Low-energy Ion-atom Collisions Research Articles

Proposal for measuring line profiles and angular distributions of the radiative-electron-capture transition in low-energy ion-atom collisions: Beyond the impulse approximation

We propose to measure line profiles and angular distributions of the radiative-electron-capture transition in collisions of sub-MeV/$u {\mathrm{Ar}}^{18+}$ with helium. We estimate the line profile and the angular distribution using hydrogenlike and Hartree-Fock wave functions and show that the asymmetry of the line profile and the deviation of the angular distribution from the ${sin}^{2}{\ensuremath{\theta}}_{\mathrm{t}}$ law are magnified in the sub-MeV/$u$ region. Finite detector resolution and ion-beam monochromaticity are considered, and the results show that by using a usual bent-crystal spectrometer and a group of commercial silicon drift x-ray detectors one will be able to obtain sufficiently precise line profile and angular distribution to distinguish different theoretical descriptions of target atoms. The enhanced sensitivity to different theoretical approaches at low energies suggests that the proposed experiment will provide insights into the electromagnetic transition of an electron between a pair of relatively moving nuclei.

Impact parameter sensitive study of inner-shell atomic processes in the experimental storage ring

In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for low-energy (heavy-) ion-atom collisions. The experiment was performed with bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms, resulting in a symmetric collision system. This choice of the projectile charge states was made in order to compare the effect of a filled K-shell with the empty one. The projectile and target X-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35–70fm.

Measurement of the Low-Energy Rb+–Rb Total Collision Rate in an Ion-Neutral Hybrid Trap**Supported by the National Natural Science Foundation of China under Grant Nos 11604334, 61227807 and 61575108, and the Tsinghua Initiative Scientific Research Program under Grant No 2013THZ02-3, and the Beijing Natural Science Foundation under Grant No 1164016.

An ion–neutral hybrid trap is built to study low-energy ion–atom collisions. The ion–neutral hybrid trap is combined with two spatially concentric traps: a linear Paul trap for the ionic species and a magneto-optical trap (MOT) for the neutral species. The total ion–atom collision rate coefficient between87Rb atoms and optically dark 87Rb+ ions is measured by monitoring the reduction of the steady-state MOT atoms by sequentially introducing various mechanisms, namely photoionization and ion–atom collisions. In contrast to other experiments, a concise experimental procedure is devised to obtain the loss rates of the MOT atoms due to photoionization and ion–atom collisions in one experimental cycle, and then the collision rate of cold 87Rb atoms with 87Rb+ ions is deduced to m3/s with K measured by the time of flight of the ion signal. The measurements show good agreement with the collision rate derived from the Langevin model.

Relativistic calculations of inner-shell atomic processes in low-energy ion–atom collisions

Relativistic calculations of inner-shell atomic processes in low-energy Ne–F8+(1s) and Xe–Xe53+(1s) collisions are performed. The method of calculation exploits the active-electron approximation and is based on the coupled-channel approach with atomic-like Dirac–Sturm–Fock orbitals, localized at the ions (atoms). The screening density-functional theory is applied for description of the interaction with passive electrons. The role of relativistic effects is analyzed.

Sturmian theory of electron energy distributions in low energy ion–atom collisions

The energy and angular distributions of electrons ejected from targets and projectiles in the collisions of slow H and H− with He targets has been calculated exactly in a model that treats electron interactions with neutral targets via zero-range interactions. These calculations employ two-center Sturmian functions for collisions at non-zero impact parameters. The computed distributions are compared with recent measurements for He targets. Total ionization cross sections are also computed and compared with experiment over a broad energy range for proton, alpha particle and neutral hydrogen impact on helium atoms. The calculations are in generally good agreement with experiment.

Hyperspherical close-coupling calculations for charge-transfer cross sections inHe2++H(1s)collisions at low energies

A theory for ion-atom collisions at low energies based on the hyperspherical close-coupling (HSCC) method is presented. In hyperspherical coordinates the wave function is expanded in analogy to the Born-Oppenheimer approximation where the adiabatic channel functions are calculated with B-spline basis functions while the coupled hyperradial equations are solved by a combination of R-matrix propagation and the slow/smooth variable discretization method. The HSCC method is applied to calculate charge-transfer cross sections for He{sup 2+}+H(1s){yields}He{sup +}(n=2)+H{sup +} reactions at center-of-mass energies from 10 eV to 4 keV. The results are shown to be in general good agreement with calculations based on the molecular orbital (MO) expansion method where electron translation factors (ETF's) or switching functions have been incorporated in each MO. However, discrepancies were found at very low energies. It is shown that the HSCC method can be used to study low-energy ion-atom collisions without the need to introduce the ad hoc ETF's, and the results are free from ambiguities associated with the traditional MO expansion approach.

Ejected-electron spectrum in low-energy proton-hydrogen collisions

The behavior of the ejected electron spectra resulting from low-energy ion-atom collisions relevant to momentum imaging experiments is explored and the origin of the oscillatory structures these spectra display as a function of collision energy is discussed. This is aided by consideration of the time-dependent, electronic Sch\odinger equation that is solved at a fixed impact parameter for 1--25 keV proton impact of atomic hydrogen utilizing the lattice, Fourier collocation technique and split-operator time propagation. At a large internuclear separation after the collision the bound states of the target and projectile are projected out for $nl4$ and the resulting continuum wave function is examined. Techniques, such as multigridding, are investigated to extend the propagation of the wave function to significantly larger final distances.

Harmonic oscillator Green’s function

The Green’s function for the harmonic oscillator in three dimensions plays an important role in the theory of atomic collisions. One representation of low-energy ion–atom collisions involves harmonic oscillator potentials. A closed-form expression for the harmonic oscillator Green’s function, needed to exploit this representation, is derived. This expression is similar to the expression for the Coulomb Green’s function obtained by Hostler and Pratt. Calculations of electron distributions for a model system of ion–atom collisions are reported to illustrate the theory.

Rapid oscillation of electron distributions in low-energy ion–atom collisions

The Sturmian theory with only a few Sturmian functions gives a complete ab initio description of electron distributions in ion atom collisions. The interference of the contribution from σ and π Sturmian functions gives rise to rapid oscillation of electron angular distributions with impact velocity v. These rapid oscillations measure the real part of the adiabatic eigenvalues in the harmonic oscillator region, which occurs only at complex R.

State selective Rydberg charge transfer and ionization in low energy ion–atom collisions

The Classical Trajectory Monte Carlo (CTMC) simulation method with a core modified interaction potential has been used to study the single charge transfer in Na +and Ar + ions colliding with a variety of state selected Na Rydberg atom targets ( n=24, 28, 33, 40 and l=2) in the reduced velocity region v=0.2–2.0. The experimentally observed structures in the total capture cross section versus reduced velocity curves are reproduced by CTMC method. The n-distribution of final capture state has got two peaks viz. first one at n f= n i and the second one at a higher n f depending on the initial angular momentum in the velocity regime 0.4–0.6. These structures have been explained in terms of quasimolecular-ion formation and a classical model proposed by Roy et al. (B.N. Roy, D.N. Tripathi, D.K. Rai, Phys. Rev. A 5 (1972) 1252). The CTMC ionization cross section results are benchmarked with the recent experimental measurement of Makarov et al. (O.P. Makarov, D.M. Homan, O.P. Sorokina, K.B. MacAdam, in: F. Aumayr, G. Betz, H.P. Winter (Eds.), Proceedings of the 20th International Conference on the Physics of Electronics and Atomic Collisions, Vienna, 1997, p. FR052) for Na +–Na(24 d).

The new ECR ion source of the ATOMKI: A tool to generate highly charged heavy ion plasma and beam

Between 1993 and 1996 a 14 GHz electron cyclotron resonance (ECR) ion source has been designed and built in the ATOMKI. The ion source is aimed for the production of highly charged heavy ion beams from a wide range of the elements. One of the most important planned applications in the near future is its usage for low energy ion-atom collisions. This ion source is the first ECR ion source in Hungary and in Middle-Eastern Europe as well. The first plasma was successfully generated in February 1996 and the first extracted beam is expected in the fall of the year. This paper briefly reviews the history and progress of the ECR program and shows the ECR ion source itself. The results of some indirect plasma measurements (X-ray spectra) are shown and discussed.

CONTINUUM X-RAYS PRODUCED BY 60 keV- AND 80 keV- PROTON BOMBARDMENT

We measured continuum x-rays from an aluminium target bombarded with 60 keV- and 80 keV- proton beams. On the basis of the PWBA theory, we calculated the thick target yields of atomic bremsstrahlung and nuclear bremsstrahlung produced in very low energy ion-atom collisions and compared with the experiment. The present theory predicts that the main component of continuum x-rays produced in such low energy ion-atom collisions is the nuclear bremsstrahlung. The theory presents the yields of continuum x-rays about 5 times larger than the experimental ones, however, reproduces well the spectral shape.

Collisions of highly charged ions with electrons, atoms and surfaces

At the Oak Ridge Multicharged Ion Source Facility, an experimental atomic collisions physics program is centered around a recently upgraded Electron Cyclotron Resonance (ECR) multicharged ion source. The 10 GHz CAPRICE source has been in operation since October 22, 1992, and has provided more intense, higher charge ion beams than our previous ECR ion source. Intense metallic beams have recently become available with the installation of a metallic oven on the source. In addition to measurements of electron-impact excitation, carried out in collaboration with the Joint Institute for Laboratory Astrophysics (JILA), experiments are presently on-line to study electron-impact ionization, low-energy ion-atom collisions, and ion-surface interactions. A brief summary of our various activities with an emphasis on the new capabilities is presented.

Coulomb "path" interference in low-energy ion-atom collisions.

A new interference mechanism, analogous to ``classic'' double-slit electron scattering, has been identified in low-energy ion-atom collisions. This Coulomb ``path'' interference results from the existence of two trajectories, indistinguishable with respect to laboratory energy and emission angle, along which ejected autoionizing electrons may be scattered by the attractive Coulomb potential of the slowly receding spectator ion. A simple semiclassical description is presented. Calculated model line shapes are in excellent ageement with the strong angular dependence of the interference structure observed in the He-target 2${\mathit{s}}^{2}$ $^{1}$S autoionizing line shape measured near 0\ifmmode^\circ\else\textdegree\fi{} following 10-keV ${\mathrm{He}}^{+}$+He collisions.

Anisotropy Parameters and Adiabatic Approximations for Two-electron Atoms and lons

In dynamial processes atomic systems evolve from a condensation region at small distances where all particles are close together to an asymptotic region where some of the constituent particles are free and accessible to measurement. This dynamical evolution is characterised by the Jost matrix. Evaluation of the Jost matrix generally involves complex calculations, but considerable simplification is achieved when the evolution can be described in terms of adiabatic or diabatic potential curves. For low energy ion-atom and atom-atom collisions standard molecular potential curves have long been used. For low energy electron-atom scattering and photo-ionisation similar molecular-like potential curves have been proposed. There is no a priori justification for the adiabatic approach in these latter systems, thus confrontation with experiment is crucial for further development of this theory. Anisotropy parameters represent a particularly appropriate probe of the various adiabatic representations. This is illustrated by studies of photo-ionisation of helium at the n = 2 threshold. Potential curve crossings are important here and their relevance to the anisotropy parameters is illustrated

Triple-center treatment of ionization inp−Hcollisions

A triple-center atomic-state method is proposed for treating ionization in low-energy ion-atom collisions. This method intrinsically accounts for the Wannier mechanism, in which the electron will not have been removed in a slow collision unless it is asymptotically at the point of unstable equilibrium between the nuclei (the chosen third center). When applied to p-H collisions, the method shows that most of the ionized electrons stay near the third center and that direct Coulomb ionization and charge transfer to the continuum: accounted for by normal double-center methods: are less important: The triple-center method dramatically raises the cross section above that obtained using a double-center pseudostate basis, bringing it closer to the single early experimental curve at energies below 15 keV.

Empirical State-Selection Rules for Electron Capture in Low Energy-Ion-Atom Collisions

Experimental evidence for state-selective one-electron capture in low energy ion-atom collisions shows these processes probably to follow some empirical rules which are presented and discussed. Making use of these selection rules we analyze some recent results on one-electron capture in Ne+/Ne2+-Li and O+-Li collisions which all involve probably transfer for a single "active" electron only.

K-L and L-K Vacancy Sharing in Ion-Atom Collisions

The molecular model has been very successful in explaining the production of atomic vacancies in low energy ion-atom collisions. One area of active investigation involves the sharing of inner-shell vacancies between two collision partners. The sharing results when a vacancy, which has been produced at some smaller internuclear distance, is transferred from one molecular orbital (MO) to another during the separation of the two collision partners. K-shell vacancy sharing in near symmetric collisions due to radial coupling between the 1sσ and 2pσ MOs has been studied extensively. Meyerhof derived a simple formula using a parameterization of the Demkov formalism, which was very successful in explaining a large body of data on K-K vacancy sharing. In this paper I will summarize the current situation involving sharing of K and L vacancies in highly asymmetric collisions (Z1/Z2 ≥2).

Interference Structure in Ion-Atom Collisions

In low-energy ion-atom collisions, electronic excitation occurs dominantly via non-adiabatic transitions between quantum states of the quasimolecule (collision complex) formed momentarily during the collision. A growing list of cases have been found by several groups in which collision-induced optical emission of particular target or projectile spectral lines oscillates as a function of bombarding energy. These "Rosenthal oscillations" result from multiple excitation paths where there is long range mixing. In the light emitted from certain states of helium atoms excited in He+-He collisions, we have seen these oscillations emerge dramatically in the velocity-dependence of the polarization fraction of the light, even though they were not obvious in the total emission cross-section. In particular, from the sign of the polarization of light from the 3, D states, one can infer that rotational coupling in the excitation is nearly maximal, leading to strong collisional alignment. The interpretation of these oscillation patterns is discussed with particular reference to polarization effects. Possible applications to spectroscopy of collision complexes and to the analysis of the first few monolayers of solid surfaces will be described.

Symmetric charge transfer in low-energy ion-atom collisions

Previous calculations of ion-atom interactions by the pseudopotential and asymptotic methods are used in the computation of the cross section for symmetric charge transfer at energies below 1 keV. The results for ${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{Rb}}^{+}$, ${\mathrm{Cs}}^{+}$, and ${\mathrm{Ca}}^{+}$ ions are compared with data obtained in beam experiments, and by optical-pumping techniques. The difference in the cross sections for $^{2}P_{\frac{1}{2}}$ and $^{2}P_{\frac{3}{2}}$ ions of ${\mathrm{Kr}}^{+}$ and ${\mathrm{Xe}}^{+}$ at thermal energies is studied, and the predictions are compared with recent mobility measurements. Cross sections are obtained for ${\mathrm{U}}^{+}$-U collisions, and the dependence of the thermal cross section on the polarizability is described. Symmetric charge transfer of the negative ions ${\mathrm{H}}^{\ensuremath{-}}$, ${\mathrm{Na}}^{\ensuremath{-}}$, and ${\mathrm{Cs}}^{\ensuremath{-}}$ is discussed briefly.