Coincident detection of electrons ejected at large angles and target recoil ions produced in multiply-ionizing collisions for the 1MeV/uO[q+](q=4,7) + Ar collision system

We have designed and constructed a new multi-spectrometer system for use in ion-atom collision experiments. This new multi-spectrometer system can detect electrons ejected at 0°, 180°, and into an angular range from about 45° to about 135° measured relative to the incident beam direction as well as the recoil ions produced in ion-atom collisions. A charge state analyzer allows one to determine the exit charge state of the projectile after interacting with the target gas. A new computer-based data acquisition system has been designed and built and its associated software has been developed in order to collect and store the large amount of data that can be acquired with this new apparatus.

In our experiments we have measured, in coincidence, the angular distributions of electrons ejected into an angular range from about 45° to about 135° relative to the incident beam direction and the target recoil ions produced in multiply ionizing collisions for the 1MeV/uO^q+ (q=4,7) + Ar collision systems. These measurements have been performed for 179 eV, 345 eV, and 505 eV electrons. Additionally, we have measured the energy distributions of electrons ejected into specific angular regions. The goal of the experiments is to determine whether or not binary encounter processes alone can explain the angular distributions of electrons ejected at large angles in multiply ionizing collisions.

Ar LMM satellite Auger electrons appear as a peak in the energy spectrum of electrons ejected at large angles. The center of this peak is found at an electron energy of 179 eV. Electrons of 179 eV energy, ejected at large angles, are preferentially produced in coincidence with recoil ions of charge state 4+. The angular distribution measured for 179 eV electrons is essentially flat for ejection angles that are greater than 90° and exhibits a peak in the forward direction. The flat angular distribution for electrons ejected at angles greater than 90° is due to the Ar LMM satellite Auger electrons. Electrons of 345 eV energy and 505 eV energy ejected at large angles are preferentially produced in coincidence with recoil ions of charge state 3+. The angular distributions for these electrons are strongly peaked in the forward direction. There are essentially no electrons ejected at angles larger than 90°. These results are consistent with the dominant production mechanism for electrons ejected at large angles being a binary encounter process. No evidence is seen for "hot" electrons, which are energetic electrons ejected at large angles from the incident beam direction in addition to binary encounter electrons or Auger electrons. Differential cross sections have been calculated from these angular distributions. They are on the order of 10^-21cm²/eV ster.