Production of doubly excited projectile states in collisions of 0.1 MeV/u Ag⁺⁴ ⁺ Ions with He, H₂ ₊ and Ar gas targets

An important problem in atomic dynamics is the structure of very high doubly excited states, in which two electrons are excited away from the rest of the electrons in the ionic core, making their mutual interaction and correlation as important as their individual interaction with the remaining core. In this work, we study the formation of doubly excited projectile states near the double escape threshold in collisions of 0.1 MeV/u Ag⁺⁴ ions with He, H₂, and Ar gas targets. We detect projectile states in which one electron occupies a high Rydberg state and a second electron (a cusp electron) occupies a low lying continuiim state. Cusp electrons are characterized by a velocity which is nearly equal in magnitude and direction to the projectile velocity. Cusp electrons are energy analyzed with a 30° parallel plate energy analyzer, and Rydberg electrons are field ionized in a spherical ionizer before being energy analyzed in a 160° spherical sector analyzer. Standard coincidence measurements between a cusp and a Rydberg electron signal the production of a doubly excited projectile state. The cross sections for producing these doubly excited projectile states are on the order of 10^-20 cm². The production probabilities are measured as a function of cusp electron laboratory frame energy with the detected Rydberg electrons arising from a fixed band of principal quantum numbers. The laboratory frame energy distributions of cusp electrons associated with these doubly excited states are then compared to the energy distribution of cusp electrons measured without imposing a Rydberg electron coincidence requirement. For the He target, the shapes of the two energy distributions are the same. However, the width of the energy distribution of cusp electrons measured in coincidence with Rydberg electrons is smaller than the width of the cusp electron energy distribution without a coincidence requirement for the H₂ and Ar gas targets, the difference being greatest for Ar. The width of the cusp electron energy spectra measured in coincidence with a Rydberg electron decreases as the fraction of doubly excited states formed by double excitation/ ionization of the projectile decreases and the fraction formed by double target-electron capture increases.