R.D. DuBois

Ionization and charge transfer in He2+-rare-gas collisions.

Cross sections for all significant channels leading to positive-ion and free-electron production in He/sup 2 +/-rare gas collisions are presented. The energy range studied is 15-200 keV. It is shown that the electron production is dominated by the single charge transfer plus ionization channels: even for a helium target. It is also shown that charge transfer plus an additional ionization can be more probable than charge transfer by itself. For double capture from argon and krypton, this is observed to occur throughout the entire energy range studied.

Electron-electron interactions in fast neutral-neutral collisions☆

Abstract Differential electron emission is studied for 50–500 keV H + and H atom impact on helium. Using the first Born formulation, it is shown that projectile electron-target electron interactions are expected to dominate the differential cross sections for low energy target electron emission induced by fast neutral projectile impact on any target. Measurements of the 15° electron emission were made in order to investigate this prediction. For low impact energies, a constant ratio between the hydrogen atom and proton impact cross sections was found for emitted electron velocities less than half the projectile velocity, V P . But as the collision energy increased, for electron velocities less than 0.25 V P , the cross section ratio increased as the emitted electron velocity decreased. This is interpreted as a signature of projectile electron-target electron interactions becoming dominant for distant collisions between neutral particles.

The influence of projectile electron-target electron interactions on electron emission in fast ionizing collisions

Abstract The influence of projectile-electron-target-electron (e-e) interactions leading to ionization is examined, both theoretically and experimentally, for various collision systems. Among our conclusions, we find that e-e interactions dominate the low energy electron emission in fast collisions between neutral atoms. This results in a linear, as opposed to a quadratic, projectile nuclear charge dependence for the total target ionization cross sections.

CLOSE ENCOUNTERS OF THE MULTIPLE-CAPTURE KIND - CA IONS IN AR

Abstract Collisions in which a fast highly charged ion passes within the orbit of inner-shell electrons of a target gas atom are selected by emission of an X-ray from the projectile or target. Measurement of the projectile charge state after the collision, in coincidence with the X-ray, allows measurement of the charge-transfer probability during these close collisions. When the projectile velocity is approximately the same as that of the target electrons, a large number of electrons can be transferred to the projectile in a single collision. Results for 47–160 MeV Ca 17+ + Ar show that the relative electron-capture probability in a close encounter decreases with increasing projectile velocity.

Recent studies of simultaneous ionization and charge transfer in helium ion-atom collisions☆

Abstract In our laboratory, we are interested in ionization of atomic and molecular targets induced by light ion impact. Generally these collisions are studied by monitoring the ejected electron spectra with the assumption that each ionizing event leads to a single secondary electron. By measuring the target ionization charge states produced by directly ionizing and charge changing collisions, we have obtained cross sections for direct ionization, pure charge transfer, and charge transfer plus ionization thus making it possible to distinguish how target ionization occurs. It was found that charge transfer plus ionization can sometimes be more probable than pure charge transfer alone. This is important because one mechanism which has until recently been disregarded as a means of producing free electrons is the possibility of capturing an electron from the target and simultaneously ionizing one or more target electrons. For ion impact velocities comparable to, or less than, the outer shell bound electron velocities, we have found that charge transfer plus ionization can be a substantial, if not principal, means of producing free electrons.

Coincidence techniques to study electron emission in ion-atom collisions

Abstract Electron emission as a function of emission angle and electron energy provides a very sensitive probe for testing our theoretical understanding of ion-atom collisions. For fast proton impact on gaseous targets the Born approximation has been shown to work quite well [1]. However, for structured particle impact a comparison with theory is more difficult. The experimental observation of the total electron emission must be compared with the sum of the theoretical calculations for projectile-only, target-only, and simultaneous projectile-target ionization. Thus a set of experiments will be discussed in which the electron emission is being recorded in coincidence with the post-collision projectile charge state. He+ -Ar is being studied for impact energies between 1 and 2 MeV. Coincidences between emitted electrons and stripped projectiles provide information about projectile ionization while information about simultaneous projectile-target ionization is obtained from stripped projectile-target ion coincidences.

Positron differential studies: Comparison to photoionization

In this paper we study the transition from adiabatic to sudden double ionization of valence electrons of atoms by positron impact. Double ionization by a charged particle is usually represented in terms of the shake-off (SO), twostep 1 (TS-1), and two-step 2 (TS-2) mechanisms. The relative contribution of those mechanisms is determined for double ionization of neon. For argon, the SO mechanism is shown to fit the electron impact data but not the positron data. Different post-collision interactions for positron and electron impact or the transition from the adiabatic (TS-1) to the sudden regime (SO) which could imply different interferences between single-and double-interaction mechanisms are possible interpretations for the observed differences.

Scaling of cross sections in ion-atom collisions

Differential electron emission at 15{degrees} is investigated for 50-500 keV/amu hydrogen impacting on He and H targets. From hydrogen particles (H, H{sup +}, H{sub 2}{sup +}, H{sub 3}{sup +}) ratios of cross sections relative to proton impact data, it is shown how bound projectile electrons influence the differential electron emission and where different ionization mechanisms are important. It is demonstrated that the H{sub 2}{sup +} and H{sub 3}{sup +} molecular ions interact as though they are composed of independent nuclei with the appropriate number of bound electrons, the electronic structure of the components appearing to be unimportant.

Multiple Ionization of Atomic Targets by Proton Impact

We have combined experimental measurements of multiply charged ion production, doubly differential electron emission cross sections and Auger electron spectra produced in fast-proton noble-gas collisions with theoretical inner and outer shell ionization cross sections in an effort to provide a unified description of the ionization process. A detailed analysis is provided for 1-4 MeV proton ionization of krypton. Our measured relative yields of Kr+2 and Kr+3 ions are in approximate agreement with the photoionization results of Krause and Carlson. If we assume nearly half of the 3d vacancies created in Kr are filled through multiple (3dNNN) Auger transitions, the M-subshell ionization cross sections that we derive from our Auger electron spectra are in good agreement with theoretical calculations. The measured absolute yields of Kr+2 and Kr+3 are, however, a factor of 2 smaller than those derived from the decay of inner shell vacancies. This discrepancy leads us to believe that multiple ionization in the initial interaction is a significant contribution to the observed yields.