State-selective electron capture in collisions of multicharged ions with Rydberg atoms

Atoms in a Rydberg state should be exceptional targets for a strongly-enhanced electron-capture process in collisions with multicharged ions since a quadratic increase of the cross section is expected when the binding energy of the target electron decreases. Simultaneously, because of the quasi-resonance of the capture process, very high orbitals should be populated. Laser excitation of the target atom to a Rydberg state, combined with a new detector based on field ionization and energy spectrometry, has made it possible to detect very high Kr^7+*(n) Rydberg states after single electron capture. A wide n distribution, up to n ≈ 100, of the capture cross section has been observed. Several collision systems composed of a Kr⁸⁺ projectile and a well-defined Rydberg state Rb*(n_ip) of the target atom have been investigated with various kinetic energies and various initial n_ip states (15 < n_i < 25). The production of doubly excited Kr^6+**(n_o, n) ions, after a two-electron capture has been investigated as well, and can be explained by two successive collisions with Rb(5s) and Rb*(n_ip) atoms, leading to n_o = 10 and a wide distribution of n states.