J. Gavin

Positron and electron impact double ionization of argon: How 1st- and 2nd-order mechanisms influence the differential electron emission

Double-to-single–ionization ratios for electron emission as a function of the angle are measured for 200, 500, and 1000 eV positron and electron impact on argon. Both the sign of the projectile charge and the impact energy are shown to influence the angular dependences. By combining these ratios for positron and electron impact, information about how first- and second-order double-ionization mechanisms interfere and contribute to the total differential electron emission at different collision velocities is obtained.

Differential ionization studies for positron impact

Recent studies of differential ionization by positron impact being performed at the University of Missouri-Rolla are described. Recoil ion-scattered positron and recoil ion-scattered positron-ejected electron coincidences are measured to provide doubly and triply differential ionization data (DDCS and TDCS) for e+ - Ar collisions. The DDCS and TDCS studies are for 750 and 200 eV impact respectively. For the TDCS studies the positron scattering angles are between approximately ± 5o and electrons ejected with energies less than 15 eV and angles between 90o ± 50o with respect to the beam direction are detected. Examples of TDCS data for the scattered positron and the ejected electron are presented.

Projectile Charge Effects in Differential Ionization by Positrons and Electrons

Differential data for single and double ionization of argon by positron and electron impact are presented and compared. For single ionization, coincidences between scattered projectiles, ejected electrons and recoil ions are measured as functions of the projectile scattering angle and energy loss. Differences associated with the sign of the projectile charge are indicated with regard to the scattering angle, the energy loss, and the relative intensities for binary and recoil events. Ejected electron‐recoil ion coincidences are also measured as a function of the observation angle along the beam direction. From these, double to single ionization ratios are determined and compared. Differences in the magnitudes and angular dependences for positron and electron impact are attributed to the interference between the TS‐1 and TS‐2 double ionization mechanisms.

Differential Cross Sections for Ionization of Argon by 1 keV Positron and Electron Impact

Differential information was generated by establishing coincidences and imposing conditions on data recorded for target ions, scattered projectiles, and ejected electrons, as a function of projectile energy loss and scattering angles; in order to describe the interaction between a positron (electron) 1 keV beam and a simple Ar jet. Single ionization triply differential cross section (TDCS) results exhibit two distinct regions (lobes) for which binary (events arising from 2-body interaction) and recoil (events which can only be produced by many-body interactions) interactions are associated. Results indicate that binary events are significantly larger for positron impact, in accordance with theoretical predictions. A similar feature is found for different energy losses and scattering angles. Intensity of the recoil lobe for both projectiles, positron and electron, is observed to depend on the energy loss and scattering angle. Also, it can be noticed that for positron impact the recoil interactions intensity is larger than that observed for electron impact.

1st and 2nd order contributions to the double ionization of argon resulting from positron and electron impact

Angular dependences for electron emission resulting from single and double ionization of argon are presented for 200, 500, and 1000 eV positron and electron impact. From these data, double to single ionization ratios are determined as a function of the emission direction. At each impact energy, these ratios have different angular dependences positive and negative particle impact. The positron and electron induced ratios are analyzed to provide information about how first- and second-order double ionization mechanisms interfere and contribute to the differential electron emission.

Triply differential ionization of Ar by 500 eV positron and electron impact

Coincidences between recoil ions-ejected electrons and recoil ions-scattered projectiles have been used to study the kinematics of electron and positron impact ionization. Triply Differential (TDCS) data for 500 eV positron and electron impact on Ar are presented here as function of scattering angle for a given range of energy losses. Binary and recoil interactions can be distinguished allowing us to determine the relative intensity between those interactions. Preliminary integration of the data indicate an enhancement of the binary region for positron interaction while for electron impact the intensity of the recoil and binary interactions is comparable.