photoionization; Reynolds Number; Velocity Distribution; Astrophysics; ISM: CLOUDS; ISM: MAGNETIC FIELDS; ISM: MOLECULES; MAGNETOHYDRODYNAMICS: MHD; TURBULENCE; WAVES full text sources ADS | data products SIMBAD (7)

Effect of interference of quantum states on the broadening of energy levels in a strong electromagnetic field

The wave equation, written down for ψ-functions averaged over the gauge, is solved within the resonance approximation for a number of low levels of a hydrogen-like atom, located in a strong electromagnetic field and undergoing impact action. It is shown that the interference of quantum states, resulting from the interaction of the atom with its surrounding, may lead, depending on the conditions, to a further dependence of the broadening or narrowing of spectral line contours on the external field intensity.

Dynamical chaos in the highly excited states of a quantum Daffing oscillator in an external harmonic field

The energy spectrum of the highly excited states of a quantum Daffing oscillator is considered. It is shown that the energy levels satisfy En>En+1 and En → -F2/8ω2, when n → ∞, where F2 and ω are the intensity and frequency of the external field. It is suggested that this property of the spectrum is the quantum manifestation of a stochastic attractor in the classical nonlinear limit. This property also takes place for large F2.

The Average Free Energy and the Effective Mean Field Hypothesis

In this paper, an improvement in the variational-cumulant expansion method is made. For any model in lattice field theories, the average free energy can be calculated rigorously according to the generalized cumulant expansion, and the intensity of the effective external field can be determined by the effective mean field hypothesis. As a test, the Ising model is studied, and the results take a visible turn for the better.