Electron Capture To The Continuum Research Articles

Experimental evidence of the target excitation in transfer ionization of He2+ on argon collisions

We have investigated the dependence of electron capture to the continuum (ECC) on the longitudinal momentum of recoil ions in the transfer ionization of 120 and 300 keV He2+ on argon collisions. Compared with theoretical lines predicted by the kinematical relationship between longitudinal momentum of recoil ions and ECC electrons, it is shown that the electron is mainly transferred to the first excited state of He+ ions. The experimental data also show that the electron capture to the ground state of He+ is accompanied by target excitation, where the target excitation results from direct excitation in the transfer ionization processes.

Multiphoton ionization of a metastable hydrogen atom by electron and positron impact and charge asymmetry

Charge asymmetry is studied theoretically for multiphoton ionization (nγ e±, e e±) of a hydrogen atom from its metastable (2S) state using coplanar geometry. The external laser field is chosen to be single-mode, spatially homogeneous, linearly polarized with laser intensity that is quite high by laboratory standards. The continuum states of the impinging electron (e) or positron (e+) are represented by plane wave Volkov states, while the wavefunctions for the ejected electron and the scattered electron/positron are chosen to be of Coulomb–Volkov type. The dressed wavefunction for the metastable (2S) hydrogen is constructed in the framework of degenerate perturbation theory. Charge asymmetry is noted in the triple differential cross sections (TDCS) of the two projectiles both in the field-free (FF) and laser-assisted situations. The positron binary peak intensity is much higher than that of the electron, while the reverse is true for the recoil peak. As for the laser modifications, the single-photon TDCS exhibits a distinct four lobed structure in contrast to the FF (two lobed) and is highly suppressed w.r.t. the latter, while the multiphoton TDCS usually tends to approach the FF (with some exceptions), obeying the famous Kroll–Watson (KW) sum rule for both projectiles. For e+ impact, the electron capture to the continuum (ECC) effect is also studied (for the symmetric geometry), where enhancement is noted in both the single and multiphoton ECC peaks thereby deviating from the KW sum rules.

Longitudinal Recoil Ion momentum distributions for proton-argon ionizing collisions at intermediate energies

Proton-argon ionizing collisions at intermediate energies are studied by means of the classical trajectory Monte Carlo method (CTMC). In particular, we focus on the longitudinal and transverse recoil momentum distributions for the process. We compare the profile of the longitudinal momentum distribution at the abrupt rise region corresponding to the electron capture to the continuum (ECC) processes, with those obtained for simpler targets like H and He. Selected subsets of the data show that the recoiling multielectronic ion plays a major role in the low energy electron emission, influencing the angular asymmetry of the electronic emission.

Projectile focusing near the recoil-ion threshold

The post-collisional interactions in ion-atom ionization collisions are studied around the electron capture to the continuum (ECC) process. For this purpose, a suitable double differential cross section is introduced, involving the longitudinal recoil-ion momentum and the projectile transverse momentum transfer. Using the fact that the ECC process is closely related to the threshold in the longitudinal momentum distribution, we study this distribution as a function of the projectile scattering angle. Using the CDW-EIS approximation we theoretically find a focusing (defocusing) effect as we get closer to the distribution threshold for proton (antiproton) impact on He atoms.

Cusp formation in classical trajectory Monte-Carlo calculations of single atomic ionization by the impact of neutral projectiles

The Classical-trajectory Monte-Carlo (CTMC) model provides an excellent description of the electron capture to the continuum (ECC) cusp in atomic ionization collisions whenever the electron-projectile interaction is of a Coulomb or even of a dipolar type. However, in this communication we show that this description fails for the case of a polarizability potential, such as in the one produced by a neutral He (21S) outgoing projectile. Actually the CTMC calculation predicts an ECC peak that is much broader and smaller than for a Coulomb interaction, a result that differs from experimental data and quantum-mechanical calculations.

Divergency problem of the electron capture to the continuum cusp: Classical trajectory Monte Carlo simulation and experimental data

The question whether the velocity of the projectile and that of the electrons at the maximum of the ECC (electron capture to the continuum) cusp are equal is analyzed by means of classical mechanical calculations. The analysis is made for collisions of 20 keV protons with He atoms. Our theoretical results as well as experimental data do not support the large energy shift of the ECC cusp observed recently by Shah et al. [Phys. Rev. A 67, 010704(R) (2003)].

On the divergency problem of the ECC cusp: A final state interaction theory

We demonstrate, as asserted against common knowledge in recent papers by Illescas et al. [Phys. Rev. A 65 (2002) 030703] and Shah et al. [Phys. Rev. A 67 (2003) 010704], that the ECC (Electron Capture to the Continuum) cusp does not diverge and does not exactly peak at the projectile velocity. However, at the same time, we show that to experimentally observe these effects the equipment resolution or the projectile velocity would have to be reduced below prohibitively small values. In particular, both effects would not be observable at the lowest energy that was considered by Illescas et al. and Shah et al.

Shifting of the electron-capture-to-the-continuum peak in proton-helium collisions at 10 and 20 keV

A refined theoretical approach has been developed to study the double-differential cross sections (DDCS's) in proton-helium collisions as a function of the ratio of ionized electron velocity to the incident proton velocity. The refinement is done in the present coupled-channel calculation by introducing a continuum distorted wave in the final state coupled with discrete states including direct as well as charge transfer channels. It is confirmed that the electron-capture-to-the-continuum (ECC) peak is slightly shifted to a lower electron velocity than the equivelocity position. Comparing measurements and classical trajectory Monte Carlo (CTMC) calculations at 10 and 20 keV proton energies, excellent agreement of the ECC peak heights is achieved at both energies. However, a minor disagreement in the peak positions between the present calculation and the CTMC results is noted. A smooth behavior of the DDCS is found in the present calculation on both sides of the peak whereas the CTMC results show some oscillatory behavior particularly to the left of the peak, associated with the statistical nature of CTMC calculations.

Theoretical study of cusp-electron emission from O8+ + Ar collisions

Cusp-electron emission via processes electron capture to the continuum (ECC) and transfer ionization (TI) has been investigated theoretically in 0.3–2 MeV/amu O8+on Ar collisions. Doubly differential cross-sections for ECC and TI were calculated by using the classical trajectory Monte Carlo method within the framework of the independent particle approximation. The TI/ECC cusp-intensity ratio was found to have a maximum at about 1 MeV/amu impact energy, in accordance with the previous experimental observation. The maximum is due to the increasing contribution of the L shell of argon to the cusp-electron production with increasing collision velocity.

Correlated eikonal initial state in ion-atom collisions

An approximation is developed to deal with the ionization of atoms by bare charged ions. In this method the transition amplitude describing the three-body final state is evaluated using a continuum correlated wave and that for the initial state by an analytical continuation of the ${\ensuremath{\Phi}}_{2}$ model to complex momenta. This procedure introduces in the atomic bound state a kinematical correlation with the projectile motion. Doubly differential cross sections (DDCS's) are computed for collisions of ${\mathrm{H}}^{+}$ and ${\mathrm{F}}^{9+}$ ions with He atoms. Results for the DDCS's in the forward direction are compared with experimental data and other theoretical models. We find an enhancement of the distribution for low energy electrons and that the asymmetry of the electron capture to the continuum (ECC) peak is correctly described.

Classical description of the electron capture to the continuum cusp formation in ion-atom collisions

Classical calculations are used to describe the dynamics of the electron capture to the continuum (ECC) cusp formation in ${\mathrm{H}}^{+}+\mathrm{He}$ collisions. We illustrate the frontier character of the ECC electrons between capture and ionization, and confirm that it is a temporary capture, through projectile focusing, that is responsible for the ECC cusp. Furthermore, the cusp is not a divergence smoothed by the experiment, and is slightly shifted from the impact-velocity value because of the residual pull from the target after ionization. This shift is larger the smaller the nuclear velocity.

Study of the transfer ionizationprocess by observing the electron cusp in 100-300 keVHe2+ + He collisions

Cusp-electron emission in He2+ on He collisions has beeninvestigated in the energy range 100-300 keV. By detecting theelectrons in coincidence with the charge-state-analysed outgoingHe2+ and He+ ions, the processes of electron capture to thecontinuum (ECC), and ECC accompanied by bound-state capture (transferionization, TI) were identified, respectively. The ratios of yieldsfor the TI and ECC cusps are found to increase steeply withdecreasing projectile energy. At 100 keV the obtained TI/ECC ratio(1.42±0.14) is almost two times larger than the correspondingratio measured in a previous experiment for an Ar target at the sameimpact energy. The measured data are compared with calculationscarried out in the independent particle model with use of theclassical trajectory Monte Carlo method and thecontinuum-distorted-wave theory. The disagreement observed betweenthe theory and experiment indicates the presence of electroncorrelation effects in TI.

Abrupt rise of the longitudinal recoil ion momentum distribution for ionizing collisions.

We report on the experimental observation of an abrupt rise in the longitudinal momentum distribution of recoil ions created in proton helium collision. The details of this structure can be related to electrons traveling with the velocity of the projectile [electron capture to the continuum (ECC)]. The longitudinal as well as the transverse distribution of the recoil ions can be explained as a continuation of the momentum distribution from ions resulting from electron capture illustrating the smooth transition from the capture to bound states of the projectile to the ECC.

Energy spectra of forward-ejected electrons in multiple ionization of Arby 1 MeV protons

Time-of-flight (TOF) techniques have been used to measuresingly and multiply ionized Ar recoils in coincidence withenergy-analysed, forward-ejected electrons in 1 MeVH+-Ar collisions. Electron energies up to2800 eV and charge states 1-4 are covered. It is shown experimentallythat the peak of binary-encounter (BE) electrons in the electron energyspectrum is associated almost entirely with single ionization of Ar,whereas the peak of electron-capture-to-the continuum (ECC) electrons isassociated with multiple ionization. This shows that the BE electronsoriginate exclusively from the valence shell of Ar (the M shell) and theECC electrons predominantly from an inner shell (the L shell).

Electron-capture-to-the-continuum by positron impact

The process of electron-capture-to-the-continuum (ECC) is discussed for positrons and other projectiles. The work leading to the first observation of this phenomenon in the case of positron impact is outlined. The resulting first experimental triple-differential cross-section is compared with recent theories.

Electron capture to the continuum induced by dipolar interaction

A three-body classical trajectory Monte Carlo method has been used to investigate the process of electron capture to the continuum (ECC) by dipole particles impinging on hydrogen atoms. The projectile consisted of two particles of opposite charge separated by a fixed distance. The orientation of the dipole was also fixed. The calculations were carried out for an impact velocity of 1 au. It was found that the energy spectrum of the electron ejected via the dipole interaction exhibits a cusp peak at 0° observation angle. The cusp formation was studied as a function of the length and orientation of the dipole. A particularly intense cusp was observed when the positive charge was the leading particle in the dipole. Analysis of individual electron trajectories revealed that quasi-stationary two-centre orbits play an important role in the formation of the ECC peak.

Local saddles and local maxima in double differential cross sections of single ionization in ion-atom collisions

The ejected-electron spectrum for H+ incident on H2 and He at 40 keV is examined experimentally and theoretically. The continuum-distorted-wave eikonal-initial-state (CDW-EIS) model is adopted to study the ionization process. Experimentally a 260 mm diameter hemispherical analyser with half-angle acceptance of 1.5° and energy resolution E/E of 0.012, is used to measure the velocity distribution of the emitted electrons. The analyser and the surrounding interaction region are double shielded against the Earth's magnetic field by use of µ-metal enclosures. The observations and calculations, for double differential cross sections (differential with respect to electron momentum and emission angle), are in very good agreement and show a spectrum dominated by a well defined electron capture to the continuum (ECC) peak although existence of saddle-point electrons at 0° is not confirmed.

Complete single-ionization momentum spectra for strong perturbation collisions

The combination of recoil ion and ionized electron momentum spectroscopy provides an unparalleled method to investigate the details of ion-atom collision dynamics in kinematically complete experiments. To predict single ionization scattering behavior at the level now realized by experiment, the classical trajectory three-body Monte Carlo method has been used to obtain complete momenta information for the ionized electron, recoil ion, and projectile in the collision plane defined by the incident projectile and outgoing recoil ion. Strongly coupled systems were considered where the charge state of the projectile divided by the speed of the collision $q/v$ is greater than unity. Illustrated are 3.6-MeV/nucleon ${\mathrm{Se}}^{28+}$ and 9.5-MeV/nucleon ${\mathrm{Ni}}^{26+}$ collisions on He where experimental data are available. The theoretical results are in good agreement with these data and calculations have been performed for 165-keV/nucleon and 506-keV/nucleon ${\mathrm{C}}^{6+}+\mathrm{He}$ to compare results for the same $q/v$ perturbation strengths, but at much lower velocities. In all cases the ejected electrons are found to be preferentially emitted opposite to the recoil ion in the projectile-recoil collision plane. The 165-keV/nucleon ${\mathrm{C}}^{6+}$ spectra are especially rich in that electron capture strongly contributes to the overall electron loss process. Here, the electron capture to the continuum (ECC) spectrum is observed to have not only the known asymmetry in the longitudinal direction, but also has an almost complete asymmetry in the collision plane opposite to the recoil ion. Collision plane spectra differential in the transverse momenta of the recoil ion depict the transition from soft electrons for low transverse recoil momenta, to two-center, and ECC electrons for increasing transverse recoil ion momenta.

Evidence for electron capture to the continuum by protons scattered at non-0° angles from Ar atoms

The process of electron capture to the continuum (ECC) has been studied at non-0\ifmmode^\circ\else\textdegree\fi{} electron emission angles in a type of experiment in which the electrons ejected in a given direction are detected in coincidence with the outgoing projectiles scattered on the target nucleus in the same direction. Triply differential cross sections (differential with respect to the energy and angle of the ejected electron and the angle of the scattered projectile) have been determined for 75-keV proton on Ar collisions at scattering angles 3.7\ifmmode^\circ\else\textdegree\fi{}, 5.0\ifmmode^\circ\else\textdegree\fi{}, and 8.1\ifmmode^\circ\else\textdegree\fi{}. The occurrence of the cusp peak in the coincidence spectra gives an evidence for the existence of the ECC process at non-0\ifmmode^\circ\else\textdegree\fi{} emission angles. The experimental data are compared with results of first-order distorted-wave Born and classical trajectory Monte Carlo (CTMC) calculations.

Cusp-electron production due to transfer ionization in 25–250 keV/amu He 2+ + Ar collisions

Cusp-electron emission has been investigated in 25–250 keV/amu He 2+ on Ar collisions. Two processes which contribute to cusp electrons for this collision system, Electron Capture to the Continuum (ECC) and Transfer Ionization (TI), were distinguished by detecting the electrons in coincidence with the charge-state analyzed outgoing He 2+ and He + projectiles, respectively. The ratios of the electron yields for the TI and ECC channels were determined over the energy range investigated. These ratios can provide information on the role of the electron-electron interaction in the two-electron TI process. The measured TI ECC ratios were compared to the measured total double- to single-bound state capture cross-section ratios. These ratios are found to be consistent with a simple relationship based on the independent particle model, indicating a lack of electron-correlation effects for cusp-electron production by TI in these collisions.