bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of Zeff for low-energy positron scattering by the hydrogen molecular ion
Authors: Armour E. A. G. , Carr J. M.
Published: April 2011

The Kohn variational method has recently been applied to the calculation of the addition to the Coulomb phase shift, in positron scattering, by the hydrogen molecular ion below the positronium-formation threshold at 9.45 eV. In this paper the wave function obtained for the lowest spheroidal partial wave of [Formula: see text] symmetry is used to calculate the contribution to Zeff from this symmetry. The results are significantly larger than those obtained using the Coulomb–Born approximation.
Excitation of the 3D states of helium by electrons and positrons
Authors: Verma Surbhi , Srivastava Rajesh
Published: April 2011

Distorted-wave approximation calculations were carried out for the electron-impact excitation of 31,3D states from the 11S ground state of helium. Results are presented at an electron energy of 40 eV for the Stokes parameters P1-P4; coherence parameters Pl, γ, [Formula: see text], ρ00; and differential cross sections where most other theoretical and experimental data are available for comparison. The calculation is further extended to the positron-impact 11S–31D excitation in helium and comparison with the corresponding electron-impact results is discussed.
Excitation from metastable 23S states of helium by electrons and positrons
Authors: Verma Surbhi , Srivastava Rajesh
Published: April 2011

Our recent distorted-wave approximation calculation for the electron-impact excitation cross sections of the helium atom from its 23S state to upper states was extended to obtain alignment and orientation parameters for n3P (n = 2, 3, 4) excitations. Further, similar results are obtained by considering the excitation by impact of positrons. The calculated alignment and orientation results are compared with the only available theoretical electron-impact results obtained using first-order many-body theory.
Positron lifetimes in molecular crystals
Authors: Ishibashi Shoji , Tokumoto Madoka , Kinoshita Nobumori , Terada Norio , Ihara Hideo , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa , Anzai Hiroyuki
Published: April 2011

Positron lifetime measurements were carried out on molecular crystals of C60, TTF-TCNQ, and (BEDT-TTF)2Cu(NCS)2 utilizing a pulsed variable-energy positron beam. Each lifetime spectrum is well described by a single component. Their values are 390, 338, and 357 ps, respectively. In addition, positron lifetimes were calculated theoretically. The electronic density was obtained by the superposed-neutral-atom model or quantum-chemical molecular-orbital calculations with various basis sets. The lifetime enhancement due to the positron–electron correlation was taken into account through an enhancement factor as a function of the high-frequency dielectric constant in the local density approximation or a gradient correction in the generalized gradient approximation. The results are compared with the measured lifetimes.
Structure in differential transfer ionization: A Thomas resonance?
Authors: Falke T. , Raith W. , Weber M.
Published: April 2011

The differential cross section for Ar(e+, Ps e−)Ar++ was measured for the first time. The outgoing Ps atom was detected in time correlation with the Ar++ ion to identify this particular cross section. At an emission angle of 45°, a distinct dip in this transfer ionization, the formation of Ps combined with the ejection of an electron, was observed. The width of this structure is not wider than our effective angular acceptance of ±2.5°. No such feature is observed in the transfer reaction, usually referred to as Ps formation. Thomas scattering is suggested as a possible cause for this phenomenon.
Positron impact ionization of He, Ne, and Ar
Authors: Campeanu R. I. , McEachran R. P. , Stauffer A. D.
Published: April 2011

An ionization model containing a realistic description of the final state of the system, but employing only plane and Coulomb waves, is applied in positron ionization of helium, neon, and argon. This model was shown to agree perfectly with the earlier helium experiment of Fromme et al. for positron impact energies between 30–150 eV. Cross sections calculated with this model for impact energies up to 500 eV compare well with recent helium, neon, and argon measurements. Our work also shows that by improving the representation of the outgoing positron and electron one achieves a better agreement with the high-energy experimental cross sections.
The history, development, and future prospects for (e,2e) spectroscopy
Authors: McCarthy Ian E.
Published: April 2011

Electron momentum spectroscopy of atoms, molecules, and solids is based on (e,2e) reactions that observe the distribution of recoil momenta for energy-resolved states of the residual system. It is interpreted simply in terms of the momentum-space orbitals of the independent-particle model. The relevant ideas originated in nuclear physics. The earliest experiments observed that strongly excited final states belong to orbital manifolds that extend the independent-particle ideas to correlated systems. Some weakly excited final states do not belong to orbital manifolds. They give sensitive information about target ground-state correlations. The energy-momentum distribution of valence bands is observed for solids. Calculations for atoms, molecules, and crystals converge to the experimental result as the structure calculation is improved.
Studies of the single and double photoionization of two-electron systems at the Advanced Light Source
Authors: Lubell Michael S.
Published: April 2011

The Advanced Light Source (ALS) at Lawrence Berkeley Laboratory is a "third generation" synchrotron radiation machine that incorporates wigglers and undulators to enhance the spectral brightness of the emitted radiation. Commissioned in October 1993, its performance to date has been nothing short of spectacular. Its demonstrated characteristics make it an ideal tool for carrying out high resolution measurements of photoionization studies of two-electron atomic and ionic systems. ALS Beamline 9.0.1, a facility dedicated to the study of photoprocesses in atoms, molecules, and ions, utilizes an undulator with an 8 cm period and a monochromator with spherical gratings to produce light at energies between 20 and 300 eV. At a photon energy of, 50 eV, the beamline is designed to deliver about 1.5 × 1013 photons/s into a spot measuring approximately 50 × 800 μm with a resolving power of 10 000. Measurements of low-lying autoionizing resonances in He, carried out shortly after the ALS was commissioned, provided the first confirmation of the essential beamline characteristics. The 1.0 meV line width, more recently observed for the 2p3d double-excitation state of He at 64.12 eV, demonstrates that the actual resolving power of the beamline far exceeds the design value. The properties of the ALS, combined with improvements in the reliability and operating characteristics of ion sources, make it possible to extend high-resolution synchrotron experiments beyond the study of neutral systems. Investigations of low-Z ions are important, because they provide an easy way to vary the relative contribution of the electron–electron interaction. An international collaboration has recently been formed to perform such studies. Its list of priorities includes the doubly excited spectra and the threshold region for two-electron photoionization in Li+ and H−. Metastable He*, formed by resonant charge transfer from He+, will provide access totriplet states.
Recent experiments in atomic double photoionization
Authors: Samson James A. R. , He Z. X. , Moberg R. , Stolte W. C. , Cutler J. N.
Published: April 2011

Studies of double photoionization of atoms produced by single photon impact have greatly improved our knowledge of electron correlation processes. Such experimental studies have also revealed interesting phenomena to challenge theoreticians. This brief review will cover incident photon energies in the range from about 40 to 254 eV. Some of the areas that will be covered include autoionization of double excited neutral states into the double ionization continuum and threshold measurements ranging from the Wannier law to post-collision interactions.
Theory of double photoionization in "two-electron" systems
Authors: Sadeghpour H. R.
Published: April 2011

Theory of double photoionization of atoms and molecules with two active electrons is reviewed. The emphasis is on advances toward theoretical understanding of new developments in laboratory observations.
Electron momentum spectroscopy experiments and calculations for the production of excited states of He+ and
Authors: Lermer N. , Todd B. R. , Cann N. M. , Brion C. E. , Zheng Y. , Chakravorty S. , Davidson E. R.
Published: April 2011

The (e,2e) cross-section for transitions to the n = 2 final state of He+ and the 2sσg, final state of [Formula: see text] have been measured, relative to the cross-section for the transitions to the respective ground state ions, using a highly sensitive momentum dispersive multichannel electron momentum spectrometer. The experimental results for He are compared with plane wave impulse approximation (PWIA) cross-section calculations carried out using two previously published GI wavefunctions and also with two cross-section calculations based on explicitly correlated wavefunctions with energy errors of less than 10 nHartree. The H2 results are compared with calculations by J.W. Liu and V.H. Smith Jr. (Phys. Rev. A, 31, 3003 (1985); erratum: Phys. Rev. A, 39, 3703 (1989)). For both He and H2, significant differences are observed between the measured relative cross-sections and those calculated using the PWIA. While the measurements for He differ from previous work, the results for H2 are consistent with some earlier measurements.
Insights into theoretical quantum chemistry from electron momentum spectroscopy
Authors: Davidson Ernest R.
Published: April 2011

Most basis sets used in quantum chemistry are designed to get the correct charge and momentum density in the region important for covalent bonding. The (e,2e) cross section measured by electron momentum spectroscopy (EMS) emphasizes the low-momentum, large r, region of the wave function. Improving the description of this part of the wave function for water has resulted in good agreement with (e,2e) data. Because the hydrogen bond is sensitive to the long-range tail of the wave function, this has simultaneously led to an improved description of the hydrogen bond in the water dimer. The satellite region of the binding energy spectrum gives information about the excited states of the cation that is not available at present from any other form of spectroscopy. Calculations seeking agreement with the binding-energy spectra and the momentum distribution associated with satellite peaks have led to the most complete catalog of the cation excited states for ethylene. Here we report the assignment of the excited states based on the dominant part of the wave function rather than focusing on the small coefficients that describe the intensity borrowing from the primary holes. We also examine the adequacy of the assumption that every Dyson orbital is similar to one of the Hartree–Fock orbitals.
Green's function calculations for doubly ionized molecular states and simulation of Auger spectra
Authors: Tarantelli F. , Sgamellotti A. , Cederbaum L. S.
Published: April 2011

The two-particle Green's function algebraic diagrammatic construction method for computing molecular double ionization spectra is described. In this context, key aspects of molecular Auger spectroscopy, as hole-localization effects, nuclear dynamics effects, and transition rates are discussed. Illustrative applications are reviewed.
Inner shell (e, 2e) processes
Authors: Whelan Colm T. , Rasch J. , Walters H. R. J. , Keller S. , Ast H. , Dreizler R. M.
Published: April 2011

The great advantage of coincidence measurements is that by suitable choice of the kinematical and geometrical arrangement one may probe delicate physical effects that would be swamped in less differential experiments. The calculation of the triple-differential cross section for the inner shell ionization of high-Z elements at relativistic energies presents a serious challenge to theory. Relativistic effects enter in both the kinematics and the nature of the target, and the large atom exerts a strong distorting influence. We consider such processes in a range of geometries and kinematics. We report the results of fully relativistic distorted wave calculations. Results are presented for silver, gold, and uranium targets and suggestions for further experimental arrangements proposed where delicate relativistic effects should be readily detectable.
Ionization of heavy inert gases by spin-polarized electrons
Authors: Madison D. H. , Kravtsov V. D. , Jones S. , McEachran R. P.
Published: April 2011

In the collision of a spin-polarized electron with an atom, it is only natural to assume that any observed difference between spin-up and spin-down electrons must originate from spin-dependent forces in the interaction. However, it has been known for sometime that, for inelastic electron-atom scattering, a non-zero spin asymmetry can result from the coulomb interaction ignoring spin-dependent forces on the projectile. In this paper, it is demonstrated that the same type of effect may be expected for ionization of the heavier inert gases. Theoretical results are compared with recent unpublished experimental measurements.
Theoretical developments in (e,2e) studies of excited states and in (e,3e) spectroscopy
Authors: Popov Yu. V. , Cappello C. Dal
Published: April 2011

The theory of single and double ionization of atoms deals with one of the most difficult problems in quantum mechanics: the scattering of a few charged particles. A large number of different (e,2e) experiments and theoretical calculations have helped us to understand the main physical mechanisms and their effect on the shape of triple differential cross section (TDCS). Recently the first deeply asymmetric (e,2e) experiments, leaving the residual ion in an excited state (which we indicate in this paper by (e,2e)*), and (e,3e) experiments have been performed. These offer new challenges to the theory. A very preliminary survey of main theoretical methods currently used to explain the experimental measurements is presented here. It will be shown that small differences in the choice of initial and final state models employed by different authors lead to large effects in both the shape and absolute size of the TDCS in the case of excitation ionization, even if these models give almost identical results for the (e,2e) case. A few physical mechanisms contributing to the (e,2e)* process are discussed in this paper. Special attention is given to the multichannel close-coupling method. (e,3e) experiments allow us to study the final state wave function with two continuum electrons. We obtain two unexpected results. First, we found that the two-step mechanism contribution is comparable and even bigger than that of shake-off. Second, the algorithms exploiting the angular decompositions of many-body continuum wave functions do not work in the case of long-range potentials; this is a result of the failure of the widely used diagonalization approximations in this case. The physical considerations that support these and other results are presented in this paper.
Single, double, and triple photoionization of molecules using synchrotron radiation
Authors: Masuoka Toshio
Published: April 2011

The recent advances concerning production and dissociation dynamics of multiply-charged molecules using synchrotron radiation are discussed with emphasis on dissociative double photoionization in the valence region. In the determination of the ratio of double to single photoionization, both molecular and dissociative processes are properly included. Kinetic-energy release and anisotropic fragmentation in dissociative double photoionization of carbon monoxide are also discussed.
A polarization trip from Québec to Vancouver
Authors: Kessler Joachim
Published: April 2011

Exactly 30 years ago the ICPEAC was held for the first time in Canada. At this conference in Québec first results on spin-polarization phenomena in low-energy electron scattering were presented. In the following years spin-polarization effects played an increasing role at the ICPEAC and its satellite conferences. This contribution gives a perspective of this development in the past 30 years. It outlines fundamental directions of development, shows how they interact, and gives an account of their usefulness for a better understanding of collision physics. In the first part, processes are discussed where the polarization effects are caused by either the spin–orbit interaction or the exchange interaction alone. Experimental results on such processes can be used to study the role of these spin-dependent interactions in electron-atom collisions. In recent years more complicated processes have been studied where the two spin-dependent effects can no longer be disentangled. Examples of such processes are elastic scattering from atoms with unsaturated spins, inelastic scattering, and spin-selected coincidence experiments. Major progress in spin-dependent scattering from chiral molecules will also be reported. The enormous progress of this field in the past 30 years could only be made by a number of novel developments in the production and detection of electron polarization.
Interference effects between electron exchange and the spin–orbit interaction
Authors: Raith W. , Baum G. , Tondera M.
Published: April 2011

In accordance with predictions made in 1974, recent measurements demonstrated that the elastic e–Cs scattering is richer in physics than a simple addition of Mott scattering and nonrelativistic exchange would provide. Now it is time to take a closer look at the general analysis of Burke and Mitchell and think about further experiments that will stimulate improvements in calculations and lead to a deeper understanding of the underlying physics.
Calculation of orientation and alignment parameters in e-He excitation
Authors: Fursa Dmitry V.
Published: April 2011

Convergent close-coupling (CCC) calculations are used to obtain electron–photon coincidence parameters for 21P,31,3P, and 31,3D states of helium in the intermediate energy region. Results of the CCC calculations are compared with experiment and other calculations. Consistent agreement of the CCC calculations with experiment has been achieved for these states at all scattering angles.
Chiral effects in the scattering of electrons by molecules
Authors: Thompson D. G.
Published: April 2011

There are a number of scattering phenomena that depend on the "handedness" of the scattering system. In this paper we concentrate on the elastic scattering of polarized electrons by closed-shell molecules and consider mainly the property of electron optic dichroism. The effects can be fully analysed using the general invariance properties of the scattering matrix under spatial inversion, time reversal, and rotations. However, there is still considerable discussion about the physical mechanism causing the effects. Among the models proposed are two involving the spin–orbit interaction; in one the bound orbitals are perturbed, and in the other the incident electron. In this paper we show how these two approaches can be combined in a unified treatment. We set up the scattering equation including exchange and spin–orbit in a consistent manner and identify which terms contribute to the chiral effects.
Complete experiments in electron–atom impact excitation: present status and future prospects
Authors: Andersen Nils , Bartschat Klaus
Published: April 2011

An overview is given of the present understanding of excitation in electron–atom collisions, with particular emphasis on the extent to which a "perfect scattering experiment" in the Bederson sense has been achieved. Recent experimental and theoretical results are put into a common framework, generalizing the ideas and systematics, presented in a review paper, of the case of excitation by spin-polarized electron beams. For various levels of complexity, complete sets of coherence parameters are suggested, and their relationships to generalized Stokes parameters and generalized STU parameters are pointed out.
Theoretical study of slow He+–Na(3s, 3p) collisions
Authors: Fritsch Wolfgang
Published: April 2011

Electron transfer processes in He+–Na(3s, 3p) collisions are studied with the semiclassical close-coupling method. The electron dynamics is described with a number of antisymmetrized two-electron configurations of atomic-orbital type. Contrary to assumptions in earlier work, capture to the 31,3L He states is found to be important in the keV energy region. The calculated transfer cross sections agree well with recent data on Na(3p)–Na(3s) cross-section ratios and on the Na(3p0)–Na(3p1) anisotropy parameter.
Alignment effects in electron transfer: Experimental studies for singly charged ion collisions with Na(3p) atoms
Authors: Thomsen Jan W.
Published: April 2011

This paper reports experimental results for the influence of target excitation and orbital alignment on the electron-transfer process in ion–Na(3s, 3p) collisions. Systematic experimental investigations with a range of simple ions reveal a similar behavior of the electron-transfer cross-section parameters as a function velocity. The experimental findings are compared with atomic and molecular orbital calculations.
Decoupling of measured spherical tensor components of resolved J states of collisionally excited (Ar)+
Authors: Yenen O. , Moudry B. W. , Jaecks D. H.
Published: April 2011

We measured Stoke's parameters P1, P2, P3, and P4 of excited Ar+ in coincidence with the scattered He for the two-electron process[Formula: see text]From the measured Stoke's parameters of the 4610 Å (1 Å = 10−10 m) radiation of the 2F(J = 7/2) to the 2D(J = 5/2) transition and the 4590 Å radiation of the 2F(J = 5/2) to 2D(J = 3/2) transition we determined the alignment and orientation parameters of the upper states using the Fano–Macek formalism. These parameters were then converted to the appropriate spherical tensor components in the J representation to obtain, in general eight values of [Formula: see text]. Since these states are best described by LS coupling, we decoupled the spherical tensor components of the density matrix of each J state in terms of the spherical components of the density matrices of the total orbital angular momentum L and total spin 5. The unknowns in the expansion are [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], with [Formula: see text] because of reflection symmetry. We used Mathematica to calculate the decoupling coefficients and solve these nonlinear equations. The decoupling also allows us to optically determine the octopole moments of the total orbital momentum provided that [Formula: see text] is nonzero. When [Formula: see text], the two sets of equations are linearly dependent. In this case, the determination of a single set of [Formula: see text] specifies [Formula: see text].
Alignment of H(2p) in H+–H collisions
Authors: Hippler R. , Siegmann B. , Lin C. D. , Toshima N. , Wang J. B.
Published: April 2011

The integral alignment of H(2p) in H+–H collisions is re-examined. The previously observed differences between experiment and theoretical calculations cannot be attributed to cascade effects from the H(n = 3) states.
Recent developments in the theory of angular distribution and spin polarization of Auger electrons
Authors: Lohmann B.
Published: April 2011

The general theory of angular distribution and spin polarization of Auger electrons is reviewed. The angular distribution of open-shell atoms is investigated in more detail. Whereas the KLL Auger lines have to be isotropic for closed-shell atoms, this is no longer valid for open-shell systems. The angular distribution of the KLL spectra of sodium and laser-excited sodium is discussed. Large anisotropy parameters are predicted for the unresolved initial- and final-state multiplets of the laser-excited sodium KLL spectrum. An outlook is given for the angular distribution of Auger electrons emitted from diatomic molecules. The case of electron-impact ionization is discussed. The physical importance of a coherent excitation process is emphasized within a simple example. For the case of the rare gases a simpler experiment is discussed to measure the in-reaction plane components of the spin-polarization vector. Within the LS-coupling approximation theoretical predictions for the spin-polarization vector for Auger transitions on argon are given. A degree of spin polarization of up to 50% is predicted.
Fragmentation of planetary three–body Coulomb states
Authors: Seng M. , Halka M. , Heber K.-D. , Sandner W.
Published: April 2011

Using a novel combination of several experimental techniques for state selective ion and electron spectroscopy, we investigated the decay dynamics of "n1l1n2l2" (4 < n1 < n2 < 80, l1 ≥ 3, l2 > 5) planetary states in strontium after five-laser excitation. As a result, at least two dominant and competing fragmentation mechanisms (autoionization and fluorescence–autoionization cascades) could be identified. Quantitative investigation of these mechanisms and their decomposition into different channels yield a fairly complete picture of the fragmentation dynamics in highly excited three–body Coulomb states.
Superelastic electron scattering from calcium and lithium
Authors: Teubner P. J. O. , Karaganov V. , Law M. R. , Farrell P. M.
Published: April 2011

Superelastic scattering experiment were performed on optically pumped calcium atoms at energies of 25.7 and 45 eV referred to the ground state. Orientation and alignment parameters derived from these experiments are compared with the predictions of several theories based on a distorted-wave formalism. The agreement between theory and experiment is unsatisfactory at the lower energy at all scattering angles. At the higher energy agreement improves at small scattering angles but is poor at middle angles. The results of our quantum electrodynamical calculation on optical pumping in lithium are compared with our observations. We find such good agreement between theory and experiment that we explore the possibility of superelastic scattering experiments on lithium atoms that are optically pumped with single-frequency laser light. A two-frequency pumping system is described and its use in the observation of superelastic scattering from lithium is discussed. Orientation and alignment parameters are presented at an equivalent energy of 21.8 eV for small angles. They are compared with those predicted by two close-coupling calculations. Excellent agreement is found between the present work and the convergent close-coupling theory of Bray.
Experiments with low-energy electrons scattered by polarized, excited sodium
Authors: Vušković L.
Published: April 2011

Several crossed-beam experiments were performed to acquire information concerning collisions of low-impact-energy electrons with ground-state or laser-excited sodium atoms prepared in 32P3/2, F = 3 (MF = +3 or −3) polarized states. Results of these experiments are the azimuthal asymmetry of the differential cross sections between two polarized states in the elastic electron collision (3P → 3P) at selected scattering angles, absolute differential cross sections for elastic, inelastic, and superelastic scattering obtained with no normalization procedure involved, and total ionization cross sections in the threshold energy range.
Gravitational scattering of massive particles on a background with axial symmetry
Authors: Radu D , Tatomir D
Published: April 2011

Using the S-matrix formalism and the Feynman diagram technique, the gravitational scattering of the minimally and non-minimally coupled scalar, spinor, vector, spin-vector, and spin-2 massive particles, in a background described by KerrNewman geometry is studied for any value of the scattering angle. We find that the differential cross sections of the scalar, spinor, and vector particles in the backward direction and ultrarelativistic case are finite and consequently the backscattered particles must have the opposite helicity, whereas for the spin-vector and spin-2 particles in the same case, the differential cross sections are clearly infinite. It has been shown, for the particular case when the angular momentum of the scatterer vanishes (i.e., for the Schwarzschild geometry) and in the small-angle approximation and ultrarelativistic limit as well, the differential cross sections are all of the same type, i.e., in this special limit case the gravitational particle scattering is spin independent. PACS Nos. 03.80+r, 11.80-m, 03.70+k
Le développement d'une nouvelle procédure d'autoprotection dans le cadre de la méthode des taux de réaction effectifs
Authors: Saygin H
Published: April 2011

The major problem of the self-shielding methods based on the equivalence principle is the difficulty of calculating the dilution cross section σe with adequate precision. We have proposed a new self-shielding procedure that contains a new technique to calculate dilution cross section. We have compared our method to the generalized Stamm'ler method. Accuracy of each computing approach is determined using reference results obtained from a micro-group slowing-down code named CESCOL.
Scattering from a spherical shell with a circular aperature using neural network approach
Authors: Hamid A -K
Published: April 2011

A neural network is developed for the problem of scattering from a spherical shell with a circular aperture. The neural network is used to predict the backscattering cross section without the need to solve the vector field equations. The neural network is trained to model the nonlinear relation between the electrical radius of the sphere, half aperture angle, and the backscattering cross section using experimental data. This method is proven to be faster than standard methods and consumes less CPU time. Numerical results are presented graphically for backscattering cross section with electrical radii and half aperture angles different from those are used for training.

PACS Nos. 41.10H, 41.90
Article
Authors: Ononiwu J S
Published: April 2011

A phenomenological one-parameter model potential that includes sp-d hybridization and core--core exchange contributions is used to calculate the phonon dispersion curves in the lattice dynamics of the body-centered cubic alkaline-earth metal, barium. There is good overall agreement between theory and experiment, and in particular, in the [xi00] direction we obtained frequencies of the transverse dispersion curves that are lower than those of the longitudinal dispersion curves along the [100] symmetry direction thereby restoring the normal ordering of the branches.PACS Nos.: 63.20D
Article
Authors: De S S , Ghosh A K , Bera M
Published: April 2011

Some physical characteristics of photoluminescence spectra in GaAs--(Ga,Al)As quantum-wells under steady optical excitation conditions are presented. They are based on the dependence of photoluminescence on laser intensity. The variations of carrier density with laser intensity and electron--hole recombination decay time are compared with earlier experimental results. PACS Nos. 73.20 Dx, 73.20 Mf
Article
Authors: Burrows B L , Cohen M , Feldmann T
Published: April 2011

Eigenfunctions of a generic double-well potential are calculated and found to be of two distinct kinds. The transition from delocalized to localized form, driven by time-independent and time-dependent perturbations, is examined in detail. It is shown that a polar diagram provides a useful description of the localization effects. PACS Nos. 3100, 3110, and 3115
The ponderomotive four-momentum
Authors: Gao J , Bagayoko D , Guo D -S
Published: April 2011

Using classical mechanics and classical field theory, we confirm that the ponderomotive momentum exists as a counterpart of the ponderomotive energy to form the ponderomotive four-momentum when an electron interacts with a light plane wave. As a solution to the equations of motion, a time-dependent four-momentum on the electron (rest) mass shell is obtained in a coordinate-free form. By taking out the quiver motion, we find that the nonoscillating part of the four-momentum is not on the electron mass shell. We show mathematically that the decomposition of the nonoscillating four-momentum into an on-mass-shell four-momentum and a lightlike four-momentum along the light-propagation direction is unique. The time-component of the lightlike four-momentum is exactly the ponderomotive energy, while the space-component is the ponderomotive momentum. We discuss the critical role of the ponderomotive four-momentum in lightelectron scattering processes due to the lightlike property and the uniqueness of the decomposition of a four-momentum. Two pieces of experimental evidence of the ponderomotive momentum are identified in the discussion.

PACS No. 32.80Rm
Infrared behavior of Yennie gauge QED at two-loop order
Authors: Adkins Gregory S , Zhang Yu
Published: April 2011

We show, by explicit calculation, that the on-shell renormalization constants of Yennie gauge QED are infrared finite to two-loop order.PACS Nos.: 12.20.Ds and 11.10.Gh
Article
Authors: Ayuela A , Lopez J M , Alonso J A , na V Lua
Published: April 2011

(NaCl)nNa+ clusters whit n up to 15 have been studied using the ab initio Perturbed-Ion model, which is based on the Hartree--Fock method and the Theory of Electronic Separability. The energetics and structural properties are discussed in detail in the size range n <=10. Even for those small sizes, a certain tendency to form fragments of the rock-salt lattice is evident, although the excess repulsion of the cations induces distortions. We have also studied the addition and the removal of one or two NaCl molecules to (or from) (NaCl)13Na+.PACS Nos. 31.20Gm and 36.40+d
Article
Authors: Galstyan T V , Drnoyan V
Published: April 2011

Light-induced molecular orientation switching and large intrinsichysteresis in a homeotropic cell of nematic liquid crystals areobtained. Both the switching threshold and the hysteresis width areeffectively controlled by the elliptical symmetry of a noncoherentoptical field. We confirmed the predicted doubling of the switchingthreshold in the case of the light with circular symmetry and vanishingangular momentum.PACS No. 78.47
Single quantum pair annihilation of a bound particleparticleantiparticle system
Authors: Darewych J W , Biswas P
Published: April 2011

A relativistic coupled channel formalism is presented for the description of a quasi-bound three-body state.The annihilation of a particleparticleantiparticle system into a particle and a single quantum is studied in the scalar Yukawa model, in which scalar "nucleons" (ϕ) and "antinucleons" ([Formula: see text]) interact via scalar "meson" χ exchange. A three-Fock-state trial state is used to derive variational equations that describe the quasi-bound [Formula: see text] system as a resonance in the χ ϕ collision channel. These equations are solved approximately to yield the [Formula: see text] decay width. This width is found to be negligible compared with the two-quantum pair annihilation width if the coupling is weak (α << 1), but is found to be of comparable magnitude for strong coupling (α [Formula: see text] 1). Relativistic effects on the width are pronounced and cannot be neglected at strong coupling. Order of magnitude comparison is made with the Ps¯ system for the massless-χ case, and to nuclear [Formula: see text] systems for the massive-χcase.

PACS Nos. 11.10Qr and 11.10st
Review/Synthèse Synchrotron radiation in atomic physics
Authors: Crasemann B
Published: April 2011

Much of present understanding of atomic and molecular structure and dynamicswas gained through studies of photon--atom interactions. In particular,observations of the emission, absorption, and scattering of X rays havecomplemented particle-collision experiments in elucidating the physics ofatomic inner shells. Grounded on Max von Laue's theoretical insight andthe invention of the Bragg spectrometer, the field's potential underwent astep function with the development of synchrotron-radiation sources. Notablycurrent third-generation sources have opened new horizons in atomicand molecular physics by producing radiation of wide tunability andexceedingly high intensity and polarization, narrow energy bandwidth, andsharp time structure. In this review, recent advances insynchrotron-radiation studies in atomic and molecular science are outlined.Some tempting opportunities are surveyed that arise for future studiesof atomic processes, including many-body effects, aspects offundamental photon--atominteractions, and relativistic and quantum-electrodynamic phenomena.PACS Nos.: 32.20J, 32.20R, and 07.65E
Article
Authors: Denny M
Published: April 2011

We derive equations of motion describing curling rock dynamics. These are solved analytically for centre of mass motion, and for angular motion. The resulting relationship between linear and angular speeds agrees with observation. Physical mechanisms are discussed that give rise to asymmetry of the coefficient of friction. It is found that a small left--right asymmetry, described by a single parameter, is all that is required to explain how a curling rock curls. Conclusions are presented and compared with the results of previous work. A discussion of further work completes the paper.PACS No. 46.10
Formal theory of multiphoton double ionization: Sequential and direct
Authors: Altzman Jerry B , Mittleman Marvin H
Published: April 2011

The rate parameters for double multiphoton ionization are obtained as functions of T matrices, which are derived from the S matrix by a generalization of the Fermi Golden Rule. The different T's appear, and are distinguished from each other as coefficients of different energy singularities, principal parts as well as delta functions, in S. These are connected to the rate parameters, which appear in threecoupled-rate equations. New terms are shown to contribute to the direct ionization process and new interference terms are also found to contribute to the direct ionization rate. A new interpretation of the shake-off term is given, and it is found to be a sequential process.

{PACS Nos.: 32.80F, 32.80K}
Article
Authors: Ahmed MES , Attia H A
Published: April 2011

The steady laminar flow and heat transfer of an incompressible, electrically conducting, non-Newtonian fluid in an eccentric annulus are studied in the presence of an external uniform magnetic field. The inner cylinder is subject to a constant heat flux while the outer cylinder is adiabatic and the viscous and Joule dissipations are taken into consideration. A numerical solution for the governing partial differential equations is developed and the influence of the magnetic field on both the velocity and temperature distributions are discussed.PACS Nos.: 47.65, 47.50, 47.15C
CallanSymanzik function in QCD
Authors: Nigam B P
Published: April 2011

The perturbative expansion of the CallanSymanzik function β(g) in QCD is extended to orders g7 and g9 using the scheme of the integral equation defining the "running coupling" g(t). PACS Nos.: 11.10Hi, 12.38Bx
Article
Authors: Liu F H
Published: April 2011

The conditional moments of the charge distributions of the projectile fragments in sulphur fragmentation at 200 GeV/N are obtained using the simple Monte Carlo partition method. Thus, the logarithmic correlations between the conditional moments can be observed. The distributions of conditional moments are investigated. The results of the nuclear diffractive excitation, electromagnetic dissociation, and Monte Carlo methods are compared.PACS Nos.: 25.70Np and 25.70-z
Isochronous anharmonic oscillations
Authors: Mohazzabi Pirooz
Published: April 2011

The problem of a particle oscillating without friction on a curve in a vertical plane (referred to as a vertical curve) is addressed. It is shown that there are infinitely many asymmetric concave vertical curves on which oscillations of a particle remain isochronous. The general equation of these curves is derived, and a one-to-one correspondence between these curves and one-dimensional potentials is established. The results are compared with the existing literature, and an interesting nontrivial special case is discussed. Some issues regarding interpretation of the results in the context of action and angle variables are also addressed. PACS No. 03.20
Article
Authors: Gagné M C , Boulanger J S , Douglas R J
Published: April 2011

The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 1016 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium-fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 muK using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.PACS Nos.: 06.20, 06.30, and 32.80P
Article
Authors: Kent R D , Schlesinger M , Wybourne B G
Published: April 2011

The role of Lie groups and algebras in symmetry-based models of the genetic code is considered. The two schemes, based upon the symplectic group Sp(6) and the exceptional group G(2) are shown to correspond to different embeddings in the group SO(14). Some possible alternative schemes are sketched. Problems with considering codons being represented as fermionic or bosonic are noted. A complete listing is given of all 64-dimensional irrecducible representations that can arise in the symmetric and alternating groups.

PACS Nos.: 87.10 and 02.20
Article
Authors: Burgess M , Carrington M , Kunstatter G
Published: April 2011

The initial value problem of wave motion is considered for two immiscible layersof incompressible fluids in the presence of a porous wave maker immersed vertically in thetwo uids, which are periodic in the horizontal direction along the wave maker as well as intime. The resulting motion is investigated using the method of generalized functions, and anasymptotic analysis for large values of time and distance is given for the depression of thefree surface and the surface of separation between the two fluids. The generated fluid patternis discussed for different values of the wave number of the oscillations of the wave makervelocity in the horizontal direction.

PACS No.: 47.35
Article
Authors: Toyama F M , Nogami Y
Published: April 2011

This is a sequel to two earlier papers in which the behaviour of the nonlinear Dirac soliton in an external potential in 1+1 dimensions is examined. Regarding the Lorentz transformation properties there are three types of potentials: vector (V), scalar (S), and pseudoscalar (PS). In this paper we consider the PS potential. Unlike the V and S potentials that we considered earlier, the PS potential has no obvious counterpart in classical mechanics. Still the soliton behaves like a point particle in relativistic classical mechanics with a certain Hamiltonian. Close scrutiny, however, reveals that the soliton undergoes Zitterbewegung.

PACS Nos.: 02.30, 03.40Kf, 03.65-w
Article
Authors: Polat B
Published: April 2011

The diffraction of acoustic plane waves by a semi-infinite parallel platewaveguide satisfying Dirichlet (soft) and Neumann (hard) boundary conditionson its exterior and interior faces, respectively,is investigated rigorously by using the Fouriertransform technique in conjunction with the mode-matching method.This mixed method offormulation gives rise to ascalar Wiener-Hopf equation of the second kind, the solutionof which contains infinitely many constants satisfying an infinite system of linearalgebraic equations.A numerical solution of this system is obtained forvarious values of the thickness of the parallel plate waveguideand the incidence angle, through which the effect of these parameters on the diffraction phenomenon is studied.PACS No.: 43.20
Article
Authors: Wang Lixen , Jones W E
Published: April 2011

In-situ real time CARS (coherent anti-Stokes Raman spectroscopy) spectra of the nu1 mode of SF6 were recorded in a discharge tube with varying delay times between the discharge pulse and the CARS detection pulse. Theoretical CARS spectra were calculated for the nu1 mode of SF6 at various vibrational temperatures. Based on relationships between line width, frequency shift, and temperature, a best fit method was developed to extract the temperatures from experimental line shapes instead of peak intensities. The kinetic behaviour of the vibrational temperature of SF6 during the discharge pulse was thus obtained. This method is useful for examination of chemical and physical processes in which the total population in a specific vibrational mode changes.

PACS No.:33.20f
Article
Authors: Witwit M RM , Gordon N A
Published: April 2011

A determination of the eigenvalues for a three-dimensional system is made by expanding the potential function V(x,y,z;Z2, λ,β)= Z2[x2+y2+z2]+λ {x4+y4+z4+2β[x2y2+x2z2+y2z2]}, around its minimum. In this paper the results of extensive numerical calculations using this expansion and the Hill-determinant approach are reported for a large class of potential functions and for various values of the perturbation parameters Z2, λ, and β. PACS No.:03.65
Article
Authors: Noël J- MA , Patitsas A J
Published: April 2011

The properties of the modes of vibration in an ideal fluid bounded by rigid infinitely long eccentric cylindrical surfaces are studied using the translationaddition theorem for the ordinary Bessel functions. The lack of sensitivity of the circumferential modes, both as to the value of their eigenfrequencies and their standing wave patterns, to the amount of eccentricity constitutes the focus of this study. The degree of sensitivity is measured relative to that of the thickness modes. The effect of eccentricity on the degree of excitation of these modes is also examined when the fluid is excited by a δ-function harmonic infinite line source. PACS. No.:46.30M
Article
Authors: Eldabe Nabil T , Sallam Sallam N
Published: April 2011

The problem of two-dimensional magnetohydrodynamic flow and heat transferthrough a non-Newtonian viscoelastic incompressible porous fluid obeying the rheologicalequations of state due to Walters is considered. The fluid is confined between two parallelplates, the lower is a stretching plate and the upper is a stationary porous plate, in thepresence of a uniform magnetic field. The approximate solutions of the governing nonlinearequations are obtained using the perturbation technique. The solutions for the flow andheat transfer characteristics are evaluated numerically for different parameters, such asthe magnetic parameter M* , the elastic parameter K, the injection parameter λ, andthepermeability parameter ε. The analysis presented in this paper is more general than anyprevious investigation.PACS No: 47.65+a