Binary Encounter Electron Production Research Articles

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}

A new hemispherical analyser with 2-D PSD and focusing lens for use in 0° electron spectroscopy

A new electron spectrograph has been developed to study electron emission in ion–atom collisions and in particular for use in 0° Auger projectile spectroscopy. It incorporates a hemispherical analyser, a 2-dimensional position-sensitive detector (PSD) with multichannel plates and a resistive anode encoder. A novel feature of this analyser is the use of an asymmetric entrance aperture and a non-zero entrance potential to improve focusing by correcting for fringing field effects. A four-element lens mounted at the entrance of the analyser, provides a virtual slit, focusing and decelerating incoming electrons for improved energy resolution and reduction of the ion beam scattering at the spectrometer entrance. The analyser has an acceptance energy range of 20%. As a first test of the apparatus, the production of Binary Encounter electrons from 20 MeV F9+,8++H2 collisions was measured. The bare F9+ results were used for an in situ calibration of the 2D-PSD overall efficiency. The F8+ data were found to be in good agreement with the elastic electron scattering model.

Recoil-ion charge-state-resolved electron-production cross sections at 55 degrees for 1 MeV/u C5+ on He and Ar.

Recoil-ion charge-state-resolved doubly differential cross sections for ejecting electrons at \ensuremath{\sim}55\ifmmode^\circ\else\textdegree\fi{} with respect to the incident beam direction in collisions between 1 MeV/u ${\mathrm{C}}^{5+}$ projectiles and Ar and He targets have been measured. Electrons with kinetic energies between 100 and 1250 eV have been detected. A prominent feature in the electron energy distributions is the binary-encounter peak. Experimental results are compared with binary-encounter electron production cross sections obtained using the impulse approximation and with theoretical predictions from many-body classical trajectory Monte Carlo calculations. An enhancement in the fraction of electrons detected with singly charged He recoil ions and a corresponding decrease in the fraction detected in coincidence with doubly charged He recoil ions as a function of the electron energy have been observed near the binary-encounter electron energy. This structure has been predicted by recent many-body classical trajectory Monte Carlo calculations. \textcopyright{} 1996 The American Physical Society.

0 degrees binary encounter electron production in 30-MeV Oq++H2, He, O2, Ne, and Ar collisions.

Double-differential cross sections (DDCS's) for the production of binary encounter electrons (BEE's) were measured for collisions of 30-MeV ${\mathrm{O}}^{\mathit{q}+}$ projectiles with ${\mathrm{H}}_{2}$, He, ${\mathrm{O}}_{2}$, Ne, and Ar targets with q=4--8 and an electron ejection angle of \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} with respect to the beam direction. Particular interest focused on (a) the evaluation of the contributions of the different electron subshells of the multielectron targets, ${\mathrm{O}}_{2}$, Ne, and Ar; (b) the study of the well-known enhancement of the BEE DDCS's with decreasing projectile charge-state q; here this dependence was tested for higher collision energies and new targets; (c) the study of the dependence of the BEE peak energy on the particular target and projectile charge state. Results were analyzed in terms of the impulse approximation, in which target electrons in the projectile frame undergo 180\ifmmode^\circ\else\textdegree\fi{} elastic scattering in the field of the projectile ion. The electron scattering calculations were performed in a partial-wave treatment using the Hartree-Fock model. Good agreement with the data was found for the ${\mathrm{H}}_{2}$ and He targets, while for the multielectron targets ${\mathrm{O}}_{2}$, Ne, and Ar only electrons whose velocity was lower than the projectile velocity needed to be included for good agreement. All measured BEE DDCS's were found to increase with decreasing projectile charge state, in agreement with other recent BEE results. The BEE peak energies were found to be independent of the projectile charge state for all targets utilized. \textcopyright{} 1996 The American Physical Society.

Zero-degree binary encounter electron production in 30 MeV bare O8+ in collisions with H2, He, Ne, Ar, Kr and Xe

Double differential cross-sections for the production of binary encounter electrons were measured for collisions of 30 MeV bare O8+ projectiles with H2, He, Ne, Ar, Kr and Xe targets at an electron ejection angle of θ = 0° with respect to the beam direction. Results were analyzed in terms of the impulse approximation (IA), in which target electrons in the projectile frame undergo 180° Rutherford scattering in the field of the bare projectile ion. Excellent agreement with the data was found for the H2 and He targets, while for the multi-electron targets good agreement was established only when target electrons whose velocities were lower than the projectile velocity were included in the calculation.

Binary encounter electron production at relativistic velocities

Relativistic binary-encounter electrons produced in very fast collisions of bare and H-like Ar with carbon foils are studied as a function of laboratory frame emission angle. It is found that the velocity of the binary-encounter peak is consistent with a relativistic billiard ball model. The cross section for binary-encounter electron production is not in good agreement with a simple Rutherford scattering model. It is also found that the ratio of H-like to bare projectile cross sections is 1 at 0 degrees and becomes less than 1 at large emission angles.

Diffraction structures in binary encounter electron spectra: A dominant feature for projectiles above Cl and a tool to study the ionization mechanism

The energy and angular distributions of electrons produced in collisions of about 0.3 MeV/u ions on H 2 have been measured over a broad range of projectiles (Cl, Cu and I) and observation angles ( θ L) of 0° to 70° with respect to the beam direction. A double peak structure is observed in the binary encounter electron (BEe) region for projectiles heavier than Cl. This structure can be understood in the impulse approximation as elastic scattering of quasi free target electrons in the screened projectile potential. The angular distribution of BEe production is investigated for different projectiles, and the violation of a q 2-scaling is observed not only for 0° but for the whole angular range covered.

Transition from quantum to quasi-classical behaviour of the binary encounter peak in collisions of 0.6 to 3.6 MeV amu-1 I23+ and Xe21+ with He and Ar

Double differential cross sections are reported for the production of binary encounter electrons in collisions of 0.6 MeV amu-1 I23+ and 1.4, 2.4, and 3.6 MeV amu-1 Xe21+ projectiles incident on He and Ar targets. Electron energy spectra were measured between 0 degrees and 45 degrees in the case of the two lower projectile energies, and between 17.5 degrees and 60 degrees for the two higher projectile energies. The data are compared with quantum mechanical impulse approximation and classical trajectory Monte Carlo calculations. While the quantum model calculation predicts a rapid disappearance of diffraction effects in the binary encounter peak with increasing projectile energy, these remain visible in the experimental results up to the highest energy measured. The necessity of including multiple target ionization involving inner shell electrons in the theoretical description of the collision process is demonstrated by the classical trajectory Monte Carlo calculation, which accounts well for the shape of the 2.4 and 3.6 MeV amu-1 cross sections, except at angles where diffraction effects are manifest. Systematic shifts of the binary encounter peak position towards lower energies with increasing emission angle were observed for all projectile energies.

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.

Binary peak electrons observed at 0° for 2–4 MeV/u F q+, Si q+ and Ni q+ ions in collisions with a He target

Binary encounter electrons originating from collisions of 2 MeV/u F q+ , 4 MeV/u Si q+ and Ni q+ ions impact on a He target have been measured at zero degree with respect to the beam direction. The dependence of the binary encounter peak on the projectile charge states was studied. As earlier observed by Richard et al. [J. Phys. B23 (1990) L213] using fluorine ions, we found that the binary encounter electron production increased for decreasing charge state for the present ion species and energy. For fluorine ion impact, the enhancement of the binary encounter electron production becomes weaker at 2 MeV/u compared to the results of Richard et al. (1 and 1.5 MeV/u). A simple estimation based on potential screening and collision diameter for the enhancement is presented and the results reproduced the experimental values well.

Diffraction effects in binary encounter electron production from collisions of partially stripped ions

A systematic investigation of binary encounter electron production from collisions of partially stripped ions with hydrogen and noble gases is currently under way. We present experimental double-differential binary encounter electron cross sections as well as model calculations for 0.6 MeV amu −1 Cu 19+, I 23+ and Au 29+ ions colliding with argon. The results show how, with increasing the screening of the projectile nuclear charge, quantum diffraction effects manifest themselves in the angular behaviour and shape of the binary encounter peak.

Binary encounter electrons in ion-atom collisions

The theory of binary encounter electron production is reviewed. The elastic differential cross sections for electrons scattered from Au q+ ( q = 9, 11 and 19) are compared for an impact energy of 329.1 eV. Some examples are presented where theory is compared to the experimental data of Au +-neon collisions for selected angles in the laboratory frame.

Effect of exchange on the binary-encounter-electron double-differential cross section in Cq+-H2 collisions at 0.75 MeV/amu.

The production of binary-encounter electrons in collisions of ${\mathrm{C}}^{\mathit{q}+}$ ions with molecular hydrogen has been studied at an impact energy of 0.75 MeV/amu. The measured double-differential cross section (DDCS) at a forward angle (${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=0), with respect to the beam direction, is in excellent agreement with the theoretical cross sections evaluated in the impulse approximation by using the elastic-electron--ion differential cross section with the Compton profile of the target electrons. The theoretically calculated DDCS is extremely sensitive to the impact energy. Hence a precise measurement of the projectile beam energy is required to compare predicted values with the data. The agreement between theory and experiment demonstrates the importance of exchange in the description of binary-encounter electrons and that the exchange increases with the number of electrons on the projectiles.

Diffraction in the binary encounter electron peak observed in collisions of 0.6 MeV amu-1 I7+ I23+ and Au11+ projectiles with He and Ar

Relative double differential cross sections for electron emission from collisions of 0.6 MeV amu-1 I7+ and I23+ projectiles with Ar and Au11+ projectiles with He were measured for electron energies from 100 eV to 2000 eV and for angles from 0' to 50'. Experimentally observed sudden shifts of the position of the binary encounter peak were interpreted as resulting from quantum interference in the scattering of target electrons from the partially stripped projectile ion. To this end calculations for the elastic scattering of free electrons initially at rest in the laboratory reference frame from the screened projectile field as well as model calculations of binary encounter electron production taking into account the target Compton profile were performed and compared with the experimental results. Good agreement was obtained for the angular locations of these shifts as well as for the angular distribution of the binary encounter electron yield.

Binary-encounter electron production in energetic heavy-bare-ion-atom collisions.

The production of binary-encounter electrons in heavy-ion\char21{}atom collisions at high impact velocities is studied. The theoretical model used is the continuum-distorted-wave\char21{}eikonal-initial-state approximation. Deviations from classical predictions for the height and position of the associated peak in double-differential cross sections are determined. A simple adiabatic resonant-tunneling model is developed to interpret the physical meaning of the shift in the position of the binary-encounter peak.

Binary Encounter Peaks for 0° Electrons in Collisionsbetween 2 MeV/amu Siq+and He

Binary encounter electrons originating from collisions between 56 MeV Si q + and He have been measured at zero degrees with respect to the beam direction. The dependence of the binary-encounter peak on the projectile charge state q =6, 12, 13 and 14 (fully stripped Si) was studied. As earlier observed by Richard et al . (J. Phys. B 23 (1990) L213) using fluorine ions, we found that the binary-encounter electron production increased for decreasing charge state. This dependence which reflects the importance of outer screening has recently been treated in several theoretical papers and the present results are compared to estimates by K. Taulbjerg (private communication).

Strong dependence of zero degree binary encounter electron production on the initial and final charge state

Doubly differential cross sections d2 sigma /dEed Omega e and fractional probabilities for binary encounter (BE) electron emission at 0' electron emission angle as a function of initial (qi) and final (qf) projectile charge state have been measured for 0.53 MeV u-1 Fqi on H2. The data confirm the surprising decrease of the partial cross section with increasing charge state of the incoming projectile qi reported by Richard et al. (1990). The authors show by coincident electron projectile detection that the pure ionization channel qi=qf strongly dominates the differential cross section at theta e=0' and projectile capture and loss channels are of negligible importance. In view of a calculation by Olson et al. (1990) they conclude from their data that 0' BE electron emission depends strongly on the shape of the screened projectile potential varying with qi.