J. Desesquelles

Collision Spectroscopy of Quartet and Doublet Transitions in doubly excited Ne7

We present the observation of VUV transitions between doubly excited states of Ne7+. Spectra were selectively produced by single electron capture in a cesium vapor by Ne8+ ions prepared in the metastable state 1s2s 3S by a previous collision of a Ne9+ beam with a gas target. The observed lines were assigned to 1s2l2l → 1s2lnl and 1s2l3d → 1s2lnf electric dipole transitions. Furthermore, the forbidden electric quadrupole transition 1s2s2p 4Po-1s2p3d 4Fo has been identified at 78.71A.

Ar8+ - C60 collisions at v = 0.4 a.u. electron capture and target fragmentation

We report experiments concerning collisions of slow Ar8+ ions on a gaseous C60 target. We measured recoil ion mass spectra in coincidence with the final charge state analysed projectiles Ar7+, Ar6+, Ar5+ and Ar4+ successively and we were able to establish correlation between low energy fragments produced by break up of molecular C60r+ ions. We also measured the relative cross sections of multicaptured processes, especially the stabilization ratio of two electrons in double capture process which has been found Rs2 = 5%. The r-electron capture cross sections for r = 1-5 are compared with predictions of the C.B.M. barrier model.

Double Rydberg States in Stabilized Triple Capture in Bare Ions (Z = 6-10) on Rare Gas Target Collisions

We report converging evidence from two different coincidence experiments of the formation of triply excited states C3+(2, n, n) and Ne7+(3, n, n) with two n, n (n and n >> 3) Rydberg electrons. They are formed by triple electron capture in collision of C6+ and Ne10+ bare ions with Ar at few keV. This detailed study was triggered by measurements of surprisingly large cross sections of stabilized triple electron capture (STC) for which a full radiative decay leaves the three electrons bound to the projectile. Both the energy gains and the visible photon spectra support the interpretation in term of a large population of triply excited double Rydberg states among which some states decay without autoionization. For both systems, possible population mechanisms are discussed in term of purely atomic mechanisms occurring in the post collisional evolution of the quasi symmetrical triply excited states initially populated by the collision.

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 R2s(R2s = σS.D.C./(σS.D.C. + σA.D.C.); σS.D.C. and σA.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 R2s 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.

Doubly excited states in lithiumlike Si XII, S XIV and Ar XVI

The 3d–4f transitions between doubly excited states have been studied in Si XII, S XIV, Ar XVI. Moreover three 3d – 5f and 4f–5g lines have been identified in Si XII.

Fast beam quantum beat study of fine structure evolution in an electric field

Stark shifted energy splittings have been calculated in He I (n3D) as a function of static electric field strength. Quantum beats have been observed after beam-foil interaction in 4He I (4 and 53D) for a series of electric fields applied parallel to the helium beam. Frequencies are determined by fitting the theoretical expressions for time evolution of fine structure. Tensor polarisabilities have been deduced.