Showing papers on "Dipole published in 1985"

First observation of an extremely large‐dipole infrared transition within the conduction band of a GaAs quantum well

Abstract: A new type of optical transition in GaAs quantum wells has been observed. The dipole occurs between two envelope states of the conduction‐band electron wave function, and is called a quantum well envelope state transition (QWEST). The QWEST is observed by infrared absorption for two structures with 65‐A‐thick‐ and 82‐A‐thick wells. The transitions exhibit resonant energies of 152 and 121 meV respectively, full width at half‐maximum linewidths as narrow as 10 meV at room temperature, and an oscillator strength of 12.2. The material is anticipated to have subpicosecond relaxation times and be ideal for low‐power optical digital logic.

Dressed-atom approach to atomic motion in laser light: the dipole force revisited

Abstract: We show that the dressed-atom approach provides a quantitative understanding of the main features of radiative dipole forces (mean value, fluctuations, velocity dependence) in the high-intensity limit where perturbative treatments are no longer valid. In an inhomogeneous laser beam, the energies of the dressed states vary in space, and this gives rise to dressed-state-dependent forces. Spontaneous transitions between dressed states lead to a multivalued instantaneous force fluctuating around a mean value. The velocity dependence of the mean force is related to the modification, induced by the atomic motion, of the population balance between the different dressed states. The corresponding modification of the atomic energy is associated with a change of the fluorescence spectrum emitted by the atom. The particular case of atomic motion in a standing wave is investigated, and two regimes are identified in which the mean dipole force averaged over a wavelength exhibits a simple velocity dependence. The large values of this force achievable with reasonable laser powers are pointed out with view to slowing down atoms with dipole

The energy and elastic dipole tensor of defects in ionic crystals calculated by the supercell method

Abstract: The energy, the elastic dipole tensor and the entropy of point defects in ionic crystals are usually calculated by the Mott-Littleton approach, which treats a single defect in an infinite crystal. The authors suggest that there may be advantages, particularly for the dipole tensor and the entropy, in performing the calculations in periodically repeating geometry. They examine how this 'supercell' method can be used for the energy and the dipole tensor, paying attention to the problem that for charged defects the repeating unit carries a net charge, so that the Coulomb energy is divergent. They test the supercell method on a number of both charged and uncharged defects, and show that the results for the energy and dipole tensor are in close agreement with those obtained by the conventional approach.

Electrostatic effects in proteins

Abstract: 1 PERSPECTIVE AND OVERViEW 387 2 PROTEIN STRUCTURES AND ELECTROSTATIC INTERACTION 389 2.1 Charge Site Geometry and Dielectric Value 389 2.2 Intrinsic Versus Effective and Observed pK Values 389 2.3 Solvent Accessibility and Effective pK Values 390 2.4 Peptide Dipole Partial Charges 394 3 COMPUTATION OF LONG-RANGE ELECTROSTATIC INTERACTIONS 395 3.1 Microscopic Electrostatic Calculations 395 3.2 Uniform and Nonuniform Dielectric Continuum Methods 397 3.3 Continuum Models with Dielectric Boundaries 399 4 SUMMARY 404 5 EXAMPLES OF PROTEIN ELECTROSTATIC FIELD CALCULATION 406 5.1 Effective pK Values 406 5.2 Protein Electrostatic Stabilization 408 5.3 Recognition and Binding 413

The solution of the hypernetted‐chain approximation for fluids of nonspherical particles. A general method with application to dipolar hard spheres

Abstract: In this paper we describe a general method for the numerical solution of the full hypernetted‐chain (HNC) theory for fluids characterized by angle‐dependent pair potentials. This method is also applicable to the closely related reference hypernetted‐chain (RHNC) approximation. The only formal restriction is that the pair potential and correlation functions must be expandable in a basis set of rotational invariants. We present explicit numerical solutions of the RHNC theory for dense dipolar hard sphere fluids and detailed comparisons are made with previous theories and computer simulation results. It is found that the full RHNC theory generally improves upon the previous reference linearized and quadratic HNC approximations. The values given by the RHNC theory for the static dielectric constants are smaller than those given by these earlier approximations and are in much better agreement with computer simulations.

Pseudo-spectral dipole oscillator strengths and dipole-dipole and triple-dipole dispersion energy coefficients for HF, HCl, HBr, He, Ne, Ar, Kr and Xe

Abstract: Pseudo-dipole oscillator strengths and excitation energies, which are discrete representations of original continuous dipole oscillator strength distributions (DOSDs), are tabulated for the ground state HF, HCl and HBr molecules and Ne, Ar, Kr and Xe atoms. These pseudo DOSDs are used to evaluate the dipole-dipole and triple-dipole dispersion energy constants for all possible two- and three-body interactions between HF, HCl, HBr, He, Ne, Ar, Kr and Xe. The importance of these results, and of the original DOSDs, is discussed briefly.

Density-matrix theory of semiconductor lasers with relaxation broadening model - Gain and gain-suppression in semiconductor lasers

Abstract: The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Density-matrix theory of semiconductor lasers with relaxation broadening model-gain and gain-suppression in semiconductor lasers

Abstract: The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Electromagnetic radiations from rocks

Abstract: To test the possibility of the emission of electromagnetic waves from rocks, experiments have been made to measure the electric field by using mainly granite samples that were struck together or struck by a hammer or a weight and were fractured by a bending moment. The wide-band (10 Hz to 100 kHz) waveforms of electric signals were digitally recorded. Roughly four kinds of signals have been observed: 30 kHz, 5 kHz, 10 Hz and, in addition, intermittent pulses. Using these measurements of the electric fields, the average electric dipole moment generated was estimated to be 10−14 C m. The possibilities of producing such a dipole moment are discussed in terms of contact electrification (or separating electrification) and piezoelectrification, and both possibilities are shown.

Quantum oscillator in a blackbody radiation field

Abstract: The quantum Langevin equation is used to calculate an exact expression for the free energy of a quantum oscillator interacting, via dipole coupling, with a blackbody radiation field. In particular, we obtain a temperature-dependent shift in the free energy. This result may then be used to obtain corresponding results for the energy, the partition function, and other thermodynamic quantities.

Bounds to two- and three-body long-range interaction coefficients for S-state atoms

Abstract: New upper and lower bounds to the van der Waals C6, C8, and C10 coefficients for hydrogen, noble gas, alkali, and alkaline earth atoms are determined by using Pade approximants to bound the dynamic multipole polarizabilities. Also, the nonadditive, three‐body coefficients involving dipole, quadrupole, and octupole interactions are bounded.

Enhanced fields on rough surfaces: dipolar interactions among particles of sizes exceeding the Rayleigh limit

Abstract: Amplified electromagnetic fields generated by a surface of finitely sized metal or dielectric particles are calculated. Regular arrays of particles produced by lithographic techniques and stochastic particle distributions that occur, e.g., in island films, are discussed. Retarded dipolar interactions between the particles are explicitly taken into account. Particles of finite size are considered for which dynamic depolarization and radiation damping effects are important. Limits of validity of the present approach are indicated. The total electromagnetic field from the surface is calculated by superposition: Scalar potentials characterizing a single particle are convoluted with a distribution function describing the particle positions. The surface Hertz vector is obtained from the single-particle Hertz vector by convolution with a two-dimensional Shah function representing the array or with the autocorrelation function of the stochastic surface. A plane-wave description of the dipolar fields is used, whereby the convolution is transformed into a simple multiplication in Fourier space. Cylindrical, general spheroidal, and spherical shapes are considered for the individual particle. Particle dipole moments are obtained by a self-consistent procedure. Dipolar interactions result in shifts and broadening of the particle plasmon resonances, which are responsible for the local intensity enhancement. A set of universal curves is given from which shift and broadening can be calculated for particles of all sizes and shapes. Extrema in the dipolar interactions arise when grating orders change from radiative to evanescent character. The strong variation of the Raman enhancement with angle and wavelength in the vicinity of these extrema is clearly predicted from the Hertz-vector calculation. The formalism described permits one to calculate electromagnetic properties of the surface and enhancement factors for any electromagnetic process occurring at or near the surface. As examples, the calculation of reflectivities for s-and p-polarized excitation and surface-enhanced Raman-scattering cross sections are discussed.

Dipole and slot elements and arrays on semi-infinite substrates

Abstract: The printed dipole or slot antenna on a semi-infinite substrate and infinite phased arrays of these elements are investigated. The solution is based on the moment method in the Fourier transform domain. The generalized impedance or admittance matrix can be expressed in rapidly converging infinite-integral or infinite-summation forms, allowing the accurate determination of the current distributions. Using the present formulation, the input impedance, resonant length, and radiation pattern for the isolated antennas, and the reflection coefficient for infinite phased arrays, are calculated.

The electronic structure of the calcium monohalides. A ligand field approach

Abstract: The electronic structure of the calcium monohalides is addressed using a ligand field model which approximates the halide as a polarizable negative charge perturbing the one electron valence structure of the Ca+ ion. A simple, zero‐free‐parameter model is shown to predict accurately electronic energies, transition moments, permanent dipole moments, and several other molecular constants that have been experimentally determined. The molecular properties and electronic wave functions are interpreted in terms of the polarization (s/p/d/f mixing) and radial expansion (nl/n+1l mixing) of the low lying, free ion, basis functions caused by the electric field of the ligand.

l2,3 absorption-spectra of the lighter 3d transition-metals

Abstract: A theory is presented for the x-ray absorption spectra of materials characterized by bandwidths which are somewhat larger than the (quasiatomic) electron correlation energies. Assuming that an empty-band approximation is appropriate for nearly empty bands we show that the problem is much akin to the well-known Coulomb localization problem in Auger spectroscopy, with an enhanced sensitivity for exchange and multipole effects because of the dipole selection rules. Using first-principles band structure and atomic data as input for the theory, we calculate the spectra of Ca, Ti, and V and obtain good agreement with high-resolution experimental spectra.

Effect of nonuniform zeta potential on particle movement in electric fields

Abstract: The translational and rotational velocities of a colloidal particle in an applied electric field (E∞) are derived for the general case of a zeta potential (ζ) which varies with position on the particle surface. Analytical results which are correct to O(∇E∞) are presented. The particle is modeled as a rigid, nonconducting sphere of radius a, and the analysis assumes ϰa → ∞ where ϰ is the Debye screening parameter of the surrounding fluid. The electrophoretic mobility in a constant field depends on the quadrupole moment of ζ and is not just related to the area-averaged zeta potential through Smoluchowski's equation, as is commonly assumed. The dipole moment of ζ contributes to rotation in a constant field, tending to align the particle. Dielectrophoretic motion results from the dipole moment interacting with ∇E∞ for unpolarized charge distributions, and from induced dipole and quadrupole moments for particles whose surface charge is mobile.

Theoretical Gain of Quantum-Well Wire Lasers

Abstract: Gain is given theoretically for quantum-well wire lasers where electrons are confined one-dimensionally. Maximum gain is obtained for a quantum-well wire perpendicular to light propagation, due to anisotropy of the dipole moment. Although the density-of-states is infinite at subband edges, gain remains finite due to the intraband relaxation. Therefore, high gain can be obtained by reducing intraband scatterings. Gain in 100 A×100 A Ga0.47In0.53As/InP quantum-well wires is about twice that in 100 A thick conventional quantum-wells, and reduction of the laser threshold is expected.

Integrated dipole oscillator strengths and dipole properties for Ne, Ar, Kr, Xe, HF, HCl, and HBr

Abstract: Dipole oscillator strength distributions have been constructed, and used to evaluate integrated dipole oscillator strengths and a variety of dipole oscillator strength properties, for ground state ...

Calculation of the second-order electronic polarizabilities of some organic molecules. Part 1

Abstract: A version of the CNDO/S method has been re-parametrized by correlating computed and measured dipole moments and transition wavelengths for a range of organic conjugated molecules. The method has then been used to calculate the hyperpolarizability tensor, βijk, in the form related to second-harmonic generation, for an extended set of molecules of similar type. A good correlation between theory and experiment has been found. The convergence of the excited-state perturbation expansion used in the calculation of the tensor has been extensively investigated.

Zero field NMR and NQR

Abstract: Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-1/2 nuclei. Experimental results are reported or reviewed for 1H, 2D, 7Li, 13C, and 27Al nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.

Modeling of pressure-induced far-infrared absorption spectra Molecular hydrogen pairs

Abstract: Meyer et al. (1985) have calculated the accurate induced dipole moment function of H2-H2 from first principles, using highly correlated wave functions for the first time in such work. The present paper is concerned with the collision-induced translational-rotational absorption coefficient for molecular hydrogen pairs, taking into account computations on the basis of the fundamental theory considered by Meyer et al. Data have been obtained for temperatures in the range from 40 to 300 K. Criteria are developed for choosing among various model line shapes. It is found that certain models are capable of approximating the quantum profiles closely, with rms errors of only a few percent.

Interaction-induced contributions to Rayleigh and allowed Raman bands

Abstract: Molecular dynamics simulations of a model of CS2 have been analysed to calculate the shapes and intensities of the Rayleigh and allowed Raman bands of the liquid. The calculations are performed using the first order dipole induced dipole mechanism (DID). There is a clear time scale separation between the collision induced and reorientational components of the spectrum which becomes more marked at lower temperatures. The projection factors which determine the effective polarizabilities are large for CS2 at first order DID and second order DID terms calculated for the Raman polarizabilities are also substantial. An empirical renormalization scheme to incorporate higher order DID effects for the Raman intensities is described. A simple model which neglects orientational correlations and orientational translational correlations provides a semi quantitative prediction of the collision induced intensities.

Circular dichroism in photoelectron angular distributions from oriented linear molecules.

Abstract: We show that circular dichroism exists in the photoelectron angular distributions from oriented linear molecules in the electric dipole approximation. Contributions to the dichroism arise solely from interferences between degenerate photoelectron continua with m values differing by ±1. We identify specific photon-propagation, electron-detection configurations where circular dichroism will be observable. Finally, we illustrate the magnitude of this effect through ab initio calculations for photoionization out of the 4σ orbital of an oriented CO molecule.

Theoretical calculation of ozone vibrational infrared intensities

Abstract: An ab initio dipole moment function for ozone has been computed using the CASSCF (complete active space self-consistent field) method, and forms the basis for a calculation of ozone infrared band intensities. Vibrational wave functions were generated using the variational method with potential energy surfaces derived from experimental force constants. Computed values of the permanent dipole moment, dipole moment derivatives, and infrared band strengths are all found to be in remarkably good agreement with experiment. Intensities are predicted for hot bands for which experimental values are unavailable, and implications for atmospheric ozone spectroscopy are discussed. As the dipole moment matrix element signs are now established for nearly all of the observed bands, further refinement of the dipole moment function is possible.