Electron Loss To The Continuum Research Articles

Strong asymmetry of the electron-loss-to-continuum cusp of multielectronU28+projectiles in near-relativistic collisions with gaseous targets

The process of electron-loss to the continuum (ELC) has been studied for the collision systems ${\mathrm{U}}^{28+}+{\mathrm{H}}_{2}$ at a collision energy of 50 MeV/u, ${\mathrm{U}}^{28+}+{\mathrm{N}}_{2}$ at 30 MeV/u, and ${\mathrm{U}}^{28+}+\mathrm{Xe}$ at 50 MeV/u. The energy distributions of cusp electrons emitted at an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam were measured using a magnetic forward-angle electron spectrometer. For these collision systems far from equilibrium charge state, a significantly asymmetric cusp shape is observed. The experimental results are compared to calculations based on first-order perturbation theory, which predict an almost symmetric cusp shape. Some possible reasons for this discrepancy are discussed.

Complete break up of ortho positronium (Ps)-hydrogenic ion system

The dynamics of the complete breakup process in an Ortho Ps - He+ system including electron loss to the continuum (ELC) is studied where both the projectile and the target get ionized. The process is essentially a four body problem and the present model takes account of the two centre effect on the electron ejected from the Ps atom which is crucial for a proper description of the ELC phenomena. The calculations are performed in the framework of Coulomb Distorted Eikonal Approximation. The exchange effect between the target and the projectile electron is taken into account in a consistent manner. The proper asymptotic 3-body boundary condition for this ionization process is also satisfied in the present model. A distinct broad ELC peak is noted in the fully differential cross sections (5DCS) for the Ps electron corroborating qualitatively the experiment for the Ps - He system. Both the dynamics of the ELC from the Ps and the ejected electron from the target He+ in the FDCS are studied using coplanar geometry. Interesting features are noted in the FDCS for both the electrons belonging to the target and the projectile.

COMPLETE FRAGMENTATION OF ORTHO Ps − He+ ION SYSTEM AND THE ELC PROCESS

The dynamics of the complete breakup process in an Ortho Ps − He + system, including electron loss to the continuum (ELC) is studied where both the projectile and the target get ionized. The process is essentially a four body problem and the present model takes account of the two centre effect on the electron ejected from the Ps atom which is crucial for a proper description of the ELC phenomena. A distinct broad ELC peak is noted in the fully differential cross sections (FDCS) for the PS electron, corroborating qualitatively the experiment for the Ps − He system.

Fragmentation of Positronium (Ps) atom in collision with Li ion

Fragmentation of ground state Ortho Positronium (Ps) in collision with Li ion is studied, giving special emphasis on the dynamics of the electron loss to the continuum (ELC) that occurs when the electron and the positron are very close to each other in the velocity space (ve ≈ vp). The present model takes account of the two center effect on the ejected electron which is crucial for a proper description of the ELC phenomena. Shifting of the of the DDCS peak (ELC) towards higher value of the ratio ve /vp with increasing incident energy, corroborating experimental findings.

Fragmentation of positronium in collision with singly ionized lithium

Fragmentation of ground state ortho Positronium (Ps) in collision with Li ion (Li+) including the electron loss to the continuum (ELC) are studied in the framework of post collisional coulomb distorted eikonal approximation (CDEA) for the target elastic case. The present model takes account of the two center effect on the ejected e which is crucial for a proper description of the projectile ionization involving an ionic target, particularly for the explanation of the shifting of the ELC DDCS peak.

Fragmentation of positronium in collision with Li ion including electron loss to the continuum

Fragmentation of ground state ortho-positronium (Ps) in collision with Li ion (Li+) is studied in the framework of post-collisional Coulomb distorted eikonal approximation (CDEA), giving special emphasis on the dynamics of the electron loss to the continuum (ELC) that occurs when the electron (e) and the positron (e+) are very close to each other in the velocity space (υe ≈ υp). The present model takes account of the two-centre effect on the ejected e which is crucial for a proper description of the ELC phenomena. Both the fully differential cross-section (TDCS) and the doubly differential cross-section (DDCS) (energy spectra) are investigated at intermediate and high incident energies. A broad distinct ELC peak centred around υe ≈ υp is noted in the e energy spectrum in contrast to the sharp ELC peak for a heavy projectile, corroborating the experimental findings. Two salient features are noted in the present study: (i) the shift of the e DDCS peak (summed over e+ angles) towards higher ejection energy with respect to half the residual energy of the system, (ii) comparison of the e and e+ energy spectra reflect a strong e−e+ asymmetry with respect to the ratio υe/υp = 1. Both these features are in qualitative agreement with the experimental observations due to Armitage et al for Ps-He atom system and could be attributed to the post-collisional two-centre effect on e due to its parent nucleus (e+) and the screened target ion. Two different wave functions of the Li ion are chosen in order to test the sensitivity of the present results with respect to the choice of the wave function.

Break-up of positronium (Ps) atom in collision with a He+ ion

The dynamics of electron loss to the continuum (ELC) from the light neutral projectile positronium (Ps) atom in collision with He+ ion is studied in the framework of post-collisional Eikonal approximation. The full three-body interaction, which is crucial for such ELC process is taken into account in the final channel. Both the triple differential cross sections (TDCS) and the double differential cross sections (DDCS) are studied at some intermediate and high incident energies, with respect to the threshold energy for this process. A distinct broad ELC peak centred around ve (velocity of the ejected electron) ≈vp (velocity of the positron) is noted in the ejected electron energy spectrum (DDCS) for forward ejection of e/e+, corroborating the experimental findings for the ELC process. The probability of emission of the e+ at higher angles is almost negligible as compared to its forward emission. Another salient feature noted in the present work is the shifting of the ELC DDCS (summed over the electron ejection angles) peak towards higher value of the ratio ve/vp with increasing incident energy, in agreement with the experimental findings of Armitage et al for Ps–He break-up process.

Saturation effects in projectile electron loss

The behavior of the projectile electron-loss cross sections with the target atomic number, Z 2, presents a saturation as the value of Z 2 increases. This fact, which had elready been observed for the excitation of highly charged projectiles, appears in the first-order calculations for the contribution of the projectile electron-target electron interaction but not for the projectile electron-target nucleus interaction. In this work, we compare total electron-loss cross sections with Electron Loss to the Continuum (ELC) yields, for 1.0 MeV He − projectile on various targets, with atomic numbers ranging from 2 to 54. Besides a similar saturation with Z 2 observed in both cases, some differences between the data appear, such as enhanced values of the total loss for all targets when compared to the ELC and a structure between the Ar and Xe targets also in the total loss data. These differences may be assigned to the contribution due to the interaction between the projectile active electron and the target electron cloud.

Projectile energy dependence of the contributions of different mechanisms to cusp electron production in He + → Ar collisions

The energy spectra of cusp electrons from He + → Ar collisions have been measured in coincidence with the outgoing charge state selected He 0,1+,2+ projectiles in the 18.75–75 keV/u energy range. The electron capture to the continuum (ECC), electron loss to the continuum (ELC), and transfer ionization (TI) reaction channels have been identified. In the energy range studied ECC decreases from 86% to 80% and ELC increases strongly from 2.2% to 19.4% with increasing energy. At 18.75 keV/u TI contributes about 12% to the total cusp electron yield whereas at 75 keV/u this fraction decreases to 1.2%. A strong target gas pressure dependence was found for the TI fraction.

Cusp-shape studies with He+ ions at 1.41- and 2.41-a.u. impact velocities.

The electron loss to the continuum (ELC) process was investigated in ${\mathrm{He}}^{+\mathrm{\ensuremath{-}}}$(He,Ar) collisions at 1.41- and 2.41-a.u. impact velocities. The cusp-shaped energy distributions of electrons ejected into the forward direction were measured in coincidence with the scattered ${\mathrm{He}}^{2+}$ projectile ions. The shape of these cusps was studied by fitting the multipole series expansion [Meckbach, Nemirovsky, and Garibotti, Phys. Rev. A 24, 1793 (1981)] of the measured spectra using ${\mathit{B}}_{\mathit{n}\mathit{j}}$, the nth-order expansion coefficients of the jth-order Legendre polynomials, as fitting parameters. The observed skewness of the spectra (nonzero ${\mathrm{\ensuremath{\beta}}}_{1}$=${\mathit{B}}_{01}$/${\mathit{B}}_{00}$ values) indicates the importance of the higher-order theories in the description of the distribution of ELC electrons. A dip at the top of the peak (``cusp inversion''), reflected by a positive ${\mathrm{\ensuremath{\beta}}}_{2}$=${\mathit{B}}_{02}$/${\mathit{B}}_{00}$ value, was observed in the 1.41-a.u. ${\mathrm{He}}^{+\mathrm{\ensuremath{-}}}$He collision.

Production of convoy electrons by foil-excited He ions

Convoy electrons are studied, which are produced by the transmission of 50 MeV 3He 2+ ions through two thin foils of carbon, where the downstream foil is exposed to the beam transmitted through the upstream foil. By eliminating the convoy electrons emitted from the upstream foil by applying an electric field, the convoy electrons produced at the downstream foil are studied. The yields of convoy electrons produced at the downstream foil, which has a thickness of less than the mean free path for convoy electrons (∼ 3 μg/cm 2), are about twice as large as those emitted from a single foil of the same thickness. The increase is attributed to the convoy electrons produced by electron loss to the continuum (ELC) of Rydberg states of 3He + ions produced at the upstream foil.

Multipole expansion analysis of electron loss to the continuum for He+ ions colliding with rare gases

The authors present an analysis of electron loss to the continuum (ELC) for 1.6-2.8 MeV He+ colliding with He, Ne and Ar using the first six terms of the multipole expansion method. They give absolute values for B00 and discuss the way in which the method of analysis influences the values of the dipole and quadrupole parameters. The authors' dipole parameter is slightly positive for the He target and becomes progressively more negative for Ne and Ar. Their quadrupole parameter is slightly positive for He and Ne and nearly zero for Ar. For He++He the magnitude of B00 and its velocity dependence are in reasonable quantitative agreement with a recent theoretical calculation based on the second Born approximation. However their dipole and quadrupole parameters are both more positive than predicted.

Production and transport of convoy electrons in amorphous carbon foils

The production of free convoy electrons, emitted with velocities near the ion velocity in ion-solid collisions, is not well understood. Experiments concerning thickness-dependent yields have suggested the dominant mechanism for convoy production is electron loss to the continuum (ELC) in the bulk of the solid. Free electrons created in the bulk are subject to multiple elastic and inelastic scattering during transport through remaining layers of the solid. We discuss double-differential measurements of convoy electrons as a function of target thickness for fast O 5+ ion projectiles incident on carbon foils of varied thicknesses. Angular distributions confirm the ELC model for convoy production. From the radial broadening of the convoy cusps we have determined energy and angular spreading parameters due to postcollisional multiple scattering.

Electron loss to the continuum for H2+and He+ions colliding with H2, He, Ne and Ar. II. Absolute cross sections

For pt.I see ibid., vol.19, p.401 (1986). Absolute cross sections for electron loss to the continuum (ELC) have been obtained for 0.8-2.8 MeV H2+ and He+ ions incident on H2, He, Ne and Ar gases. The target consisted of a jet of gas from a multicapillary array, thereby providing a well localised and directed gas beam. A 30 degrees parallel-plate analyser having a FWHM resolution of (dE/E)=10-3 was used to study the energy spectra of the ejected electrons as detected in a cone of solid angle 1.56*10-3 sr about the forward direction. The cross sections were obtained by numerically integrating the ejected-electron velocity spectra over a width of 0.25 au on either side of the centre of the cusp-shaped peak. When defined in this way the cross sections are found to decrease slowly with increasing projectile velocity, in agreement with an available theoretical calculation and other experimental results.

Forward electron ejection in H+-He, He+-He and He2+-He collisions

The dependence of the parameters (width, intensity and shape) of the cusp in the processes of electron capture to the continuum (ECC) and electron loss to the continuum (ELC) has been studied as a function of impact velocity (from 2 to 5 au) for the simple collision systems H+-He, He+-He and He2+-He. An approximately linear increase in the width (FWHM) is found in both cases. There is no difference in this respect within the error limits for He2+ (ECC) and He+ (ELC). Regarding cusp shape, similar symmetry (ELC) and asymmetry (ECC) have been observed as in the case of heavy-ion projectiles. None of the theories satisfactorily describes the experimental absolute cross sections of ECC cusp (He2+), while in the case of the He+ projectile (ELC) rather good agreement with theory can be seen in the higher-velocity region. The deviation at lower velocities in the latter case could be explained by an ECC effect present even in the case of He+. Concerning the dependence of the cusp yield on the Zp value of the projectile, Z02.5+or-0.3 was obtained from the yield in H+-He and He2+-He collisions.

Electron loss to the continuum for H2+and He+ions colliding with H2, He, Ne and Ar. I. Electron cusp widths and shapes

Differential spectra for electron production in the forward direction have been obtained by collision of H2+ and He+ ions with H2, He, Ne and Ar gases. The measurements where made with ions of energy 0.8-2.8 MeV. The target consisted of a jet of gas from a multicapillary array, thereby providing a well localised and directed gas beam. A 30 degrees parallel-plate electrostatic analyser having a FWHM resolution (dE/E)=10-3 was used to study the energy of the ejected electrons. A cusp-shaped peak centred on ve=vi was observed for all the electron spectra, suggesting that these electrons were ejected with only a small energy relative to the parent ions. This process has been called electron loss to the continuum (ELC) since in some ways it mimics the now well established phenomenon of charge exchange to the continuum (CEC). In contrast to data for ELC from heavy ions the cusps observed here were generally asymmetric. They did, however, show a width approximately independent of projectile velocity, a feature which seems to distinguish ELC from CEC. Both the cusp shapes and widths are compared with theoretical predictions and other experimental measurements.

Charge state dependence of convoy-electron yields in fast ion-foil interaction

Sulphur ions of energy 125 MeV with initial charge states q=10+ to 16+ have been passed through 4-200 mu g cm-2 carbon foils. The generated convoy electrons which emerge from the foils in the forward direction with approximate beam velocity, have been measured. The authors find a strong dependence of the convoy electron yield on the initial charge state of the ions, suggesting that (i) convoy electrons are produced primarily by electron loss to the continuum (ELC) and (ii) the mean free path of convoy electrons is larger than that for electrons moving freely in solids.

Observation of two components in molecular cusp spectra for ˜˜MeV/U H+, He++,++ projectiles on M ⩾ 4 hydrocarbon molecules

Recently it has been realized that the cusp spectra obtained with bare projectiles and projectiles incident on gaseous targets are different in their widths and symmetries due to origin of the electrons in the cusp. For the case of bare projectiles, the electrons in the cusp are those captured frorn the target to the projectile's continuum, while in the case of the clothed projectiles the dominant contribution to the cusp yield is from projectile electrons that are expelled into the continuum during the collision. The former process is called electron capture to the continuum (ECC), while the latter is called electron loss to the continuum (ELC); together these processes constitute electron transfer to the continuum (ETC).

Convoy electron production in heavy ion-solid collisions

The properties of the sharp ν c ≌ ν cusps observed in the velocity spectrum of convoy electrons ( υ e) ejected in heavy ion-solid collisions in the ion velocity range (ν) 6–18 a.u. are compared to the properties of analogous cusps observed in binary electron capture to the continuum (ECC) and electron loss to the continuum (ELC) collisions in gases. Apart from a skew toward ν e > ν, the ν-independent convoy distributions observed are very similar to those for ELC and the cusp widths are the same in both cases. While the shape of convoy peaks is approximately independent of projectile Z, ν, and of target material, yields in polycrystalline targets (C, Al, Ag, Au) exhibit a strong dependence on Z and ν. Coincidence experiments in which convoy electrons are allocated according to emergent ion charge-state q e show a surprising independence of q e, mirroring the unweighted statistical emergent charge-state fraction Coincidence experiments on O 6+ ,7+ ,8+ ions traversing 〈 110 〉 and 〈 100 〉 channels in Au show a strong yield suppression and a dependence of yield on the channel chosen. Interpretation of these observations, comparisons to convoy production studies using protons, and a discussion of remaining puzzles is given. The history of ECC, ELC, and “wake-riding” models of convoy electron production is also reviewed.