Excitation and charge transfer in low-energyNa+-Ne collisions

Abstract

This work presents a multifaceted experimental and theoretical study of collisions of ${\mathrm{Na}}^{+}$ ions in the energy range 0.2-12 keV with Ne atoms. Four different collision experiments are reported here. With one, doubly differential cross sections in scattering angle and energy loss were obtained by electrostatic analysis for direct collisions in the energy range 0.5-3.0 keV. With a second, energy-loss spectra for the neutral component of the scattered beam were obtained by time-of-flight techniques. With a third, energy spectra of electrons ejected by autoionization decays were obtained in the impact energy range 0.5-12 keV. Finally, in a fourth experiment, cross sections were obtained for some important Neii transitions excited by collisions in the energy range 0.2-10 keV. The results of these four experiments are combined with results from previous investigations to give detailed information on the relative role of the various one- and two-electron excitation processes. The interpretation of the experimental findings is carried out within the quasimolecular framework and is based on the results of ab initio calculations for a variety of potential-energy curves of the ${(\mathrm{N}\mathrm{a}\ensuremath{-}\mathrm{N}\mathrm{e})}^{+}$ quasimolecule. It is shown that the observed processes are induced by a succession of independent transition mechanisms, namely, (i) primary single and double excitation of $\mathrm{Ne}2p$ electrons at curve crossings of the $4f\ensuremath{\sigma}$ molecular orbital with empty $\ensuremath{\sigma}$ orbitals, (ii) $\ensuremath{\sigma}\ensuremath{\rightarrow}\ensuremath{\pi}$ population transfer via rotational coupling, (iii) core rearrangement processes ($L$-vacancy sharing), and finally (iv) outer-electron (Rydberg) sharing. The estimation of the importance of the different mechanisms is made using simple models (Landau-Zener, Demkov-Meyerhof) and extrapolations of previous theoretical and experimental results on the isoelectronic Ne-Ne system. Special attention has been focused on the production of $\mathrm{Ne}(2{p}^{5}3p)$ triplet states, where it is demonstrated that simultaneous excitation of both target and projectile can well account for the Ne triplet excitation without invoking a breakdown of the Wigner spin-conservation rule.