Complex-coordinate studies of helium autoionizing resonances

Several aspects of the numerical application of the complex-coordinate method for calculating helium autoionizing resonances are discussed. A direct-search algorithm is used to locate resonance eigensolutions that satisfy the virial theorem, and the influence of the basis set on the accuracy of the calculated positions and widths is investigated. The complex-coordinate method is related to the stabilization method of Holøien and Midtdal.     

Doubly excited1 P 0 resonances in He

Doubly excited¹ P ⁰ autoionizating states in He below then=2 threshold of He⁺ ions are calculated by use of a method of complex-coordinate rotation. Hylleraas type wave functions are used to calculate resonance parameters for intrashell states (the two electrons occupy the same shell), and products of Slater-orbitals are used for intershell states (the two electrons occupy different shells). A total of 15 resonances below then=2 He⁺ thresholds are reported in this work. This includes six members in the 2snp ₊ (2≦n≦7) series, five members in the 2snp _? (3≦n≦7) series, and four members in the 2pnd (3≦n≦6) series. Comparisons are made with other theoretical calculations and with experimental observations.     

Alignment of atomic autoionizing states created by slow Na++He collisions

Ejected-electron spectra have been measured for collisions of He-atoms with Na⁺-ions, whose impact energy ranged from 1.7 to 7.0 keV. The ion impact-energy dependence of the angular-differential cross-section of the ejected electrons has been investigated for an aligned autoionizing state Na**(2p ⁵ 3s ^{2 2} P), which has been created by charge transfer from the He-atoms. The alignment of the autoionizing state Na**(2p ⁵ 3s ^{2 2} P) is discussed in relation to the scattering angles of the Na⁺-ions. A complete longitudinal alignment has been observed with respect to the quasimolecular axis.     

Alignment of atomic autoionizing states created by slow Li++Ne collisions

Ejected-electron spectra have been measured in collisions of Ne-atoms with Li⁺-ions, whose impact energy has been ranged from 0.4 to 7.0 keV. The ion impact-energy dependence has been investigated on the angular-differential cross-section of the ejected electrons from the aligned autoionizing state Ne** 2p ⁴3s ^{2 1} D. In the highest energies studied, cross section maxima have been found at both forward and backward angles with respect to the Li⁺-ion beam direction, whereas the cross sections are maximized at around right angles below intermediate energies. The data were analyzed semi-quantitatively, and the alignment of the autoionizing state was recognized to be characterized by a representative scattering angle of the Li⁺-ions.     

Scattering resonances in slow NH3-He collisions

We theoretically study slow collisions of NH(3) molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10(-4) cm(-1) to 130 cm(-1), using fully converged quantum close-coupling calculations. To describe the interaction between the NH(3) molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH(3) beam.     

Radiation amplification through autoionizing resonances formed by laser assisted dielectronic recombination in He

We present a theoretical model and calculation for the production of autoionizing resonances through laser assisted dielectronic recombination process in He, and its possible application to radiation amplification of UV light.     

An approximate description of the double capture process in He2+ + He collisions with static correlation

It is shown that the process of resonant double electron capture in high energy He²⁺+He collisions can be approximately described by a sum over products of one-electron CDW amplitudes. The summation coefficients are determined by stationary ground-state calculations with CI wavefunctions. Total and differential cross sections are calculated and compared with experimental values.     

Effects of classical nonlinear resonances in grazing diatom-surface collisions

Energy transfer between vibrational, rotational, and translational degrees of freedom of a molecule during a collision process is enhanced when the classical frequencies associated with the initial state are in the proximity of nonlinear resonance conditions. We present an analysis of the classical resonant effects in the collisions of light diatoms with periodic surfaces, and discuss the initial conditions in which these effects can be observed. In particular, we find that for grazing incidence and resonant initial values of the classical frequencies, corresponding to specific vibro-rotational molecular states and translational energies, an efficient energy transfer between the intramolecular vibro-rotational degrees of freedom and the translational degree of freedom along a symmetry direction on the surface can be found. This efficient energy transfer manifests itself in the emergence of specific peaks in the molecular diffraction patterns. The predictions of the resonance analysis are contrasted with the results of classical trajectory calculations obtained in a diatom-rigid surface collision model.     

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.     

Simultaneous electron capture and target ion excitation in collisions of C4+ and N5+ on He

Cross sections for simultaneous electron capture and target ion excitation have been measured for impact of slow He-like C⁴⁺ and N⁵⁺ ions on He. The energy of the primary ion beams has been varied over more than two orders of magnitude: 0.05–7 keV/amu. The results are discussed on basis of a slightly modified version of the dynamic classical over-barrier model for multiple electron capture. The differences in the energy dependences of the experimental results of C⁴⁺ and N⁵⁺ — He can be explained qualitatively by assuming that for N⁵⁺ binding energy sharing between the two participating electrons is of importance, particularly at the lower impact energies.     

Theoretical study of single-electron capture in the N5+ + He and O6+ + He collisions by means of ab initio methods

Partial cross sections of single-electron capture on the n = 3 levels have been determined theoretically for the N⁵⁺ + He and O⁶⁺ + He collisions by means of a semiclassical method using ab initio potential energy curves and radial and rotational coupling matrix elements. The different behavior of these two isoelectronic systems is fairly well reproduced by our calculations.     

Angle-dependent electron energy spectra resulting from autoionizing Ne⋆ (3s 3 P 2) +H(12 S) collisions

Experimental angle-dependent electron energy spectra for the autoionization complex Ne*(3s ³ P ₂)+H(1² S), leading to Penning and associative ionization, are reported. The data, measured at thermal collision energies (ē _rel～51 meV), clearly show an angular variation of the spectral shape, indicating that electrons with angular momentuml>0 participate in the autoionization process. The corresponding non-isotropic electron emission leads to a correlation between the impact parameter-dependent heavy-particle dynamics and the observed electron energy spectrum at a certain detection angle. The experimental results are qualitatively discussed in connection with previous work on the system He*(2³ S)+H(1² S). Furthermore, we present quantum mechanical model-calculations for the electron energy spectrum on the basis of available potential data.     

State-selective electron capture processes in collisions involving boron ions

A full theoretical treatment of electron capture processes using ab initio configuration interaction methods within, according to the collision energy range concerned, a semiclassical or a quantal collisional formalism including translation effects has been developed recently. An application for collisions involving boron, an important impurity in fusion reactors, is presented on examples of the ground state: B³⁺(1s²) + He, B⁴⁺(1s) + H, and the metastable ion B³⁺(1s2s) + H reactions. © 1996 John Wiley & Sons, Inc.     

Total one-electron capture cross sections for Ar q+ and I q+ ions in slow collisions on H2 and He

Argon and iodine recoil ions were produced by a 2 GeV U⁷⁵⁺ beam and total one electron capture cross sections are measured for 198 eV/q Ar ^q+ (4≦q≦15) and I ^q+ (5≦q≦27 on He andH ₂. The cross section can be approximately reproduced by 1/2 πR ² according to the classical barrier model. Theq-dependences exhibit significant fluctuations even for high charge states.     

Angular distribution of scattered projectiles following double electron capture processes at low-energy C4+ and helium collisions

A four-body classical trajectory Monte Carlo method is applied in the study of double electron capture in low-energy ${\mathrm{C}}^{4+}$ and He atom collisions. The interaction between the two target electrons is neglected throughout the collisions. The angular differential cross-sections of the scattered projectiles, following double electron capture, are calculated and compared with experimental data.     

Electron capture and target excitation in slow ion-alkali atom collisions

Experimental cross sections for single electron capture and target atom resonance line emission in impact of singly charged ions (He⁺, Ne⁺, Ar⁺, C⁺, N⁺, O⁺) on respectively Li(2s) and Na(3s) are presented and compared with semiempirical calculations of the Demkov-Olson type. It is shown that such calculations can still be useful despite involvement of metastable primary ion admixtures and several final states. In addition, observed undulations in the impact energy dependence of electron capture — and target excitation cross sections are discussed.     

Review of theories on double electron capture in fast ion-atom collisions

This work reviews quantum-mechanical four-body distorted wave theories for double electron capture in collisions between fast heavy multiply charged ions and heliumlike atomic systems. The widely used distorted wave methods of the first- and second-order in the pertinent perturbation series expansions are compared with each other. This tests the presumed importance of double continuum intermediate states of two electrons. Further, the relative performance is evaluated of the second-order theories with and without the eikonalization of the two-electron Coulomb wave functions for double continuum intermediate states. This checks the correctness and usefulness of the eikonalized Coulomb waves when two electrons participate actively to the transition from the initial to the final state of the entire system. We also analyze the significance of the contributions from excited heliumlike states especially in comparison between theory and measurement. The overall goal of the present study is to determine how much of the unprecedented experience gained over several decades in studying high-energy theories of pure three-body charge exchange could be exported directly to four-body double-electron capture without much of additional and essential eleaborations, besides the naturally increased computational demand. In particular, we address the unexpected breakdown of the continuum distorted wave eikonal initial state approximation and the anticipated success of continuum distorted wave theory for double charge exchange in ion-atom collisions at high impact energies.     

Resonances in collinear He + H2+ collisions

Reactive and non-reactive transition probabilities are reported for the collinear He + H₂⁺ collisions in the energy range 0.955–1.5 eV. An interesting mirror-image relation between states of the same symmetry is pointed out. The scattering resonances are interpreted qualitatively in terms of the bound states supported by the vibrational adiabatic potential in hyperspherical coordinates.