E.C. Montenegro

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.

Recent recoil ion momentum spectroscopy experiments at KSU

Recoil momentum spectroscopy is used to study collisions involving both fast and slow projectiles on He targets. Experiments have been performed on electron capture and loss from fast ions from the KSU LINAC and slow ions from the KSU CRYEBIS using a supersonic jets with a momentum resolution below 0.5 au. Using fast ions, the final states populated in electron capture from He by 10 MeV F8+ have been resolved with a Q-value resolution of 18 eV, sufficient to separate final channels in which the He+ ion is left excited from those in which He+ is left in its ground state. With slow ions, electron capture from He by slow bare Ne ions has been studied. A few recent results are discussed.Recoil momentum spectroscopy is used to study collisions involving both fast and slow projectiles on He targets. Experiments have been performed on electron capture and loss from fast ions from the KSU LINAC and slow ions from the KSU CRYEBIS using a supersonic jets with a momentum resolution below 0.5 au. Using fast ions, the final states populated in electron capture from He by 10 MeV F8+ have been resolved with a Q-value resolution of 18 eV, sufficient to separate final channels in which the He+ ion is left excited from those in which He+ is left in its ground state. With slow ions, electron capture from He by slow bare Ne ions has been studied. A few recent results are discussed.

Search for inelastic electrons scattered off ions in energetic ion-atom collisions

Abstract The collision of target electrons with ionic projectiles, in highly asymmetric ion-atom collisions, can be simulated within the impulse approximation (IA) by a beam of electrons scattering off the projectile ion, with a collision energy broadened by the momentum distribution (Compton profile) of the target. This description works particularly well for 180° elastic scattering of target electrons off the ion (Phys. Rev. A 41 (1990) 4816 [1]) appearing in the laboratory as the well known binary encounter electron peak. Inelastic processes due to target-electron-projectile-electron interactions (e-e) such as e-e excitation (Phys. Rev. Lett. 62 (1989) 2261 [2]) and e-e ionization (Phys. Rev. Lett. 63 (1990) 1938; J. Phys. B 24 (1991) 977; Phys. Rev. A 46 (1992) 1374; Phys. Rev. Lett. 69 (1992) 3033, 72 (1994) 3170, 72 (1994) 3166 [3–8]) are also well documented. However, for these processes, the target electron undergoing inelastic scattering has never been explicitly identified and measured in ion-atom collisions. A simple theoretical Born-IA treatment gives the expected double differential cross sections and their angular distributions for H-like ions in collision with H 2 targets. The viability of observing these effects in ion-atom collisions by electron spectroscopy is investigated.

Inclusive multi-channel measurement of the impact-parameter distribution in collisions of swift heavy ions with He

Recoil ion momentum spectroscopy is used to study the impact-parameter dependence of the single-ionization, double-ionization, single-capture and transfer-ionization channels in fast collisions of highly charged ions with He. The possibility of obtaining a relationship between the impact parameter and the transverse momentum of the recoil in inelastic collisions is investigated. It is found that a deflection function based on a static screening potential gives consistent agreement between model calculations based on the independent electron approximation and the experimental data for all channels investigated and over the whole range of impact parameters deduced from the measurement.

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.

Single capture and transfer ionization in collisions of Clq+ projectile ions incident on helium

Abstract The Kansas State University linac has been used to measure the ratio of the cross sections for the processes of transfer ionization (TI) and single capture (SC) for 2 MeV/amu Clq+ where q = 7, 9, 13, 14, and 15 projectile ions incident on a helium target. The ratio was determined using a helium gas jet target by measuring coincidences between projectile-ion and recoil-ion final charge states. The σ TI σ SC for Clq+ were compared to measurements of bare F9+ and hydrogenlike F8+ and O7+ taken at the same velocity. The ratios deviate from a q2 scaling which is predicted in the perturbative regime. This deviation is attributed to screening by the projectile electrons for low q = 7 and 9, and to the collision being non-perturbative for high q. A possible saturation effect in the ratio was observed for q ∼ 14.

Recoil momentum dynamics for the antiscreening process

Abstract The Iongitudinal momentum transferred to the target nucleus in the electron-loss process is investigated. In particular, we analyze the resulting target recoil momentum when the antiscreening contribution takes place. This contribution is due to the electron-electron interaction and gives a smaller momentum transfer to the target when compared with the screening mode, where the target nucleus is the effective agent in the loss process.