S. Grabbe

Identification of very low energy projectile autoionizing transitions in high velocity collisions using zero-degree Auger electron spectroscopy

Abstract The unusual looking “mesa”-shaped cusp observed in O3+ collisions with He [N. Stolterfoht et al., Proc. 2nd US-Mexico Symp. on Atomic and Molecular Phy. eds. A. Cisneros and T. Morgan (Instituto de Fysica, Cuernavaca, Mexico, 1986) p. 51.], has been investigated using zero-degree electron spectroscopy, in both high resolution singles measurements and lower resolution electron-projectile coincidence measurements at 10, 15 and 23 MeV. The high resolution studies indicate the “mesa” peak to be actually composed of primarily two (other than the cusp) very strong autoionizing peaks corresponding to energies of 60 and 100 meV in the emitter frame. The coincidence studies, indicate these lines to originate from excitation of the O3+ ion followed by autoionization. Ongoing Hartree-Fock-Slater calculations, severely tested at these extremely small transition energies, indicate that these lines can indeed result from the autoionization of the O3+ (1s22s2p5l) Rydberg states produced during the collision. Furthermore, the unusually sharp edges of these lines giving rise to the characteristic “mesa”-shape look, can be explained in terms of the kinematic constraints imposed by the energy and angular acceptance range of the spectrometer.

Forward-backward asymmetry in the inelastic scattering of electrons from highly charged ions

Energy-dependent, absolute differential cross sections for 1s2l excitation in F8+ by the scattering of quasi-free target electrons have been measured. The formation and subsequent decay of the F7+ (3lnl´) (n3) doubly excited intermediate states gives rise to a resonant inelastic scattering amplitude which is coherent with the nonresonant inelastic scattering. A strong forward-backward asymmetry was found for the inelastically scattered electrons due to the interference between these two scattering amplitudes. Close-coupling R-matrix calculations show excellent agreement with the experiment and thereby demonstrates a method for obtaining high-resolution, angle-resolved differential inelastic electron-ion scattering cross sections.

Elastic and inelastic scattering models in ion-atom collisions

Abstract We report on a study of resonant elastic and inelastic scattering of quasi-free electrons from O7+. Calculations are compared with recent experimental double differential electron scattering cross sections (DDCS) for 13.8 MeV O7+ + H2 at θlab = 0°. The measured DDCS of the binary encounter electrons including the 2p 2 1 D resonance are in excellent agreement with the corresponding theoretical values calculated within the elastic electron scattering model. Calculations of excitation to 2s 2p from the ground state of O7+ by electrons were performed with the R-matrix approach that included the contribution of the 3l3l′ autoionizing resonances. The experimental DDCS for 20.25 MeV O7+ + H2 at θlab = 0° clearly show the resonance features and are in reasonable agreement with the calculations using the inelastic electron scattering model.

Production of the O5+(1s2s2p 4PJ) states by electron excitation in 10–34 MeV collisions of O5+ ions with H2, He, Ne, Ar, Kr and Xe targets

Abstract Absolute double differential cross sections (DDCS) for electron production were determined for the Auger-decay of the (1s2s2sp 4P) state in 10–34 MeV collisions of O5+(1s22s) ions with H2, He, Ne, Ar, Kr, Xe targets using zero-degree projectile Auger-electron spectroscopy. Due to spin conservation, e-n interaction mechanisms are excluded to first order and thus the e-e interaction is expected to dominate the (1s 2 2s 2 S) a (1s2s2p 4 P) excitation. Auger electrons from the decay of the O5+(1s2s2p 4P) state to the O6+(1s2) ground state were observed. The electron scattering model, using calculated direct electron-ion impact excitation cross sections, was compared to the measured DDCS. These calculations included the effects of the alignment of the 4PJ ( J = 1 2 , 3 2 , 5 2 ) states and their subsequent angular-dependent Auger electron emission.

Binary encounter electrons in ion-atom collisions☆

Abstract 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.

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.

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

Abstract 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.

Two-electron excitation to Rydberg levels in fast I6+ on hydrogen collisions

Abstract The emission of electrons in the forward direction in collisions of 0.3 MeV/u I 6+ with H 2 has been studied, and strong autoionization peaks are observed on the shoulder of the cusp peak. The energies of these autoionization lines in the projectile rest frame are determined by high-resolution electron spectroscopy. Using the electron projectile final charge state coincidence technique, we probe different collision mechanisms, which create continuum electrons that are slow in the projectile rest frame. We conclude that the observed autoionization lines are due to two electron excitation to projectile Rydberg levels.