Electron Emission In Collisions Research Articles

Fast electron contribution in electronic stopping for p-He and p-Ar collisions

Our study has shown that in collisions p-He and p-Ar with energies above 50 keV fast electrons carry away as kinetic energy up to 70 % of energy released during particle stopping. Ignoring the fast electron contribution makes impossible bringing independently measured cross sections of elementary processes of ionization, excitation, stripping and charge exchange into compliance with measured cross sections of electronic stopping. Fast electron emission in collisions of energetic ions with solids may induce extra radiation effects (create additional vacancies and defects in solid).Mean kinetic energies of ejected electrons We obtained from the analysis of cross section ratio agree well with directly measured energy spectra of the emitted electrons. The dynamic ionization theory implies the We proportionality to initial velocity v. In the considered cases, such a dependence takes place at the velocities below 2 a.u. At higher velocities, the observed dependence is successfully described in the framework of the Born approximation.

Electron emission in collisions between dressed ions and multielectronic targets: the role of the projectile distortion

SynopsisA further improvement of the CDW and CDW-EIS distorted wave models for ionization by dressed ion impact is made by taking into account the projectile form factor into the projectile distortions.

Electron emission in collisions between atoms and dressed projectiles

We present theoretical results for electron emission in collisions between helium atoms and dressed projectiles at high energies. Double-differential cross sections (DDCSs) as a function of the emitted electron energies and angles are calculated. In our study we have applied the three-body formalism using the three-Coulomb wave (3CW–3B) model. The interaction between the dressed projectile and the active electron in the target has been approximated by a model potential having both a long-range Coulomb potential part and a short-range part. However, the active electron in the target has been treated as hydrogenic. We have also studied the projectile charge state dependence of the DDCS. Our theoretical results are compared with available experimental data as well as other theoretical calculations. The comparison shows a good agreement between the present calculations and the measurements. The obtained results are also compatible with other theoretical findings.

Forward electron emission in collisions of He2+ ions with Ar atoms with simultaneous capture of two electrons

The energy spectrum of the electrons ejected in forward direction from Ar atoms with simultaneous bound-state capture of two electrons has been measured by impact of 30 keV/amu ${\mathrm{He}}^{2+}$ ions with the aim to clarify the discrepancy between the results of two previous experiments concerning the existence of the cusp-shaped peak for electrons with velocity close to the ion velocity. A peak structure has been observed which is dominated by a broad cusp attributed to the zero-energy electron emission in the projectile-centered reference system at the $2\phantom{\rule{0.16em}{0ex}}^{3}S$ excitation threshold of ${\mathrm{He}}^{0}$. Further narrower peaks have been identified as results of the decay of autodetachment resonant states of ${\mathrm{He}}^{\ensuremath{-}}$ populated during the collision by three-electron bound-state capture.

Thermodynamic model for electron emission and negative- and positive-ion formation in keV molecular collisions

A statistical-type model is developed to describe the ion production and electron emission in collisions of (molecular) ions with atoms. The model is based on the Boltzmann population of the bound electronic energy levels of the quasi molecule formed in the collision and the discretized continuum. The discretization of the continuum is implemented by a free electron gas in a box model assuming an effective square potential of the quasi molecule. The temperature of the electron gas is calculated by taking into account a thermodynamically adiabatic process due to the change of the effective volume of the quasi molecule as the system evolves. The system may undergo a transition with a small probability from the discretized continuum to the states of the complementary continuum. It is assumed that these states are decoupled from the thermodynamic time development. The decoupled states overwhelmingly determine the yield of the asymptotically observed fragment ions. The main motivation of this work is to describe the recently observed $H^-$ ion production in $OH^+ + Ar$ collisions. The obtained differential cross sections for $H^-$ formation, cation production and electron emission are close to the experimental ones. Calculations for the atomic systems $O^+ + Ar$ and $H^+ + Ar$ are also in reasonable agreement with the experiments indicating that the model can be applied to a wide class of collisions.

Electron emission in collisions of fast highly charged bare ions with helium atoms

We have studied the electron emission from ground state helium atom in collision with fast bare heavy ions at intermediate and high incident energies. In the present study, we have applied the present three-body formalism of the three Coulomb wave (3C-3B) model and the previously adopted four-body formalism of the three Coulomb wave (3C-4B). To represent the active electron in the helium atom in the 3C-3B model, the initial bound state wavefunction is chosen to be hydrogenic with an effective nuclear charge. The wavefunction for the ejected electron in the exit channel has been approximated to be a Coulomb continuum wavefunction with same effective nuclear charge. Effectively the continuum-continuum correlation effect has been considered in the present investigation. Here we have calculated the energy and angular distribution of double differential cross sections (DDCS) at low and high energy electron emission from helium atom. The large forward-backward asymmetry is observed in the angular distribution which is explained in terms of the two-center effect (TCE). Our theoretical results are compared with available experimental results as well as other theoretical calculations based on the plain wave Born approximation (PWBA), continuum-distorted wave (CDW) approximation, continuum-distorted wave eikonal-initial state (CDW-EIS) approximation, and the corresponding values obtained from the 3C-4B model [S. Jana, R. Samanta, M. Purkait, Phys. Scr. 88, 055301 (2013)] respectively. It is observed that the four-body version of the present investigation produces results which are in better agreement with experimental observations for all cases.

Electron emission in collisions between atoms and dressed projectiles

SynopsisDouble-differential cross sections (DDCS) for electron emission in ionization under the impact of dressed projectiles has been calculated within the framework of three-Coulomb wave (3CW) model. The interaction between the dressed projectile ion and the active electron is approximated by a model potential. In order to take into account of the effect of the passive electrons, we have also constructed a model potential which satisfies initial condition and the corresponding wave function has been calculated from the model hamiltonian of the active electron in the target.

Electron emission in collisions of dressed ions with He atoms

We present theoretical and experimental results for angular and energy distributions of electrons emitted in collisions of B2+ with He atoms. In particular, we analyze the cusp formed by projectile Electron Loss to Continuum (ELC) and Electron Capture to Continuum (ECC). The cusp shape dependence on the electronic initial state is discussed and compared with our previous measurements on Li+ + He. The projectile incident energy was about 414 keV/u and the electron detection angle was varied between 0° and 150°. The experimental data is compared with three-body quantum Continuum-Distorted-Wave calculations.

Experimental and theoretical results on electron emission in collisions between He targets and dressed Liq+ (q = 1, 2) projectiles

We investigate experimentally and theoretically the electron emission in collisions between He atoms and (q = 1, 2) projectiles at intermediate to high incident energies. We report on measured absolute values of double-differential cross-sections, as a function of the emitted electron energy and angle, at a collision energy of 440 keV u−1. The different contributions from target ionization, projectile ionization, and simultaneous target–projectile ionization are calculated with the quantum-mechanical continuum distorted wave and continuum distorted wave–eikonal initial state models, and with classical trajectory Monte Carlo simulations. There is an overall good agreement of the calculations with the experimental data for electron emission cross-sections.

Electron emission in collisions between dressed Alq+ ions with He targets

Experimental and theoretical results are presented for electron emission in collisions between dressed Alq+ ions and atomic He targets. The experimental data are compared with a four-body CTMC model and two distorted wave models, namely the CDW and CDW-EIS. The contribution to total electron emission spectra from the ionisation of each collision center and as well as the simultaneous ionisation can be assessed separately.

Electron emission in collisions between dressed Alq+ ions with He targets

Experimental and theoretical results are presented for electron emission in collisions between dressed Alq+ ions and atomic He targets. The experimental data are studied by means of a four-body CTMC model and two distorted wave ones, namely the CDW and CDW-EIS ones. The contribution of electron emission from the ionisation of each collision aggregate and also the simultaneous ionization can be studied separately.

Experimental and theoretical results on electron emission in collisions between partially dressed ions with He targets

Experimental and theoretical results for electron emission in 440 keV u−1 Li+ with He targets are presented. Theoretical cross-sections are obtained using extensions of the continuum distorted wave and the continuum distorted wave-eikonal initial state models to the case of dressed projectiles and a four-body classical trajectory Monte-Carlo. The contributions of electron emission from the different aggregates of the collision system are investigated.

Energy and angular distributions of the low-energy electron emission in collisions of 4 MeV/u bare F ions with He atoms

The energy and angular distributions of double-differential cross sections (DDCS) of electron emission from He in collisions with 4 MeV/u F9 + ions are reported. The derived single-differential distributions and the total cross sections are also reported. The measured distributions of the low-energy electrons between 1 and 400 eV over a wide angular range between 20° and 160° are compared with the state-of-the-art quantum mechanical models. The first Born (B1) and the continuum distorted wave-eikonal initial state (CDW-EIS) approximations are used for this purpose. The DDCS for a given angle was found to fall by a few orders of magnitude over the electron energy range studied. The CDW-EIS model provides excellent agreement with the energy distributions and the angular distributions. The electron energy dependence of the forward–backward asymmetry parameter shows monotonically increasing behaviour. This has been explained very well in terms of the CDW-EIS model, which includes the two-centre effect. A large deviation from the B1 is also observed. We have also derived the single-differential distributions in terms of the angle as well as the electron energy. These distributions are also well reproduced by the CDW-EIS model.

The dependence of the forward-backward asymmetry parameter on the molecular orientation in angular distributions of emitted electrons in the H2 ionisation induced by bare ion impact

We have studied the angular distributions of electron emission in collisions of bare ions with H2 oriented target using molecular wave functions calculated with the Schwinger variational iteractive method within the Continuum Distorted Wave-Eikonal Initial State approximation (CDW-EIS-SVIM). By calculating the forward-backward asymmetry parameter for different molecular orientations we show that this parameter depend not only on the perturbation strength, Sp = Zp/vp, where Zp and vp are respectively the projectile charge and velocity (two-centre effect) and on the outgoing electron velocity, but may also depend on how the molecule is aligned with respect to the projectile incident axis. Doubly differential cross sections (DDCS) for fixed in space molecular target were calculated. The asymmetry parameter was compared with both experimental and theoretical orientation averaged results.

Role of Auger-type emission from diatomic molecular targets interacting with fast multicharged ions

Multiple electron emission in collisions between bare multiply charged ions and diatomic molecules is studied. The role played by Auger-type emission in cross sections for fixed number of ejected electrons is investigated. It is shown that this postcollisional reaction gives the main contribution to these cross sections at high enough impact velocities. This behavior is accentuated as the ionization degree increases.

Low-energy electrons emitted in collisions of heavy ions with solid state targets

Detailed knowledge of low-energy electron emission in collisions of swift ions with solids and gases is of utmost importance for the modeling of the dose distributions and track structures at small distances (<20 nm) in mixed radiation fields, particularly for the Local Effect Model, LEM, developed and used at GSI for the prediction of ion radiation effects in ion radiotherapy and related fields. At GSI, we have launched a project aimed at systematic investigations of the energy and angular distributions of low energy (sub-keV) electrons emitted from solids. We have commissioned an electrostatic toroidal spectrometer at the UNILAC accelerator, and we have developed a method to identify, study and analyze problems caused by the condition and constitution of the target foils and their surfaces. For this, we combined measurements of electrons emitted in collisions of low-energy electron beams (500 eV, 1 keV) with results of dedicated Monte Carlo simulations. We utilized the findings of these studies for the investigation of electrons emitted from the same targets in ion-atom collisions for the data analysis, and we report on results of our first measurements with carbon projectiles.

Coherent electron emission from molecules induced by swift ion impact

Interference effects for electron emission in collisions between molecules and fast highly charged ions are studied using a nonperturbative semiclassical approach. Angle- and energy-differential results are presented and analyzed as a function of the projectile charge and velocity, as well as the geometric characteristics of the molecular target. They agree qualitatively with the available experimental data and validate the observed primary interference patterns. Contrary to previous theoretical predictions a strong backward-forward asymmetry of the interference structures is found and discussed in detail.

Angular distribution of low-energy electron emission in collisions of 6-MeV/u bare carbon ions with molecular hydrogen: Two-center mechanism and interference effect

We report the energy and angular distribution of electron double differential cross sections (DDCS) in collision of $6\text{\ensuremath{-}}\mathrm{MeV}∕\mathrm{u}\phantom{\rule{0.3em}{0ex}}{\mathrm{C}}^{6+}$ ions with molecular hydrogen. We explain the observed distributions in terms of the two-center effect and the Young-type interference effect. The secondary electrons having energies between 1 and $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are detected at about 10 different emission angles between $30\ifmmode^\circ\else\textdegree\fi{}$ and $150\ifmmode^\circ\else\textdegree\fi{}$. The measured data are compared with the state-of-the-art continuum distorted wave-eikonal initial state and the first Born model calculations which use molecular wave function. The single differential cross sections are derived and compared with the theoretical predictions. The oscillations due to the interference effect are derived in the DDCS ratios using theoretical cross sections for the atomic H target. The effect of the atomic parameters on the observed oscillations is discussed. An evidence of interference effect has also been shown in the single differential cross section. The electron energy dependence of the forward-backward asymmetry parameter shows a monotonically increasing behavior for an atomic target, such as He, which could be explained in terms of the two-center effect only. In contrast, for the molecular ${\mathrm{H}}_{2}$ the asymmetry parameter reveals an oscillatory behavior due to the Young-type interference effect superimposed with the two-center effect. The asymmetry parameter technique provides a self-normalized method to reveal the interference oscillation which does not require either a theoretical model or complementary measurements on the atomic H target.

Two center electron emission in collisions of fast ions with H, and H 2: Interplay between interference and Compton profile effect

Young type interference effect has been studied in case of particle induced ionization of H 2. Oscillations are derived by comparing the measured DDCS for H 2 with the calculated DDCS for H. The effect of the Compton profiles of H 2 and H on the interference structure is also studied. A theoretical model based on molecular distorted wave calculation explains the experimental results qualitatively.

Electron emission in collisions of highly charged ions with atoms and diatomic molecules

A short review of theoretical models previously used to study single and multiple electron ionization in collisions of bare fast highly charged ions with atomic and diatomic molecular targets is given. Electron emission with simultaneous electron capture by the projectile is also considered. The principal mechanisms producing the different reactions are revisited. It is emphasized that two-center descriptions are necessary for an adequate description of the existing experimental data. The role of electron correlation in the initial bound state and during the collision process is analyzed as well as the presence of effects associated with the two-center character of diatomic molecular targets.