Double electron capture into highly charged ions: correlated or independent?

Abstract

We propose an independent-electron model for describing q u a n t i t a t i v e l y the population of doubly excited ( r~' ) s t a t e s formed by two-electron capture into m u l t i p l y charged ions. Population p r o b a b i l i t i e s c a l c u l a t e d by t h i s model reproduce experimentally determined ones if one assumes that the classical electronic angular momentum is preserved, in magnitude and also in orientation. Correlation during the collision need not be invoked for the interpretation of the spectra. I IntroductionAlthough correlation has a strong influence on the formation of stationary states, the question remains whether it is also important during the process of two-or many-electron transfer and if it directs the system into certain states [1,2] -e.g. states with such a mixing of configurations that they resemble to a large extent a triatomic molecule performing a rotational or vibrational motion. In order to obtain a quantitative basis for this discussion we have developed a model which predicts population probabilities for the various doubly excited states from an independent transfer of two electrons from the target to the projectile. II ExperimentWe have measured energy spectra of autoionization electrons resulting from collisions of bare and He-like ions on the two-electron targets He and molecular hydrogen at energies below 10 keV/amu. Electrons ejected from doubly excited (3~3~') projectile states were analyzed in a cylindrical mirror analyzer [2] at an emission angle of 50 ° with respect to the projectile beam direction. Fig.l shows a spectrum resulting from collisions of 60 keV Z3C6+ on molecular hydrogen. A fit of these spectra was made, in which energetic positions and lifetimes of the states involved were taken from theory [3,4] and in which only the peak intensities were used as fit parameters. Peak shapes are strongly influenced by post collision interaction and this is taken into account in the calculations. The possibility of obtaining good fits in this way confirms the accuracy of the theoretical energies and lifetimes. In order to obtain state populations from peak intensities one has in principle to know the angular distributions I(~) of ejected electrons. However we have performed calculations which indicate that I(O) is mainly governed by the second order Legendre polynomial P2. Our detection angle of 500 is sufficiently close to the magic angle of @=54.70 for which P2 is zero, that the intensities found in the fit reflect the population probabilities to a large extent. Ill Model calculationsIn a first step we calculate amplitudes for population of a certain configuration (n6m,n'~'m') for each impact parameter b as