Intermediate Impact Energies Research Articles

New approach on molecular ionization by ion impact: the CDW-EIS-SVIM model

We have extended the continuum distorted wave eikonal initial state approximation for single ionization of atoms by ion impact to the molecular case where a model potential for the molecular target is introduced. The continuum wavefunctions of the ejected electrons are then calculated in the static-exchange potential field generated by the residual molecular ion. The Schwinger variational iterative method was used in the calculation of these continuum wavefunctions. We apply this model to the single ionization of the hydrogen molecule (H2) by different bare projectiles. Doubly differential cross sections are calculated and compared with available experimental data for intermediate impact energies.

Fragmentation induced by charge exchange in collisions of charged alkaline clusters with alkali atoms

We present a theoretical study of the dissociative charge transfer processes induced by collisions between doubly-charged alkaline clusters and alkaline atoms at slow and intermediate impact energies. Charge-exchange cross sections have been evaluated for the collisions of Li312+ and Na312+ clusters with neutral alkaline atoms (Cs and Na) at impact energies between 500 and 4000 eV. The branching ratios of the evaporation processes have been calculated within the framework of the microcanonical statistical theory of Weisskopf. The key ingredient of this model is the cluster vibrational density of states. An approach based on quantum tight-binding Hamiltonian is introduced, allowing us to evaluate this quantity at a microscopic level, including quantum vibrational effects at low temperatures. Comparison with previously reported results obtained using a macroscopic description of the level density are presented and discussed.

Coplanar symmetric and asymmetric electron impact ionization studies from the 1b1 state of H2O at low to intermediate impact energies

(e, 2e) ionization differential cross sections are presented for incident electron energies ranging from 15 eV to 95 eV above the ionization threshold of the 1b1 molecular state of H2O. Experimental results and theoretical analysis were derived for three energies in a coplanar symmetric geometry, and for three energies in an asymmetric geometry. The experimental data show a wide variation in the cross section over this range of energies, whereas the theoretical analysis carried out using a sophisticated molecular DWBA model, which includes the final state post collision interaction (PCI), shows best agreement at lower energies. The experimental techniques used to collect the data are described here as well as an improved theoretical approach using elastic scattering cross sections to evaluate the accuracy of the distorted waves utilized in the calculation of the ionization cross sections.

Four-body continuum-distorted-wave model for charge exchange between hydrogenlike projectiles and atoms

The four-body continuum-distorted-wave model is formulated and used to investigate single electron capture in fast collisions of hydrogenlike projectiles with one- and multi-electron targets. The presented method of an analytical calculation of scattering integrals yields the total cross sections in terms of four-dimensional numerical quadratures. As an illustration, total cross sections are computed for electron capture processes in ${\mathrm{He}}^{+}\text{\ensuremath{-}}\mathrm{H}$, ${\mathrm{He}}^{+}\text{\ensuremath{-}}\mathrm{He}$, and ${\mathrm{Li}}^{2+}\text{\ensuremath{-}}\mathrm{He}$ collisions at intermediate and high impact energies. The contribution from the interelectron interaction during the collision is evaluated. The prior and post total cross sections are found to be in good agreement with the available experimental data.

Recommended data for capture cross sections in B5+ + H collisions

Recommended values for state selective capture cross sections are presented for the collision B5+ + H(1s) in the energy range from 0.05 to 1000 keV amu−1. Special attention is focused on capture processes to n = 7 states of B4+, which play an important role in spectral diagnostics in fusion plasmas. In order to completely cover the intermediate impact energy domain, quantal, semi-classical and classical treatments have been employed for low, low-intermediate and intermediate-high energies, respectively. We also give some guidelines about the domain of accuracy of the methodologies employed. Additionally, preliminary cross sections of the B5+ + H(2s) collision are also provided.

Second-order effects in (e, 2e) ionization–excitation of H2

Ionization–excitation by electron impact of diatomic species in general and hydrogen molecule in particular presents very interesting mechanisms which can be observed in coincidence (e, 2e) experiments. The dissociative character of the final excited state makes these types of experiments easier to perform than those of atomic targets for which the coincidence measurement of the emitted electron and the photon is necessary. Recently Takahashi et al (2003) Phys. Rev. A 68 042710 have shown large disagreements between their experimental results and theoretical ones obtained by the first-order plane-wave impulse approximation at intermediate impact energies. These authors conclude that a second-order treatment, including the two-step mechanisms (TS1 and TS2), would be necessary to explain these differences. In this paper, we present a second-order Born treatment including the two-step mechanisms and study the simple ionization of H2 and the ionization–excitation of H2 to the 2Σg(2sσg) and 2Σu(2pσu) levels of H+2 by electron impact.

Positronium formation in collisions of fast positrons impacting on vapour water molecules

Positronium formation through electron capture by fast positrons impinging on vapour water molecules is studied theoretically at intermediate and high impact energies. This multi-electron system is treated within the framework of the independent electron model. The charge transfer process is described by employing the continuum distorted-wave final-state approximation. In this model, the final state of the collision is distorted by two Coulomb wavefunctions associated with the interactions of both the positron and the active electron (the captured one) with the residual ionic target. The water molecule is described by an expansion of monocentric Slater-like functions centred on the oxygen atom. Total cross sections are computed by using a partial-wave technique and compared with results obtained using the simpler Coulomb–Born approximation. Theoretical results for the production of H2O+ ions in the final channel of the reaction through charge transfer processes (positronium formation and ionization) are also presented.

Classical and quantal methods in atomic and molecular collisions

We present theoretical calculations of electron capture cross-sections for ion–atom and ion–molecule collisions in the intermediate impact energy range. The results stem from intensive computations based on quantal, semiclassical and classical approaches. We focus our attention on (i) electron transfer cross-sections for quasi-one-electron ion–atom/molecule systems and (ii) energy distributions of the fragments produced in multiply charged ion–H2 collisions. The results from the different approaches are compared systematically, also with experimental data, when available.

Influence of initial state distortion in ion–atom collisions

We have studied the influence of initial state distortion in a single ionization by ion impact. We have taken a continuum distorted wave type distortion and by taking up to the first order in its asymptotic series expansion we build an eikonal-spherical distortion. In this way the influence of each term in the transition amplitude can be stated. This approximation can be considered an intermediate one between the eikonal initial state and the continuum distorted wave approaches for initial state distortion. We have computed doubly differential cross sections for helium ionization by protons and highly charged ions at high and intermediate impact energy. We have also discussed the contribution of the different terms in electron energy spectra, specially in the vicinity of ECC peak. Very good agreement is found with the available experimental data.

Formation of Asteroid Families by Catastrophic Disruption: Simulations with Fragmentation and Gravitational Reaccumulation

This paper builds on preliminary work in which numerical simulations of the collisional disruption of large asteroids (represented by the Eunomia and Koronis family parent bodies) were performed and which accounted not only for the fragmentation of the solid body through crack propagation, but also for the mutual gravitational interaction of the resulting fragments. It was found that the parent body is first completely shattered at the end of the fragmentation phase, and then subsequent gravitational reaccumulations lead to the formation of an entire family of large and small objects with dynamical properties similar to those of the parent body. In this work, we present new and improved numerical simulations in detail. As before, we use the same numerical procedure, i.e., a 3D SPH hydrocode to compute the fragmentation phase and the parallel N-body code pkdgrav to compute the subsequent gravitational reaccumulation phase. However, this reaccumulation phase is now treated more realistically by using a merging criterion based on energy and angular momentum and by allowing dissipation to occur during fragment collisions. We also extend our previous studies to the as yet unexplored intermediate impact energy regime (represented by the Flora family formation) for which the largest fragment's mass is about half that of the parent body. Finally, we examine the robustness of the results by changing various assumptions, the numerical resolution, and different numerical parameters. We find that in the lowest impact energy regime the more realistic physical approach of reaccumulation leads to results that are statistically identical to those obtained with our previous simplistic approach. Some quantitative changes arise only as the impact energy increases such that higher relative velocities are reached during fragment collisions, but they do not modify the global outcome qualitatively. As a consequence, these new simulations confirm previous main results and still lead to the conclusion that: (1) all large family members must be made of gravitationally reaccumulated fragments; (2) the original fragment size distribution and their orbital dispersion are respectively steeper and smaller than currently observed for the real families, supporting recent studies on subsequent evolution and diffusion of family members; and (3) the formation of satellites around family members is a frequent and natural outcome of collisional processes.

The influence of cooling rate on the fracture properties of a glass reiforced/nylon fiber‐metal laminate

AbstractThe effect of varying cooling rate on the microstructure and resulting mechanical properties of a novel fiber‐metal laminate (FML) based on a glass fiber‐reinforced nylon composite has been investigated. Polished thin sections removed from plain glass fiber/nylon composites and their corresponding fiber‐metal laminates indicated that the prevailing microstructure was strongly dependent on the rate of cooling from the melt. Mode I and Mode II interlaminar fracture tests on the plain glass fiber reinforced nylon laminates indicated that the values of GIc and GIIc averaged approximately 1100 J/m2 and 3700 J/m2 respectively at all cooling rates. The degree of adhesion between the aluminum alloy and composite substrates was investigated using the single cantilever beam geometry. Here, the measured values of Gc were similar in magnitude to the Mode I interlaminar fracture energy of the composite, tending to increase slightly with increasing cooling rate. The tensile and flexural fracture properties of the plain composites and the fiber metal laminates were found to increase by between 10% and 20% as the cooling rate was increased by two orders of magnitude. This effect was attributed to over‐aging of the aluminum alloy plies at elevated temperature during cooling. Finally, fiber metal laminates based on glass fiber/nylon composites were shown to exhibit an excellent resistance to low velocity impact loading. Damage, in the form of delamination, fiber fracture, matrix cracking in the composite plies, and plastic deformation and fracture in the aluminum layer, was observed under localized impact loading. Here, the fast‐cooled fiber metal laminates offered superior post‐impact mechanical properties at low and intermediate impact energies, yet very similar results under high impact energies.

Positronium formation inpositron-simple metal cluster collisions

Positronium formation through electron capture fromground-state closed-shell sodium clusters is studiedtheoretically at intermediate impact energies. The chargetransfer process is described in the independent electronmodel by a continuum distorted-wave approximation. Inthis approximation, distortions in the final state areintroduced by using two Coulomb wavefunctions associatedwith the positron- and electron-residual targetinteractions. The cluster is described within theframework of the spherical background jellium model andthe Kohn-Sham formalism with a local-densityapproximation that includes exchange, correlation, and aself-interaction correction. Using a partial-wavetechnique, differential and total cross sections for theisoelectronic neutral and charged closed-shell sodiumclusters Na20, Na19- and Na21+are computed and compared with the predictions of theCoulomb-Born type approximation which neglects thedistortions in the final state.

Electron energy-loss spectra of coupled electronic states: Effects of Rydberg-valence interactions inO2

The concept of the ``generalized transition moment'' is extended beyond the region of applicability of the first Born approximation and is used in the analysis of electron energy-loss spectra for ${\mathrm{O}}_{2},$ presented here, which have been measured at intermediate impact energies. A coupled-channel theoretical treatment of the strongly mixed Rydberg and valence states that dominate the optically allowed spectrum is used to explain the relative intensities of many unusual features occurring in the 7--11.2-eV energy-loss region. For these electronically excited states of ${}^{3}{\ensuremath{\Sigma}}_{u}^{\ensuremath{-}}$ and ${}^{3}{\ensuremath{\Pi}}_{u}$ symmetry, the evolution of the shape of the corresponding electron energy-loss spectrum as the scattering conditions are changed is controlled essentially by one parameter: the ratio of the diabatic generalized transition moments into the Rydberg and valence components of the mixed electronic states. The generalized Rydberg transition moment is found to decrease much faster than the valence moment as the momentum transferred in the collision increases. The results of the coupled-channel analysis also indicate that the more diffuse spectral features are generally asymmetric, those in the 7--9.8-eV region changing slightly in position as the scattering conditions are altered. Thus, the Gaussian--line-shape-based models that have been employed previously in attempts to decompose the diffuse part of the electron energy-loss spectrum for ${\mathrm{O}}_{2},$ into contributions from different electronic states, are unlikely to have given meaningful results. As a consequence of the Rydberg-valence interactions, it is found that some vibrational levels of the mixed states give rise to features in the electron energy-loss spectrum which are anomalously strong at the low impact energies and large scattering angles for which normal optically allowed transitions are expected to decline in relative strength. These ``persistent lines'' are easily confused with those from the optically forbidden transitions which increase in relative intensity under such conditions. Thus, the coupled-channel technique is found to be a valuable aid to the assignment of features in the electron energy-loss spectrum.

Time-dependent screening effects in ion-atom collisions with many active electrons

Ionization and electron transfer in collisions of bare ions from neutral target atoms with many active electrons are investigated within the independent particle model. We propose a simple model for the inclusion of time-dependent screening effects and discuss the question of how to analyze the solutions of the single-particle equations in order to avoid fluctuating transition probabilities, which normally occur when the effective mean-field potential depends on the propagated orbitals. The basis generator method is used to solve the single-particle equations for the ${\mathrm{He}}^{2+}+\mathrm{Ne}$ collision system in the energy range of 5 to 1000 keV/amu. It is shown that time-dependent screening effects reduce the cross sections for ionization and capture at low and intermediate impact energies significantly. Good overall agreement with experimental data is found except for the higher final charge states of the target ion.

Projectile 1s Electron Loss in Collision Systems F8+ + C and O7+ + C

In this paper, we present our study on the K shell electron loss of hydrogen-like O and F ions by C atom in the impact energy range 20 MeV to 100 MeV. The PWBA calculations for electron loss neglecting screening by the target electrons are in good agreement with the measured cross-sections at high impact energy. Large deviations are seen in the measured loss cross-sections at intermediate impact energy and this is attributed to capture ionization (CI) which is important in nearly symmetric collision systems.

Electron-impact excitation of the(…6s5d1D2)to(…6s6p1P1)transition in barium

We have measured and calculated scattering parameters (differential cross sections and four electron-impact coherence parameters) for electron-impact excitation of the $(\dots{}{6s5d}^{1}{D}_{2})$ to $(\dots{}{6s6p}^{1}{P}_{1})$ transition in barium in the low to intermediate impact energy regime (10, 20, and 40 eV). Measurements were carried out by the superelastic scattering technique in which collisional deexcitation from the laser-excited $(\dots{}{6s6p}^{1}{P}_{1})$ state to the metastable $(\dots{}{6s5d}^{1}{D}_{2})$ level was detected by electron-energy-loss spectroscopy in a crossed-beam apparatus. Supporting calculations were carried out in the convergent close-coupling scheme. Measured differential cross sections were significantly lower than those measured for $(\dots{}{6s6p}^{1}{P}_{1})$ excitation out of the ground $(\dots{}{6s}^{21}{S}_{0})$ state and good agreement with theory was found. Measured and calculated coherence parameters show strikingly different behavior from that associated with ${}^{1}{P}_{1}$ excitations out of the ground state. Agreement between measured and calculated coherence parameters was found to be less satisfactory than in the case of the differential cross sections but was, nevertheless, encouraging considering the weak nature of the transition.

A CTMC Study of Collisions between Protons andH2+ Molecular Ions

We study numerically collisions between protons andH2+ molecular ions at intermediate impactenergies by using the Classical Trajectory Monte Carlo method (CTMC).Total and differential cross sections are computed. The results arecompared with: a) the standard one electron–two nucleonscattering, and b) the quantum mechanical treatment of theH+–H2+scattering.

Inner-shell contribution to electron capture in positron-alkali collisions

Positronium formation through electron capture from ground state alkali targets is studied theoretically at intermediate and high impact energies by means of a continuum distorted-wave approximation. In this approximation, distortions in the final state are introduced by using two Coulomb wavefunctions associated with the positron- and electron-residual target interactions. Total cross sections for lithium and potassium targets are computed. An estimation of the capture from the inner shell of lithium and potassium is given. Comparison with other theoretical results and experiments is made. The contribution from the inner shells becomes important as the impact energy increases and seems to be necessary to describe the measured cross sections.

Symmetric-eikonal theory of excitation with Hartree-Fock-Slater description of the target

The symmetric-eikonal model for single excitation of multielectronic targets is extended to include Hartree-Fock-Slater initial and final bound states. These wave functions are obtained by solving numerically the time-independent Schroedinger equation with a model potential. Total cross sections for excitation of He(1{sup 1}S) and He(2{sup 1}S) by proton impact are presented in comparison with available experimental data and other theoretical models. The model gives an accurate description at intermediate and high impact energies. Results are also presented for excitation of two-electron heavy ions and for alkaline targets. {copyright} {ital 1997} {ital The American Physical Society}

Boundary conditions of the exact impulse wave function

The behavior of the exact impulse wave function is investigated at intermediate and high impact energies. Numerical details of the wave function and its perturbative potential are reported. We conclude that the impulse wave function does not tend to the proper Coulomb asymptotic limit. For electron capture, however, it is shown that the impulse wave function produces reliable probabilities even for intermediate velocities and symmetric collision systems.