Showing papers in "Journal of Physics B in 1997"

Recoil-ion momentum spectroscopy

Abstract: High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'. Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

On the effects of the two-body non-fine-structure operators of the Breit - Pauli Hamiltonian

Abstract: We have incorporated the two-body non-fine-structure operators of the Breit - Pauli Hamiltonian, namely contact spin - spin, two-body Darwin and orbit - orbit, into the program AUTOSTRUCTURE. Illustrative results are presented, including some for reactions involving the process of autoionization.

Second-order polarization propagator approximation with coupled-cluster singles and doubles amplitudes - SOPPA(CCSD): the polarizability and hyperpolarizability of

Abstract: A new modification of the second-order polarization propagator approximation (SOPPA) is presented. In the new method, called SOPPA(CCSD), all first-order Moller - Plesset doubles and second-order Moller - Plesset singles correlation coefficients are replaced with the corresponding coupled-cluster singles and doubles (CCSD) and amplitudes. This is in contrast to the earlier coupled-cluster polarization propagator approximation (CCSDPPA) in which only one of the doubles correlation coefficients was replaced in the unrelaxed second-order density matrix. The importance of this modification is investigated for the dipole polarizability, dipole oscillator strength sum rules and second hyperpolarizability of , which is known to be a difficult case for perturbative methods. The SOPPA(CCSD) values are compared with previous and new full CI results for the same basis sets.

R-matrix calculations for polyatomic molecules: electron scattering by

Abstract: A new computer program has been developed which allows us to use the R-matrix method to study electron scattering by polyatomic molecules. Our first application is to scattering by in its linear, equilibrium geometry for energies up to 10 eV. We confirm the earlier assignment of symmetry to the resonance near 2 eV but we are unable to locate any resonance having symmetry in this energy range. We present integral and differential cross sections which are generally in excellent agreement with experiment.

Soliton dynamics in the collisions of Bose - Einstein condensates: an analogue of the Josephson effect

Abstract: The usual mean-field description of trapped atomic Bose - Einstein condensates predicts the existence of solitons in one-dimensional and quasi-one-dimensional confinement. If the net atomic pair interaction is attractive, these solitons appear as coherently propagating matter wavepackets; if it is repulsive, soliton behaviour is seen in the motion of `holes' in the condensate density, the velocity of which is determined by the phase difference between adjoining pieces of condensate, in analogy to the familiar Josephson effect. We explore the properties of these solitons via elementary analytic models and computer simulation, and make specific suggestions for their experimental observation.

Reaction dynamics of three-body dissociations in triatomic molecules from single-photon double ionization studied by a time- and position-sensitive coincidence method

Abstract: Application of the position-sensitive photoelectron - photoion - photoion (PSD-PEPIPICO) method to dissociative double-ionization processes in and at photon energies of 40.8 and 48.4 eV provides measurements of total kinetic energy releases and complete descriptions of the kinematics for three-body dissociations of the type . Comparisons of experimental observations with predictions from sequential and concerted bond breaking mechanisms suggest a unified model in which both bonds break on a time scale of , and electron ejection may be either faster (direct ionization) or competitive (indirect) on this time scale.

Investigation of high-harmonic generation from xenon atom clusters

Abstract: We report on the generation of harmonic radiation (in the 70 - 90 nm range) from clusters of Xe atoms formed in a gas jet. We find that the harmonic yield from the clusters exhibits an anomalous cubic scaling with backing pressure to the gas jet. This scaling is consistent with a cluster dipole moment resulting from collective oscillations of electrons around the central ions of the cluster. Using a nanosecond ultraviolet prepulse to dissociate the clusters, we have also attempted to compare harmonic yields from clusters with those produced from monatomic Xe, under otherwise identical conditions. Our results suggest that yields from clusters might exceed those from monomers by up to a factor of five.

Resonance parameters of photo doubly excited helium

Abstract: Using theoretical results from complex rotation calculations and data from experimental photoionization cross sections, the quantum defects, the widths, the oscillator strengths and the shape parameter of Rydberg series of autoionizing resonances in helium, excited with synchrotron radiation from the ground state, are reviewed and analysed systematically. The relation of these resonance properties to the propensity rules for radiative and non-radiative transitions in two-electron atoms is established.

An efficient approach for spin - angular integrations in atomic structure calculations

Abstract: A general method is described for finding algebraic expressions for matrix elements of any one- and two-particle operator for an arbitrary number of subshells in an atomic configuration, requiring neither coefficients of fractional parentage nor unit tensors. It is based on the combination of second quantization in the coupled tensorial form, angular momentum theory in three spaces (orbital, spin and quasispin), and a generalized graphical technique. The latter allows us to graphically calculate the irreducible tensorial products of the second-quantization operators and their commutators, and to formulate additional rules for operations with diagrams. The additional rules allow us to graphically find the normal form of the complicated tensorial products of the operators. All matrix elements (diagonal and non-diagonal with respect to configurations) differ only by the values of the projections of the quasispin momenta of separate shells and are expressed in terms of completely reduced matrix elements (in all three spaces) of the second-quantization operators. As a result, it allows us to use standard quantities uniformly for both diagonal and off-diagonal matrix elements.

Convergent close-coupling calculations of electron - beryllium scattering

Abstract: The convergent close-coupling method is extended to the calculation of electron scattering from atoms which may be modelled as two valence electrons above an inert Hartree - Fock core and applied to the beryllium atom. Elastic and excitations, as well as the total ionization and total cross section results for electron impact of the beryllium ground state, are presented at the energy range of 5 - 1000 eV. Very good agreement is found with the results of the recent R-matrix with pseudo-states calculations of Bartschat et al.

Above-threshold ionization spectrum of hydrogen using B-spline functions

Abstract: We present a method for the accurate calculation of above-threshold ionization (ATI) spectra of single active electron systems. The technique involves the expansion of the radial wavefunction onto a set of B-spline functions. Results of ATI in hydrogen are presented and particular features of the spectra are discussed. A detailed study of the sensitivity of the results to the critical parameters is given to emphasize the importance of the necessity of a high degree of convergence in such calculations. Finally, we discuss and quantitatively justify the use of methods in investigations of ATI.

One and two-colour studies of the dissociative ionization and Coulomb explosion of with intense Ti:sapphire laser pulses

Abstract: Phase control of the spatial asymmetry of the dissociative ionization channel, , of molecular hydrogen has been achieved by superposing interferometrically sub-100 fs, Ti:sapphire laser pulses at the fundamental and second harmonic frequency. The counterintuitive asymmetry previously observed with longer, less intense Nd:YLF laser pulses and HD molecules has been reproduced. However, phase modulation of total ionization and dissociation rates was not discernible. In order to further understand the process, a comprehensive study of appearance intensities, kinetic energy releases and angular distributions has been undertaken at each frequency.

Critical minima in elastic electron scattering by argon

Abstract: We determined experimentally two critical points in elastic electron scattering by argon where the differential cross section (DCS) attains its smallest values The points were found to be at and at Special attention was given to improve the angular resolution in order to determine the exact positions of the minima These minima are important because they are a sensitive test of the validity of experimental procedures, and are used to verify theoretical predictions of DCS shapes and magnitudes, and of the polarization of scattered electrons Normalized DCS were determined by measuring the angular distributions of elastically scattered electrons at incident energies of 10, 15, 20, 25, 30, 40, 50, 60, 75, 80, 90 and 100 eV in the angular range Results are compared with the available experimental and theoretical data In addition, integral, momentum-transfer, and viscosity cross sections were determined by numerical integration of the measured DCS extrapolated to and to

Convergent close-coupling calculations of electron scattering on helium-like atoms and ions: electron - beryllium scattering

Abstract: The convergent close-coupling method is extended to the calculation of electron scattering from atoms with two valence electrons above an inert Hartree - Fock core. The formalism is illustrated by calculation of electron scattering from beryllium. Integrated cross sections for scattering from the ground state up to n = 4 levels as well as the total ionization and total cross sections are presented in the energy range from 10 to 1000 eV. Good agreement is found with the available results of the RMPS calculations of Bartschat et al.

The K-shell Auger decay of atomic oxygen

Abstract: Relative photoionization cross sections for and produced by the Auger decay of a 1s hole in atomic oxygen were measured by using synchrotron radiation between 525 and 553 eV. Energies and quantum defects of the members of the two Rydberg series converging to and ionization thresholds were determined. In addition, the and ionization thresholds were calculated from the two Rydberg series. The 182 meV resolution of the monochromator allowed a detailed study over both thresholds revealing evidence for post-collision interaction and allowing a comparison of the ionization continuum above both and thresholds with that of the ionization continuum above the Ar edges. This comparison indicates that the lifetimes of the Ar(2p) and O(1s) hole states are approximately the same.

Accurate wavefunctions for the confined hydrogen atom at high pressures

Abstract: Wavefunctions are proposed for the 1s, 2p and 3d states of the compressed hydrogen atom at high pressures. It is shown that these wavefunctions give highly accurate energies in the positive energy region.

Intense laser - atom dynamics with the two-dimensional Dirac equation

Abstract: A time-dependent numerical analysis is made of atomic wavefunctions evolving under the influence of laser pulses sufficiently intense that relativistic effects are important. The investigation utilizes the two-dimensional Dirac equation to identify relativistic effects such as the contribution of the magnetic field and spin dynamics.

Single and double ionization of neon, krypton and xenon by positron impact

Abstract: Measurements of the single (σi+) and the ratio of double to single (R2) ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV By multiplying the R2 values of each target by the appropriate σi+, new data for the corresponding double (σi2+) ionization cross sections have also been obtained The ratio between positron and electron σi+ maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron-atom scattering

Hot electron x-ray diagnostics

Abstract: A new method for the determination of hot electrons in high-temperature plasmas is proposed. The presence of hot electrons leads not only to a balance shift of the ion abundance but also to a qualitative distortion. In experiments the distortion is observable through the emission of H-like and the series satellites of Li- to F-like ions. The wide range of application is demonstrated and holds even for optically thick plasmas. Transient numerical calculations are carried out for typical parameters of fusion plasmas.

Beyond the dipole approximation: angular-distribution effects in valence photoemission

Abstract: Angular distributions of valence photoelectrons showing effects due to higher-multipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250 - 1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

Photoionization - excitation of helium using an R-matrix with pseudostates method

Abstract: We have developed an alternative R-matrix with pseudostates (RMPS) method for incorporating a two-electron continuum description into the wavefunction of electron - ion collision calculations. This method is similar in spirit to various recent treatments of the pseudostate expansion, most notably the R-matrix approaches of Meyer and Greene, and Bartschat et al. Our approach differs in that we: (1) utilize a direct sum of several bases: the physical target orbitals, additional MCHF pseudo-orbitals for the He ground state, R-matrix continuum orbitals, and, on occasion, Laguerre orbitals, and (2) rely on a different approach for the creation of an orthonormal basis. Since we use the Belfast codes that are based on a Wigner - Eisenbud R-matrix treatment, we also need to introduce a modified Buttle correction. The method is first tested on the s-wave (Temkin - Poet) model for electron-impact excitation and ionization of . Then it is applied to the calculation of cross sections and angular distributions for the photoionization - excitation process (n = 2,3). We find that these results are greatly improved by the inclusion of the two-electron continuum description, which is more important for the ground state, but plays a role in the final photoionized states as well. A highly correlated multiconfiguration Hartree - Fock expansion is used to represent the ground state, and from the configuration interaction coefficients, asymptotic ratios are determined and compared with other theoretical results. Results are also compared with recent high-energy measurements for the n = 2,3 cross section ratios and angular distribution parameters. This pseudostate expansion allows us to predict photo - double-ionization cross sections, . Of particular significance, we find good agreement between our length and velocity gauge results, indicating that our ground-state correlation is sufficiently converged for the present system.

Time-dependent R-matrix theory of multiphoton processes

Abstract: A new time-dependent R-matrix theory of multiphoton processes is described which can be applied to an arbitrary many-electron atom. The theory is complementary to the R-matrix Floquet theory developed by Burke, Francken and Joachain (1991 J. Phys. B: At. Mol. Opt. Phys. 24 761) enabling processes involving higher laser field intensities and shorter laser pulses to be treated. The new theory is illustrated by analysing the multiphoton ionization of a charged particle bound initially in a one-dimensional potential well where the results are compared with an independent R-matrix Floquet calculation.

The Dirac - Coulomb Sturmians and the series expansion of the Dirac - Coulomb Green function: application to the relativistic polarizability of the hydrogen-like atom

Abstract: Two relativistic Sturmian basis sets are constructed by solving first-order Sturm - Liouville problems for the Dirac - Coulomb equation. The Sturmian expansions of the Dirac - Coulomb Green function and the reduced Dirac - Coulomb Green function are derived. Their utility is illustrated by calculating static electric dipole polarizabilities for hydrogen-like atoms with .

Observations of X-Ray Spectra from Highly Charged Tungsten Ions in Tokamak Plasmas

Abstract: X-ray spectra in the range 7 - 10 A from highly charged tungsten (Z = 74) ions were observed in the tokamak ASDEX Upgrade. Lines emitted from to were identified by comparison with ab initio calculations and with the expected ionization equilibrium charge-state distribution. Most of the features in the observed spectra coincide with predicted lines. A spectral line of As I-like could be identified as the predominant one for plasmas with 2.5 - 3 keV central electron temperature. The calculations predict clearly separated spectral lines for Kr I-like W at about 18.4 A and strong lines between 5.6 and 6.0 A for all charge states under consideration that can be used for monitoring W densities in the soft x-ray region. Tungsten concentrations in the range of were extracted from the emissivities of spectral lines from and .

Dissociation dynamics of superexcited molecular hydrogen

Abstract: The dissociation dynamics of superexcited molecular hydrogen and its isotopes has been discussed from an experimental viewpoint. Recently, more attention has been paid to doubly excited states since new experiments on ionization and dissociation via doubly excited states have started due to the large progress in synchrotron radiation light source and electron spectroscopy. This paper reviews the experimental techniques and results obtained and indicates that the competition between neutral dissociation and autoionization is a key to investigating the molecular superexcited states.

Accurate analytic wavefunctions for two-electron atoms

Abstract: Simple analytic two variational parameters wavefunctions for the ground state of , He, , , atom (ions) are proposed. These functions fulfil the cusp conditions at the singularities of the Coulombic field, and include the correct behaviour for large interparticle separations. The accuracy of the calculated energies is found to be about . These functions should be useful for the computation of the cross section of inelastic processes involving two-electron systems.

Double photoionization: I. A new parametrization of the triple differential cross section from first principles

Abstract: We have combined a laboratory frame independent particle approach with a body-fixed frame collective coordinates approach to derive new exact parametrizations of the triple differential cross section for double photoionization. The latter disentangle the kinematical from the dynamical effects to the largest possible extent. In addition, they provide an expansion of the so-called correlation factor with respect to the configurations of the electronic pair. The present approach is particularly suitable for the physical analysis of experimental data, as illustrated by the applications reported in the companion paper.

Wavelength dependence of multiphoton-induced Xe(M) and Xe(L) emissions from Xe clusters

Abstract: A direct comparative measurement of the dependence on the wavelength of irradiation of the kilovolt x-ray yields ( and ) multiphoton-induced from Xe clusters by excitation with intense femtosecond pulses at 248 and 800 nm has been made. The spectroscopic findings demonstrate that both the Xe(M) and Xe(L) emissions are strongly reduced with excitation at the longer wavelength (800 nm). The peak strengths of the Xe(M) and Xe(L) emissions are diminished by factors of and , respectively. Significant spectral differences are also observed. This sharp reduction in the amplitude of the excitation is in conflict with a thermal model for the production of kilovolt x-rays (Xe(M) and Xe(L)) from multiphoton 248 nm excited Xe clusters. These results are consistent with a dynamical mechanism of enhanced coupling which involves ordered many-electron motions in which a dephasing of the electrons can appreciably influence both the amplitude of excitation and the threshold intensity for inner-shell vacancy production. Within the framework of this picture, these experimental findings indicate an effective dephasing time for Xe clusters of - 2 fs, a range that is consistent with the measured k-space scattering dynamics of carriers in GaAs.

Positron - hydrogen scattering in the vicinity of the positronium formation threshold

Abstract: An analysis is made of the behaviour of the elastic scattering and positronium formation cross sections for s-, p- and d-wave low-energy positron - hydrogen scattering in the vicinity of the positronium formation threshold. The accurate cross sections, obtained using the Kohn variational method with elaborate trial functions containing many Hylleraas correlation functions, reveal interesting threshold structures which arise from the coupling between the positronium formation and the elastic scattering channels. Wigner's R-matrix method is used to provide a qualitative analysis of these structures. One of the most interesting features of the present results is the presence of a `rounded cusp' in the s-wave elastic scattering cross section at the threshold itself, as predicted by Wigner's threshold theory. Remarkably good fits to the partial-wave cross sections for elastic scattering, positronium formation and Ps - p elastic scattering are obtained using a five-parameter fit for each partial wave.