S.L. Varghese

Differential cross sections for single ionisation of He and D2 by fast protons

Differential cross sections for single ionisation of He and D2 by protons have been measured at impact energies of 3 and 6 MeV, and proton scattering angles between 0.1 and 1.0 mrad. A clear shoulder has been observed at an angle of 0.55 mrad. This feature in the differential cross sections is attributed to binary encounters between the protons and the quasi-free target electrons. The measured data are in good agreement in shape and magnitude with both a first Born approximation calculation of the ionisation and with a simple calculation of Rutherford scattering from free electrons.

Capture of Ar K-shell electrons by protons

The total cross section for the capture of a K-shell electron from Ar by a fast proton has been measured at kinetic energies from 1.5 to 10 MeV. To ensure that the total cross section was indeed obtained the authors also measured the K-shell capture probability as a function of impact parameter at selected energies within the interval 1.5-4 MeV. The cross sections obtained are somewhat higher than previously published cross sections. The reason for the discrepancy is discussed. The new cross sections are compared with recent theoretical cross sections.

A zero-degree tandem electron spectrometer and rydberg analyzer for projectile electron studies in ion-atom collisions☆

Abstract An electron spectrometer system consisting of a tandem 45° parallel-plate spectrometer preceded by a Rydberg analyzer has been developed to study projectile electron capture, ionization and excitation by 0° spectroscopy. This system has the ability to resolve a large number of excited and continuum atomic states of the projectile and has a low kinematic line broadening. High-n Rydberg electrons are field-ionized and decelerated in an inhomogeneous-field Rydberg analyzer. The field-ionized electrons are then energy-dispersed according to n-value. The design and performance of the system is presented with some results of continuum (cusp), Rydberg, Coster-Kronig and highly resolved (ΔE/E = 0.1–0.2%) K-Auger electrons for 0.5–1.75 MeV/amu, Fq++He, Ne, H2 collisions.

Electron capture by O8+ from aligned molecular deuterium

Abstract Velocity space distributions of deuterons from electron capture and ionization by 10 MeV oxygen ions in collision with molecular deuterium targets were measured. The experiment was performed by applying an extraction electric field perpendicular to the beam axis to project the whole velocity distribution of the recoil ions onto a two-dimensional position sensitive detector. By additional measurement of the time-of-flight of these recoil ions, we are able to reconstruct the three-dimensional velocity distribution. From the reconstructed results, the dependence of the cross sections on the alignment of the molecular axis with respect to the beam axis was determined.

Investigation of internuclear axis alignment with projectile direction in recoil ion production from ion-molecule collisions

Abstract We have developed an experimental system to measure the dependence of the cross section for the production of recoil fragments from molecular targets on the alignment of the internuclear axis of the molecule with respect to the beam. Recoil Nq+ (q = 1–5) ions, produced by a 19 MeV F9+ beam on an N2 target, are extracted in a uniform transverse electric field and allowed to fall onto a two-dimensional position-sensitive channel-plate detector. The flight time of each recoil is used to determine its charge state. The beam is charge-state-analyzed and coincidences between recoils and projectiles are measured. Information on the alignment of the molecule prior to its Coulomb explosion is extracted from the spectrum of positions with which the recoil fragments strike the recoil detector. Clear alignment effects are seen for the highest and lowest recoil charge states.

High resolution studies of electron capture and excitation by 0° projectile electron spectroscopy

Abstract A tandem 45° parallel-plate electron spectrometer has been developed for high resolution electron studies of atomic collision processes using the method of 0° electron spectroscopy. High resolution projectile K-Auger electron spectra have been obtained in a state-selective study of transfer-excitation (TE) and single excitation in collisions of 4.5–33 MeV F6+ with He and H2 targets. A high resolution measurement of 9.5 MeV F7+ with He yielded preliminary information on state-selective capture to the metastable F7+ (1s2s) state. Future plans include studies of Fq+ + T (q: 6−9; T: H2, He, Ne) to determine absolute state-selective cross sections for transfer-excitation, single and double electron capture.

Lifetime measurements of the 1P1 state of helium-like Si and Cl

K X-rays, produced by foil excitation of 48.5 MeV Cl and 47 MeV Si beams, were studied using a high resolution Doppler-tuned spectrometer. From observation of the X-ray yield at the foil as a function of target thickness, the lifetimes of the 1P1 state of helium-like Cl and Si were deduced to be 0.9+or-0.2*10-14s and 2.9+or-1.0*10-14s respectively.

Angular distributions of reaction products from low energy capture by multicharged ions

Abstract The angular deflection of the projectile which occurs when a slow multiply charged ion captures electrons from a neutral target depends sensitively on the potential curve history of the interaction throughout the encounter. We have measured angular distributions for a variety of projectiles capturing from targets of He, Ar and d2. Some of the major features may be understood in terms of classical deflection functions for the heavy particle motion. Where quantal treatments of the scattering is available, the calculations are in good agreement with the data.

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.

Coulomb explosion of D2 following capture and ionization by fast O8+ ions

Abstract In order to study two-electron processes in the collision of highly charged ions with a D 2 molecule target, measurements of cross sections for single capture, transfer ionization, single and double ionization for O 8+ on D 2 collision system were performed. During the data processing, it was found that it is very important to know the contribution to the production of D + ions from electronically excited D 2 + molecules relative to that from doubly ionized molecules. Since the D + ions from different channels have different kinetic energies, a time-of-flight technique (TOF) can be used to separate them. Therefore TOF spectra of the recoil D + ions taken with no extraction field have been measured in coincidence with the charge-analyzed projectiles for impact energies from 8 to 20 MeV. The production of D + ions from doubly ionized molecules was found to dominate at 8 MeV, while at 20 MeV the production of D + ions from electronically excited D 2 + molecules, which dissociate into D + + D, becomes important.