STUDY OF 2 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 X e and metallic vapour (strontium, barium, zinc) targets. The Stabilize d Double Capture (S.D.C.) process, in which two electrons are captured and stabilized on the projectile, has been studied by measuring the r adiative stabilization ratio R-s(2)(R-s(2) = sigma(S.D.C.)/(sigma(S.D. C.) + sigma(A.D.C.)); sigma(S.D.C.) and sigma(A.D.C.) stand for the S. D.C. and Autoionized Double Capture cross sections) and by using optic al spectroscopy of Rydberg transitions. We found that the population o f asymmetrical configurations (n, n' much greater than n) is responsib le for high radiative stabilization ratios. The population of these co nfigurations is due to Auto-transfer into Rydberg state (A.T.R) and Tr ansfer 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 fr om the A.T.R model. We also measured the stabilization ratio for highe r charge state ions Ar14+, Ar16+, Ar17+, Kr18+, Xe25+, Xe27+ on stront ium and rare gas collisions and observed a slight enhancement of R-s(2 ) values for higher charge states. The observation of high Rydberg tra nsitions for Xe27+ on Xe collision shows that the radiative stabilizat ion process is also explained by a population of asymmetrical configur ation issued from A.T.R and T.E, processes.