Atomic Collision Processes Research Articles

Multiple impacts and energy transfer in a three-body system for noncollinear collisions

Within the impulsive framework, the energy transfer processes in collisions of atoms with diatomic molecules are considered. In the case of noncollinear collisions involving multiple impacts between the particles, analytic expressions for the amount of the collision energy transferred to the internal degrees of freedom of the molecule have been derived. The limiting cases of these expressions are the well-known Mahan (a single impact) and Mahan-Shin (collinear collisions) formulas. The efficiency of energy transfer in collisions of cesium halide molecules with xenon atoms has been computed as an example; the results obtained agree well with the data of accurate trajectory calculations.

Graphical R-matrix atomic collision environment (G RACE): the problem specification stage

In this paper we introduce the concept of a graphical R-matrix atomic collision environment (G RACE). G RACE couples the graphical capability of powerful workstations with the processing power of supercomputers to provide an environment for the study of atomic collision properties and processes. At the core of G RACE is a new generation R-matrix program package, which is used to compute properties characterising electron atom and electron ion collisions. One of the motivations behind the project is to render this package simple to use by novice and experienced users alike, thereby significantly improving its usefulness to the physics community. G RACE is composed of a problem specification stage, a computation stage, and an interpretation stage. The focus of this paper is a description of the X Window graphical user interface which constitutes the problem specification stage of G RACE.

Fast lithium-beam spectroscopy of tokamak edge plasmas

Plasma-wall interaction and impurity transport processes in the outermost region of magnetically confined hot plasmas (the so-called plasma edge) must be well understood for successful development of future thermonuclear fusion reactors. To this goal, sufficiently detailed edge plasma diagnostics are in great demand. By injecting a fast Li beam into the edge plasma region, a great number of information can be obtained with excellent space and time resolution. This so-called Li-beam plasma spectroscopy gives access not only to edge plasma density profiles from the collisionally excited Li atoms, but also to the impurity concentration and temperature profiles via line emission induced by electron capture from the injected Li atoms by the impurity ions. Full utilization of all capabilities requires a reliable data base for the atomic collision processes involving injected Li atoms and plasma constituents (i.e., electrons, hydrogen ions, and relevant impurities in their various charge states), since a precise modeling of Li beam attenuation and excited-state composition has to be made for evaluating desired plasma properties from the related spectroscopical measurements. The most recent methodical improvement permits a fully consistent determination of absolute edge plasma density profiles by measuring only relative LiI line emission profiles. This is of special interest for investigating rapid edge plasma density fluctuations in connection with, e.g., ELMs, L-H mode transition, turbulence or edge cooling by impurity injection. This paper describes the capabilities of Li-beam edge plasma spectroscopy by way of illustrative examples from measurements at the tokamak experiment TEXTOR.

Influence of ionization processes on radiation defect formation

The influences of the ionization processes in the target, and of the charge state of the incident ions on atomic collision processes are studied in elementary and compound semiconductors. The radiation damaging and sputtering experiments, which are presented here, suggest that ionization enhances atomic displacement processes. Charge-state-dependent influences on nuclear and electronic processes are only observed within the first few nanometers of the ion trajectory.

Local electric fields at individual atomic surface sites: field ion appearance energy measurements

A new method of determining local electric fields is reported. Under conditions of highest field ion image contrast, absolute appearance energy measurements were carried out at a weakly corrugated step of a Rh(001) plane and at strongly corrugated step across the atom rows of a Rh(113) facet. Data of local fields, F hkl loc , are derived from appearance energies. A comparison is made with external electric field strengths, F hkl 0 , determined by electron energy spectroscopy and i- V (Fowler-Nordheim) measurements. Significantly different enhancement factors, F hkl loc / F hkl 0 , were found, 1.27 for steps at the (001) plane, and 1.54 for steps across the rows of Rh(113). In both cases the step-site atoms were covered with field-adsorbed Ne. The experimental results are discussed in view of recent self-consistent calculations of local-field distributions in the proximity of protruding surface atoms. The occurence of field ionization of image gas atoms, released from field-adsorbed states at step sites by atom-collision processes, is in accord with the present observations.

Excitation and ionization processes in slow collisions of ground-state and excited hydrogen atoms with protons.

The collision dynamics in the ${\mathrm{H}}^{+}$+H system is considered at low relative velocities within the concept of hidden adiabatic energy crossings in the complex plane of internuclear distance. Detailed information is provided on the topology of all series of hidden crossings in this system which are involved in the transitions between the states with principal quantum numbers n\ensuremath{\le}4 and in the promotion of these states into the continuum. This information is used to perform cross-section calculations for the 1s\ensuremath{\rightarrow}nl (n=2--4), n=2\ensuremath{\rightarrow}n=3,4, and n=3\ensuremath{\rightarrow}n=4 excitation transitions and for ionization from the n=1, 2, and 3 levels in the adiabatic energy region.

Calculations of generalized oscillator strength for electron-impact excitations of krypton and xenon using a relativistic local-density potential.

The generalized oscillator strength (GOS) of an atom is an essential factor in the differential cross section for inelastic scattering of fast charged particles. Recently, Takayanagi et al. [Phys. Rev. A 41, 5948 (1990)] have obtained the GOS for the excitation of atomic krypton to the 4${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{1/2}$)5s and 4${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{3/2}$)5s states from inelastic-scattering measurements using electron-energy-loss spectroscopy. The present study was undertaken with the twofold objective to determine the results of theoretical computations of the scattering parameters of the above experiment and to examine the suitability of a recently developed relativistic local-density-potential method [M. Vijayakumar, N. Vaidehi, and M. S. Gopinathan, Phys. Rev. A 40, 6834 (1989)] to study atomic-collision processes. Calculations have been done to obtain GOS for electron-krypton and electron-xenon collisions in the squared-momentum-transfer range of 0.01 to 10 atomic units. The well-known theory of Bethe has been used for the determination of the GOS in the first-order Born approximation. The present results are in fair agreement with previous Hartree-Slater and Hartree-Fock calculations and with the experimental data available in the literature. Furthermore, results of the present calculations predict that the GOS goes through a minimum, similar to the ``Cooper minimum'' in the photoabsorption cross section, as a function of the value of the momentum transfer. Experiments at slightly higher values of momentum transfer are suggested to verify the position of this minimum.

Regularities in experimental stark shifts

We have examined regularities in plasma-produced line shifts (Stark shifts) by a comprehensive analysis of literature data. Since the shifts are the result of atomic collision processes, regularities are expected from general atomic structure considerations. Specifically, systematic behavior should occur for spectral series and for corresponding transitions in homologous atoms and isoelectronic ions. Also, Stark shifts should be similar for lines within multiplets and, to a lesser degree, within supermultiplets and transition arrays. Numerous examples show conclusively that the measured data exhibit these predicted regularities. When pronounced irregularities occur, they are readily explainable in terms of special circumstances in the atomic structure.

Positron scattering from alkalis and noble gases—Suggestions for testing theoretical predictions

The reduced density matrix formalism is used to illustrate the general description of atomic collision processes, before the basic ideas behind the Distorted Wave Born Approximation (DWBA) and the close-coupling method for the numerical calculation of various observables are presented. Selected results for differential cross sections and light polarizations after electron and positron excitation are shown to illustrate the possibilities of testing the theoretical predictions.

Human exposure to large solar particle events in space.

Whenever energetic solar protons produced by solar particle events traverse bulk matter, they undergo various nuclear and atomic collision processes which significantly alter the physical characteristics and biologically important properties of their transported radiation fields. These physical interactions and their effect on the resulting radiation field within matter are described within the context of a recently developed deterministic, coupled neutron-proton space radiation transport computer code (BRYNTRN). Using this computer code, estimates of human exposure in interplanetary space, behind nominal (2 g/cm2) and storm shelter (20 g/cm2) thicknesses of aluminum shielding, are made for the large solar proton event of August 1972. Included in these calculations are estimates of cumulative exposures to the skin, ocular lens, and bone marrow as a function of time during the event. Risk assessment in terms of absorbed dose and dose equivalent is discussed for these organs. Also presented are estimates of organ exposures for hypothetical, worst-case flare scenarios. The rate of dose equivalent accumulation places this situation in an interesting region of dose rate between the very low values of usual concern in terrestrial radiation environments and the high dose rate values prevalent in radiation therapy.

STUDIES OF ATOMIC PHYSICS AT RIKEN

Experimental studies of atomic physics are performed with heavy-ion beams from accelerators at RIKEN. The variety of ion species and wide range of velocities and charge states enable us to study atomic collision processes and spectroscopy of highly charged ions. An overview of the works is presented.

Computer Simulation of Atomic Collision Processes in Solids

ABSTRACTComputer simulation is a major tool for studying the interactions of swift ions with solids which underlie processes such as particle backscattering, ion implantation, radiation damage, and sputtering. Numerical models are classed as molecular dynamics or binary collision models, along with some intermediate types. Binary collision models are divided into those for crystalline targets and those for structureless ones. The foundations of such models are reviewed, including interatomic potentials, electron excitations, and relationships among the various types of codes. Some topics of current interest are summarized.

L3-subshell vacancy state alignment in photon-atom collisions.

The differential cross sections for the emission of Ll, L${\mathrm{\ensuremath{\alpha}}}_{2}$, L\ensuremath{\alpha}, L\ensuremath{\beta}, and L\ensuremath{\gamma} groups of L x-ray lines induced in Au by 59.57-keV photons have been measured at different angles varying from 40\ifmmode^\circ\else\textdegree\fi{} to 120\ifmmode^\circ\else\textdegree\fi{} at intervals of 10\ifmmode^\circ\else\textdegree\fi{}. The L${\mathrm{\ensuremath{\alpha}}}_{2}$ line has been isolated by using preferential absorption of the L\ensuremath{\alpha} line in a Zn absorber of suitable thickness. The L x rays represented by the Ll, L${\mathrm{\ensuremath{\alpha}}}_{2}$, and L\ensuremath{\alpha} peaks are found to be anisotropic in the spatial distribution while those in the L\ensuremath{\beta} and L\ensuremath{\gamma} peaks are isotropic. The Ll peak has greater anisotropy than the L\ensuremath{\alpha} peak. The anisotropy of L${\mathrm{\ensuremath{\alpha}}}_{2}$ line is opposite to that of L${\mathrm{\ensuremath{\alpha}}}_{1}$ line. The present results contradict the predictions of Cooper and Zare [Atomic Collision Processes (Gordon and Breach, New York, 1969)], that the vacancy state produced after photoionization is not aligned irrespective of its J value, but confirm those of Flugge, Mehlhorn, and Schmidt [Phys. Rev. Lett. 29, 7 (1972)] that the vacancy state with J>1/2 is aligned.

Correlation in ion-atom collisions

Due to the strong electron-nucleus force in the target atom, many atomic collision processes are well-described within the independent particle model. This paper focuses on ion-atom collisions where dynamical correlations due to interaction between the electrons during the collision play an important role. Such correlations can be important for multi-electron transitions in weak coupling cases (high collision energies), since to first order in the projectile interaction and without correlation the cross section is zero. One example to be discussed is the double ionization of helium, where one manifestation of the correlation is the large difference in double ionization produced by protons and antiprotons. The situation for double ionization will be compared to other two electron processes, such as double excitation, excitation plus ionization, and transfer ionization.

Correlated transitions in the electron-impact ionization of Li+

Atomic collision processes that proceed through electron correlation are examined in the context of the electron-impact ionization of atomic ions. Contributions to the ionization cross section from simultaneous double excitation followed by autoionization are calculated for ${\mathrm{Li}}^{+}$ in a close-coupling approximation. Contributions from simultaneous double excitation with capture followed by simultaneous double autoionization, which proceed through triply excited states, are also calculated for ${\mathrm{Li}}^{+}$ using high-order perturbation theory. Both calculated cross sections are then compared with recent high-resolution crossed-beams experimental measurements.

Outer-shell excitation and capture processes: alignment and orientation effects

This contribution outlines some ongoing activities within a joint programme of experimental and theoretical studies of outer-shell excitation and charge transfer processes in atomic collisions. The main emphasis is presently on alignment and orientation phenomena. The aim is to reveal the shape and dynamics of the electronic charge cloud as it develops in time along the trajectory, and to understand these phenomena in terms of the underlying, basic mechanisms responsible for collisional excitation and transfer.

Plasma modeling in an electron-cyclotron-resonance ion source.

Modeling calculations of an electron-cyclotron-resonance ion source (ECRIS) are presented. The adopted model is described through an analysis of the impact of ECR heating, transport, and atomic-collision processes on the particle balance in an ECRIS. On the basis of the obtained numerical results, general relationships among externally controllable parameters, and plasma and ion-beam characteristics are derived. Comparison with experimental results that use the electron energy as the only fitting parameter supports the applied model. The predicted general trends provide some basis for future experiments.

On the calculation of electron atom collision processes using exterior complex dilated S-matrix expansions

Our work on analytically continued scattering theory based on the Schrödinger equation is reviewed. We give a brief description of how resonances, here defined as partial wave S-matrix poles, can be calculated as complex eigenvalues to the complex scaled Schrödinger equation. A Mittag-Leffler type expansion is then introduced and it is shown how one can partition a scattering cross section into contributions from isolated S-matrix poles and a background. Computationally this method has proven to be considerably faster than conventional methods. A new, faster and more accurate integration method is used. Examples of detailed previous work as well as current research are given.

Dynamics of electron correlation processes in atoms and atomic collisions

The dynamics of electron correlation processes are discussed for atoms and ion-atom collision systems. These processes are introduced in analogy with stationary electron correlation phenomena which are well known from atomic structure theory. The role of the Hartree-Fock method in finding a solution within the independent-particle model is pointed out. The concept of configuration interaction is discussed in order to clarify the verification of electron correlation processes. Configuration mixing is treated in stationary states involving bound and continuum electrons. It is pointed out that a specific electron correlation effect in a stationary state corresponds to an analogous effect in a state which evolves in time. In separated atoms emphasis is given to the Auger effect. In ion-atom collisions double ionization and double excitation are analyzed as examples for processes involving dynamic electron correlation.

Ionisation processes in collisions of open shell atoms. II. Oxygen and sulphur atom collisions with inert gases

For pt.I see ibid., vol.23, p.279 (1990). Results of an electron spectroscopy study of ionisation processes, in low keV collisions of oxygen and sulphur atoms with inert gases are presented. It is shown that ionisation close to threshold is dominantly due to population of singly excited autoionising states of the 2D(2P)nl series. At energies that are significantly above the threshold for excitation processes an additional ionisation channel which leads to a continuous electron energy distribution becomes important. A discussion in terms of the molecular orbital model is presented and indicates that quasimolecular Auger ionisation could be an efficient positive-ion production mechanism at high energies. A comparison with O- and S- collisions is conducted.