Study of two electron capture in highly charged ion-atom collisions by Rydberg spectroscopy

We present an extensive study of double electron capture processes in slow collisions of bare ions from C⁶⁺ to Al¹³⁺ with gases from He to Xe and metallic vapour (strontium, barium, zinc) targets. The Stabilized Double Capture (S.D.C.) process, in which two electrons are captured and stabilized on the projectile, has been studied by measuring the radiative stabilization ratio R²_s(R²_s = σ_S.D.C./(σ_S.D.C. + σ_A.D.C.); σ_S.D.C. and σ_A.D.C. stand for the S.D.C. and Autoionized Double Capture cross sections) and by using optical spectroscopy of Rydberg transitions. We found that the population of asymmetrical configurations (n, n' » n) is responsible for high radiative stabilization ratios. The population of these configurations is due to Auto-transfer into Rydberg state (A.T.R.) and Transfer excitation (T.E.) processes. In the case of F⁹⁺ + Ne collision system, the n' distribution of excited Rydberg states has been deduced from a precise analysis of the light emitted by Rydberg transitions. The distribution is compared with a theoretical distribution issued from the A.T.R. model. We also measured the stabilization ratio for higher charge state ions Ar¹⁴⁺, Ar¹⁶⁺, Ar¹⁷⁺, Kr¹⁸⁺, Xe²⁵⁺, Xe²⁷⁺ on strontium and rare gas collisions and observed a slight enhancement of R²_s values for higher charge states. The observation of high Rydberg transitions for Xe²⁷⁺ on Xe collision shows that the radiative stabilization process is also explained by a population of asymmetrical configuration issued from A.T.R. and T.E. processes.