Energetic Ion-atom Collisions Research Articles

Determination of the state production ratios , and from fast () mixed-state He-like ion beams in collisions with H2 targets

New results are presented on the ratio concerning the production cross sections of Li-like quartet and doublet P states formed in energetic ion–atom collisions by single electron transfer to the metastable component of the He-like ion beam. Spin statistics predict a value of Rm = 2 independent of the collision system in disagreement with most reported measurements of . A new experimental approach is presented for the evaluation of Rm having some practical advantages over earlier approaches. It also allows for the determination of the separate contributions of ground- and metastable-state beam components to the measured spectra. Applying our technique to zero-degree Auger projectile spectra from 4.5 MeV (Benis et al 2002 Phys. Rev. A 65 064701) and 25.3 MeV (Zamkov et al 2002 Phys. Rev. A 65 062706) mixed state He-like ion collisions with H2 targets, we report new values of for boron and for fluorine. In addition, the ratios of and populations from either the metastable and/or ground state beam component, also relevant to this analysis, are evaluated and compared to previously reported results for carbon collisions on helium (Strohschein et al 2008 Phys. Rev. A 77 022706) including a critical comparison to theory.

Fraunhofer-type diffraction patterns of matter-wave scattering of projectiles: Electron transfer in energetic ion-atom collisions

Fraunhofer-type diffraction patterns of matter-wave scattering of projectiles: Electron transfer in energetic ion-atom collisions

Four-body methods for high-energy ion-atom collisions

The progress in solving problems involving nonrelativistic fast ion (atom)-atom collisions with two actively participating electrons is reviewed. Such processes involve, e.g., (i) scattering between a bare nucleus (projectile) $P$ of charge ${Z}_{P}$ and a heliumlike atomic system consisting of two electrons ${e}_{1}$ and ${e}_{2}$ initially bound to the target nucleus $T$ of charge ${Z}_{T}$, i.e., the ${Z}_{P}\text{\ensuremath{-}}{({Z}_{T};{e}_{1},{e}_{2})}_{i}$ collisions; (ii) scattering between two hydrogenlike atoms ${({Z}_{P},{e}_{1})}_{{i}_{1}}$ and ${({Z}_{T},{e}_{2})}_{{i}_{2}}$, etc. A proper description of these collisional processes requires solutions of four-body problems with four active particles including two nuclei and two electrons. Among various one- as well as two-electron transitions which can occur in such collisions, special attention will be paid to double-electron capture, simultaneous transfer and ionization, simultaneous transfer and excitation, single-electron detachment and single-electron capture. Working within the four-body framework of scattering theory and imposing the proper Coulomb boundary conditions on the entrance and exit channels, the leading quantum-mechanical theories are analyzed. Both static and dynamic interelectron correlations are thoroughly examined. The correct links between scattering states and perturbation potentials are strongly emphasized. Selection of the present illustrations is dictated by the importance of interdisciplinary applications of energetic ion-atom collisions, ranging from thermonuclear fusion to medical accelerators for hadron radiotherapy.

Electron emission in energetic ion–atom collisions in the presence of coherent electromagnetic radiation

We consider the effect of a low-frequency electromagnetic field on the spectra of electron emission in energetic nonrelativistic ion–atom collisions. The field is assumed to have linear polarization and to be weak compared to the typical atomic field. The incorporation of the projectile interaction opens a new scenario to the combined study of electromagnetic and atomic interactions. Our work suggests that the electromagnetic field can have a profound effect on two of the most important structures that appear in the electron emission spectra: the electron capture into the continuum and binary encounter (BE) peaks. We show that in the BE peak region the result for the laser-assisted scattering depends on the theory applied for the collision part, the first Born approximation or the distorted wave Born approximation.

Experimental observation and theoretical calculations of triply excited2s2p22Se,2,4Pe,2Deand2p32Po,2Dostates of fluorine

An alternative experimental technique for efficient population of triply excited states in atoms and ions is presented. The states are produced by triple electron capture in energetic ion-atom collisions. The method is used to study autoionizing ${2s2p}^{22}S,$ ${}^{2}P,$ ${}^{2}D,$ ${}^{4}P$ and ${2p}^{32}P,$ ${}^{2}D$ states of fluorine. Differential cross sections, resonance energies, and Auger decay branching ratios were determined from the experiment. Results are compared to different calculations based on the hyperspherical close coupling method.

Excitation in ion-atom collisions inside subfemtosecond laser pulses.

We discuss new excitation mechanisms in energetic ion-atom collisions embedded in short laser pulses. For comparable duration and strength of the pulse and collisional interaction, the laser field will probe and modify the interaction between projectile and target. Coherence effects emerge, insight into reaction dynamics is gained, and new dynamical features are discovered. As an example, we show (i) how a propensity rule for s-p excitation can be dramatically changed, and (ii) how the presence of the laser pulse modifies the ionization process in ion-atom collisions.

Multiple-scatteringanalysis of triply differential cross sections forelectron emission in energetic ion-atom collisions

The role of multiple-scattering events has beeninvestigated in single ionization of atoms induced byenergetic ion impact. The triply differential crosssections have been evaluated as a function ofthe projectile scattering angle for fixed values of theemitted electron energy and angle within the framework ofthe distorted-wave Born approximation (DWB). It has beenshown that the double-scattering mechanism, which(sometimes) results in a characteristic double-scatteringpeak at a definite projectile scattering angle, provides asatisfactory description of the process over a wideregion of electron ejection energies and angles. Forlow-energy electron emission the role of higher orders isnot negligible, and the quantum mechanical nature of theprocess is more emphasized. At an electron velocity equalto the projectile velocity the analogy with the mechanismof electron capture into bound states has also beeninvestigated. In this region the DWB theory forionization corresponds to theOppenheimer-Brinkman-Kramers model for electron capture.

Differential electron-Cu5+elastic scattering cross sections extracted from electron emission in ion-atom collisions

We present a method of deriving energy and angle-dependent electron-ion elastic scattering cross sections from doubly differential cross sections for electron emission in ion-atom collisions. By analyzing the laboratory frame binary encounter electron production cross sections in energetic ion-atom collisions, we derive projectile frame differential cross sections for electrons elastically scattered from highly charged projectile ions in the range between 60{degree} and 180{degree}. The elastic scattering cross sections are observed to deviate strongly from the Rutherford cross sections for electron scattering from bare nuclei. They exhibit strong Ramsauer-Townsend electron diffraction in the angular distribution of elastically scattered electrons, providing evidence for the strong role of screening played in the collision. Experimental data are compared with partial-wave calculations using the Hartree-Fock model. {copyright} {ital 1999} {ital The American Physical Society}

Influence of electronic exchange on single and multiple processes in collisions between bare ions and noble-gas atoms

The time-dependent many-electron problem encountered in the theory of energetic ion-atom collisions can be approached systematically within an effective single-particle picture, based, e.g., on the time-dependent density functional theory. In the present contribution we investigate ionization and electron loss from the target for the collision systems p, $\overline{p}$, ${\mathrm{He}}^{2+}+\mathrm{Ne}$, and $p+\mathrm{Ar}$ in the energy range 10 keV/amu--1 MeV/amu. The results presented are based on a static approximation to the exchange-correlation contribution of the effective potential. The corresponding time-dependent single-particle equations are solved with the continuum distorted-wave with eikonal initial-state approximation and the basis generator method, a coupled-channel approach in terms of structurally adapted basis functions. We demonstrate that in either case the results depend strongly on the explicit form of the local static exchange potential. An orbital-dependent analysis of electron removal in ${\mathrm{He}}^{2+}+\mathrm{Ne}$ collisions shows that the quality of the exchange potential is decisive for the suppression of artificial reaction channels.

Differential electron-ion elastic scattering cross sections extracted from ion-atom collisions of 0.53 MeV/u Cu5+ on H2

A method of extracting electron-ion elastic scattering cross sections from ion-atom collisions has been developed. By analyzing the binary encounter electron (BEe) production in energetic ion-atom collisions, which is due to loosely bound target electrons ionized through direct, hard collisions with the projectile ions, differential cross sections of electrons elastic scattered from highly charged ions are derived for a broad range of scattering angles. The cross sections are observed to deviate strongly from the Rutherford cross sections, and immediately yielded an electron diffraction in angular distribution of elastically scattered electrons. Experimental data are compared with a partial-wave treatment using the Hartee-Fock model.

Two-center electron-electron interactions in collisions of fast Ne 7+ and Ne 6+ ions with gas atoms

Experimental and thoeretical studies of the two-center electron-electron interaction occurring during energetic ion-atom collisions are presented. Experiments were performed using the accelerator facility VICKSI at HMI, Berlin. Electron spectra from Auger and Coster-Kronig transitions following the excitation of a highly charged neon projectile have been studied by zero-degree electron spectroscopy. Various target atoms with atomic numbers ranging from 1–18 were used as exciting partners. We applied both impact parameter treatments and the plane wave Born approximation to describe screening effects and dielectronic two-center processes (“antiscreening”). We found that the 1s-2 l excitation of the Ne 7+ projectile at 8.5 MeV/u impact energy is mainly due to the screened target nucleus. In contrary, two-center processes are likely to be dominant in the 2s- nl excitation of the 6 MeV/u Ne 6+ projectile. The 2s- nl cross sections are roughly proportional to the number of electrons per target atom. This fact suggests that, in disagreement with first order theories, the 2s- nl excitation is significantly governed by the two center electron-electron interaction.

A comprehensive theory of electron emission in ion-atom collisions

The channel-distorted wave method is combined with a two-centre description of the ejected electron state to embrace target ionization as well as capture-to-continuum aspects of the ionization process in energetic ion-atom collisions. The target-ionization peak, the electron-capture-to-continuum cusp and the binary ridge appear in the appropriate regions of momentum space of the ejected electron but each one of these features is perturbed by the presence of the other collision partner. In the cusp and in the binary-ridge regions, the essential effect of the residual target ion is represented by a modified Compton profile in which the momentum distribution of the initial electron state has been replaced by a corresponding form factor. Similarly, in the region of low electron energies with respect to the target nucleus, the standard expression for target ionization is modulated by a suitable off-energy-shell factor. The present theory agrees well with experimental data.

Evidence for coherent electron motions in multiphoton X-ray production from Kr and Xe clusters

Studies of multiphoton-induced X-ray generation in Kr and Xe clusters give direct information concerning (1) the atom-specific energy transfer rate, (2) the dependence of the X-ray yield on the strength of the intra-cluster inelastic electron scattering cross section, and (3) the threshold intensity for X-ray generation. Measurements of these three classes of observables with subpicosecond ( approximately 300 fs) ultraviolet (248 nm) radiation at a maximum intensity of approximately 1019 W cm-2 all indicate that the non-linear coupling to the cluster has an anomalous strength with respect to that derivable from conventional single-particle interactions. Five cases have been examined (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)), spectrally spanning the range from approximately 80 eV to approximately 5 keV. In order to reconcile theoretical estimates with these experimental findings, three generalizations of the original formulation of the interaction were necessary. The most important involves an enhancement in the coupling arising from the coherent motion of the (Z) field-ionized electrons induced by the external driving field. The coherently energized electrons act like a quasi-particle possessing a charge Ze and a mass Zm, thereby presenting a sharply augmented coupling resembling that associated with energetic ion-atom collisions. The second involves the process of multiple electron ejection from an inner-shell, a mechanism that was found imperative to interpret the high level of ionization observed in the Xe M-shell. The third modification concerns multiple transits of the driven electrons in the cluster. The inclusion of these considerations consistently brings the theoretical analysis into agreement with the three measured properties. These results also indicate that energy deposition rates exceeding approximately 1 W/atom are feasible in appropriately designed molecules incorporating heavy atoms. The limiting magnitude of the excitation energy Delta Emax characteristic of the coherent coupling is estimated to be in the range Zamc2<or= Delta Emax<or=Z2amc2, a bound that can considerably exceed the K-shell binding energy of the heaviest atoms.

Time ordering of two-step processes in energetic ion-atom collisions: Basic formalism.

The semiclassical approximation is applied in second order to describe time ordering of two-step processes in energetic ion-atom collisions. Emphasis is given to the conditions for interferences between first- and second-order terms. In systems with two active electrons, time ordering gives rise to a pair of associated paths involving a second-order process and its time-inverted process. Combining these paths within the independent-particle frozen orbital model, time ordering is lost. It is shown that the loss of time ordering modifies the second-order amplitude so that its ability to interfere with the first-order amplitude is essentially reduced. Time ordering and the capability for interference is regained, as one path is blocked by means of the Pauli exclusion principle. The time-ordering formalism is prepared for papers dealing with collision experiments of single excitation [Stolterfoht et al., following paper, Phys. Rev. A 48, 2986 (1993)] and double excitation [Stolterfoht et al. (unpublished)].

Impulse approximation treatment of electron-electron excitation and ionization in energetic ion-atom collisions

The effect of electron-electron interactions between projectile and target electrons observed in recent measurements of projectile K-shell excitation [T.J.M. Zouros et al., Phys. Rev. Lett. 62 (1989) 2261] and ionization [T.J.M. Zouros et al., Nucl. Instr. and Meth. B56/57 (1991) 107 and D.H. Lee et al., Phys. Rev. A46 (1992) 1374] using 0° projectile Auger electron spectroscopy are analysed within the framework of the impulse approximation (IA) [D. Brandt, Phys. Rev. A27 (1983) 1314]. The IA formulation is seen to give a good account of the threshold behavior of both ionization and excitation, while providing a remarkably simple intuitive picture of such electron-electron interactions in ion-atom collisions in general. Thus, the applicability of the IA treatment is extended to cover most known processes involving such interactions including resonance transfer excitation, binary encounter electron production, electron-electron excitation and ionization.

Dynamic aspect of electron correlation in atomic collisions: The role of dielectronic processes

Electron correlation effects are studied in the semi-classical approximation relevant for energetic ion-atom collisions. The basic formalism for dynamic electron correlation is presented in analogy with the corresponding stationary aspect of configuration mixing. The dynamic aspect of electron correlation is shown to manifest itself in dielectronic processes, which are two-electron phenomena induced by the electron-electron interaction. First-order perturbation theory is used to describe the time-dependent coupling of states which differ by two spin orbitals. For specific cases, analytic formulas are evaluated to estimate the strength of the dielectronic processes. A brief survey over the experimental studies of electron correlation in ion-atom collisions is given. Model calculations are discussed for the process of autoexcitation and dielectronic excitation in comparison with previous experimental results.

Effect of electron-electron interaction on projectile K-shell ionization in energetic ion-atom collisions studied by 0° Auger electron spectroscopy

Cross sections for 1s ionization of the ground state of O 4(1s 22s 2) were obtained in 10–30 MeV collisions of O 4+ with H 2 and He targets. High resolution Auger electron spectroscopy was used to detect at 0° with respect to the beam direction, the electrons emitted from the Auger decay of O 5+ (1s2s 2) → O 6+ (1s 2) + e − following 1s ionization of the O 4+ ions. The electron-nucleus ( σ enI) and electron-electron ( σ eeI) ionization cross sections, due to ionization of the projectile electron either by the target nucleus or the target electrons, respectively, were independently computed. σ enI was calculated within a PWBA treatment, while σ eeI was computed within an impulse approximation (IA) treatment of electron impact ionization of O 4+. The incoherent sum σ enI + σ eei was found to be in good agreement with the data. K-shell ionization of the metastable O 4+(1s 22s2p 3P) component of the beam was also observed.

Dielectronic processes and electron correlation in energetic ion-atom collisions

Concepts of electron correlation in energetic ion-atom collisions are discussed in terms of dielectronic processes which are due to two-electron transitions produced by the electron-electron interaction. These dynamic processes are associated with configuration interaction occurring during the collision in the four-body ion-atom system or after the collision in the atomic three-body system. As in previous work, formal aspects of electron correlation in ion-atom collision are introduced in analogy with the treatment in atomic-structure theory. The full electron-electron interaction is partitioned into a mean potential of the passive electrons postulated within the framework of the independent particle model and the residual mutual interaction between two active electrons. Various cases of the interaction of electrons located initially at different nuclear centers are treated. As an example for a dielectronic process, the Bates-Griffing scattering mechanism is discussed concerning the interactions of two electrons centered at the projectile and the target. Furthermore, interferences between the nucleus-electron and electron-electron interaction are analyzed for the process of double excitation.

Electron transfer in nuclear reactions

The influence of nuclear reactions on atomic charge transfer in energetic ion-atom collisions is studied within a two-channel model for the reactions and the impulse approximation for electron capture. Large interference structures in the capture probability are found for the test reactions 12C(d,p)13C, 180(p,∝)15N, 19F(p,∝)160 and 32S(p,p′)32S.

Influence of nuclear reactions on electron transfer in energetic ion-atom collisions.

The influence of inelastic nuclear scattering on atomic charge transfer in asymmetric ion-atom collisions is described within a new theory, combining a two-channel model for the nuclear reaction with the impulse approximation for electron capture by the projectile. Test calculations of the capture probability during the reaction {sup 18}O({ital p},{alpha}){sup 15}N indicate that inelastic nuclear reactions are a more transparent probe for the atomic transition amplitudes than elastic resonances.