Fragmentation of positronium

The impulse approximation is used to calculate cross sections for fragmentation of Ps(1s) in collision with He, Ne, Ar, Kr, and Xe. Triple, double, single, and total cross sections are evaluated. Reasonably good agreement is found with the measurements of Armitage [Phys. Rev. Lett. 89, 173402 (2002)] on Ps(1s)+He(1(1)S) scattering. These absolute measurements comprise the total Ps ionization cross section and the cross section differential with respect to the longitudinal energy of the ejected positron. Characteristics of free electron and free positron scattering are explored in the double and triple differential cross sections for Ps(1s)+Xe scattering.

An investigation into the use of atomic datasets in simulations of the Ni-like gadolinium x-ray laser

The effect of differing the datasets used in the modelling of the Ni-like Gd x-ray laser (XRL) is examined through the 1.50 hydro-atomic code, EHYBRID. Two atomic datasets, including energy levels and radiative and collisional excitation rates, are used as input data for the code. It is found that the behaviour of the XRL is somewhat different than might be expected from superficial examination of the atomic data. The similarities in the gain profiles at low densities are found to have encouraging implications. in our attempts to model XRLs.

Strong parallel magnetic field effects on the hydrogen molecular ion

Equilibrium distances, binding energies and dissociation energies for the ground and low-lying states of the hydrogen molecular ion in a strong magnetic field parallel to the internuclear axis are calculated and refined, by using the two- dimensional pseudospectral method. High-precision results are presented for the binding energies over a wider field regime than already given in the literature (Kravchenko and Liberman 1997 Phys. Rev. A 55 2701). The present work removes a long- standing discrepancy for the R-eq value in the 1sigma(u) state at a field strength of 1.0 x 10(6) T. The dissociation energies of the antibonding 1pi(g) state induced by magnetic fields are determined accurately. We have also observed that the antibonding 1pi(g) potential energy curve develops a minimum if the field is sufficiently strong. Some unreliable results in the literature are pointed out and discussed. A way to efficiently treat vibrational processes and coupling between the nuclear and the electronic motions in magnetic fields is also suggested within a three-dimensional pseudospectral scheme.

Quantum manifestations of scattering orbits in the scaled spectrum of a non-hydrogenic atom in crossed fields

Evidence for scattering closed orbits for the Rydberg electron of the singly excited helium atom in crossed electric and magnetic fields at constant scaled energy and constant scaled electric field strength has been found through a quantum calculation of the photo-excitation spectrum. A particular 3D scattering orbit in a mixed regular and chaotic region has been investigated and the hydrogenic 3D closed orbits composing it identified. To the best of our knowledge, this letter reports the first quantum calculation of the scaled spectrum of a non- hydrogenic atom in crossed fields.

Single- and multiphoton detachment of K

Single- and multiphoton detachment rates have been calculated for K- using the R-matrix Floquet approach. Single-photon detachment rates, obtained at a laser field peak intensity of 10(9) W cm(-2), are discussed and compared with other theoretical work. Two-photon detachment rates at the same intensity have also been obtained, and similarities with results from earlier calculations for Li- and Na- are discussed. Three-photon rates are also presented at this laser intensity, and are compared and contrasted with those arising in the single-photon case, since both involve resonance structure with P-1(o) symmetry. The influence of resonances such as the 5s(2) S-1(e) doubly excited state and excitations of the residual atom are also considered.

Multi-state CDW approximation to electron capture during slow p-H collisions

The first complete multi-state CDW close coupling calculations which use a fully normalized basis set are performed. The results obtained at impact energies in the region of 10 keV for total and n = 2 capture cross sections are in reasonably good accord with experiment despite the fact that only the ground states of both species and the n = 2 states of the projectile are incorporated into the model. The theory has significant advantages over other atomic and molecular expansions which may require extensive bases to obtain similar accuracy.

Semiclassical analytical approach to the description of quasimolecular optical transitions

A formula was obtained that describes asymptotically forbidden quasimolecular optical transitions in the frame of the semiclassical approach. It is particularly relevant for the weak extrema in the difference between the ground- and excited- state interaction potentials. When averaged over impact parameters and velocity distribution the formula agreed reasonably well with the recent experimental data for the Ca(4(1)S --> 3(1)D) + He transition.

Angular distributions of electrons in photoionization of 3s subshell of atomic chlorine

R-matrix calculated photoelectron angular distribution asymmetry parameters, beta for Cl+ 3s3p(5) P-3(o) and 3s(2)3p(3) (D-2(o))3d P-1(o) final ionic states in photoionization of the ground state of atomic Cl are presented in the photon energy range from threshold to 80 eV. The results, characterized by prominent autoionization structures which are sensitive to multielectron correlations, are compared with those recently measured by Whitfield et al (Whitfield S B, Kehoe K, Krause M 0 and Caldwell C D 2000 Phys. Rev. Lett. 84 4818). Contrary to experiment and previous theoretical calculations, our detailed CIV3 structure calculation (Deb N C, Crothers D S F, Felfli Z and Msezane A Z 2002 J. Phys. B: At. Mol. Opt. Phys. submitted) has identified the lowest P-1(o) level of Cl+ as 3S(2)3p(3)(D-2(o))3d P-1(o) rather than 3s3p(5) P-1(o). The implications and consequences of the measured data for the 3s P-1(o) level are also discussed in the context of our calculated energies for Cl+ and beta for 3d P-1(o).

Analytical calculation of far infrared spectra of ice in terms of a molecular model

Wideband far infrared (FIR) spectra of complex permittivity e(p) of ice are calculated in terms of a simple analytical theory based on the method of dipolar autocorrelation functions. The molecular model represents a revision of the model recently presented for liquid water in Adv. Chem. Phys. 127 (2003) 65. A composite two-fractional model is proposed. The model is characterised by three phenomenological potential wells corresponding to the three FIR bands observed in ice. The first fraction comprises dipoles reorienting in a rather narrow and deep hat-like well; these dipoles generate the librational band centred at the frequency approximate to 880 cm(-1). The second fraction comprises elastically interacting particles; they generate two nearby bands placed around frequency 200 cm(-1). For description of one of these bands the harmonic oscillator (HO) model is used, in which translational oscillations of two charged molecules along the H-bond are considered. The other band is produced by the H-bond stretch, which governs hindered rotation of a rigid dipole. Such a motion and its dielectric response are described in terms of a new cut parabolic (CP) model applicable for any vibration amplitude. The composite hat-HO-CP model results in a smooth epsilon(nu) ice spectrum, which does not resemble the noise-like spectra of ice met in the known literature. The proposed theory satisfactorily agrees with the experimental ice spectrum measured at - 7 degrees C. The calculated longitudinal optic-transverse optic (LO-TO) splitting occurring at approximate to 250 cm(-1) qualitatively agrees with the measured data. (c) 2004 Elsevier B.V. All rights reserved.

Energy dependence for pair production with electron capture in relativistic heavy-ion collisions

In 'Charge transfer from the negative-energy continuum: alternative mechanism for pair production in relativistic atomic collisions', Eichler (1995 Phys. Rev. Lett. 75 3653) proposes an alternative mechanism for capture by pair production, and from it derives an analytic expression for the total cross section with a surprisingly strong energy dependence. We show that, in fact, there is no alternative mechanism; rather the above mechanism may be more transparently viewed as an ionization-like transition in one centre with inclusion of continuum distortion by the second centre. We further show that to Centre the initial and final states on the target and projectile leads to confusion in the momentum transfer vectors, and hence, respectively that the alleged high-energy behaviour is erroneous.

Equilibrium polymerization of cyclic carbonate oligomers. III. Chain branching and the gel transition

Ring-opening polymerization of cyclic polycarbonate oligomers, where monofunctional active sites act on difunctional monomers to produce an equilibrium distribution of rings and chains, leads to a "living polymer." Monte Carlo simulations [two-dimensional (2D) and three-dimensional (3D)] of the effects of single [J. Chem. Phys. 115, 3895 (2001)] and multiple active sites [J. Chem. Phys. 116, 7724 (2002)] are extended here to trifunctional active sites that lead to branching. Low concentrations of trifunctional particles c(3) reduce the degree of polymerization significantly in 2D, and higher concentrations (up to 32%) lead to further large changes in the phase diagram. Gel formation is observed at high total density and sizable c(3) as a continuous transition similar to percolation. Polymer and gel are much more stable in 3D than in 2D, and both the total density and the value of c(3) required to produce high molecular weight aggregates are reduced significantly. The degree of polymerization in high-density 3D systems is increased by the addition of trifunctional monomers and reduced slightly at low densities and low c(3). The presence of branching makes equilibrium states more sensitive (in 2D and 3D) to changes in temperature T. The stabilities of polymer and gel are enhanced by increasing T, and-for sufficiently high values of c(3)-there is a reversible polymer-gel transformation at a density-dependent floor temperature. (C) 2002 American Institute of Physics.

Hydrogen bonding and collective proton modes in clusters and periodic layers of squaric acid: A density functional study

Hydrogen bonding in clusters and extended layers of squaric acid molecules has been investigated by density functional computations. Equilibrium geometries, harmonic vibrational frequencies, and energy barriers for proton transfer along hydrogen bonds have been determined using the Car-Parrinello method. The results provide crucial parameters for a first principles modeling of the potential energy surface, and highlight the role of collective modes in the low-energy proton dynamics. The importance of quantum effects in condensed squaric acid systems has been investigated, and shown to be negligible for the lowest-energy collective proton modes. This information provides a quantitative basis for improved atomistic models of the order-disorder and displacive transitions undergone by squaric acid crystals as a function of temperature and pressure. (C) 2001 American Institute of Physics.

The role of target polarization in electron-ion recombination

We investigate the role of dynamic polarization of the target electrons in the process of recombination of electrons with multicharged ions (polarizational recombination). Numerical calculations carried out for a number of Ni- and Ne-like ions demonstrate that the inclusion of polarizational recombination leads to a noticeable increase (up to 30%) in the cross sections for incident electron energies outside the regions of dielectronic resonances. We also present a critical analysis of theoretical approaches used by other authors to describe the phenomenon of polarizational recombination.

Dielectronic recombination in He-like titanium ions

Dielectronic recombination (DR) has been studied in highly charged He-like Ti ions using an electron beam ion trap. X-rays emitted from radiative recombination (RR) and DR were observed as the electron beam energy was scanned through the resonances. Differential DR resonant strengths were determined by normalizing the DR x-ray intensity to the RR intensity using theoretical RR cross sections. KLn (2 less than or equal to n less than or equal to 5) resonant strengths were determined for He-like Ti ions. The differential resonant strengths were calibrated without reference to any theoretical DR calculations while the electron energy scale was derived with reference to the well-known energy for ionization of the He-like and H-like ions from the ground state. Calibration in this way facilitates a more exacting comparison between theory and experiment than has been reported previously. To facilitate this comparison, total and differential theoretical resonance strengths were calculated. These calculations were found to be in good agreement with the measured results.

Collision mechanisms in one-electron capture by slow H and He- like ions of C, N and O in H and H-2

Translational energy spectroscopy (TES) has been used to study state-selective one-electron capture by H and He-like ions of C, N and O in both H and H-2 within the range 250-900 eV amu(- 1). The main collision mechanisms leading to state-selective electron capture have been identified, their relative importance assessed and compared, where possible, with theoretical predictions and with any previous measurements based on photon emission spectroscopy. For one-electron capture in H-2, the relative importance of contributions from non- dissociative and dissociative capture as well as from two- electron capture into autoionizing states is found to be strikingly different for the cases considered. Our TES measurements in atomic hydrogen provide an important extension of previous measurements to energies below 1000 eV amu(-1) and show that, as the impact energy decreases, electron capture becomes more selective until only a single n product channel is significant. These product main channels are well described by reaction windows calculated using a Landau-Zener approach. However, the same approach applied to the more complex energy- change spectra observed in H-2 is found to be less successful.