Vol-II * Issue-X* Inner-and Outer-Shell Electron Dynamics in H + +Na (3s) Collisions Sanjay Bector

Vidhu Khullar
2016 unpublished
Introduction Collisions between protons and alkali atoms have been studied extensively in the past. In 1964 Donnally et al. [178] reported first measurements on cross sections for the production of metastable H(2s) in H + +Cs collisions. The study of this charge exchange reaction was motivated by the development of spin polarized ion sources. The choice for an alkali target over the earlier proposed molecular hydrogen target as donor to feed the H(2s) channel [179] was taken because for alkalis
more » ... the ionization energies are lower and the energy defects with the H(2s) state smaller. This leads to larger cross sections which maximize at lower impact energies. The latter facilitates the separation of the metastable hydrogen atoms from the beam of protons by electric or magnetic fields. Soon after, charge exchange on alkalis was proposed to be the first step in the production of polarized negative hydrogen ions [180]. From the 1970's on the motivation to study collisions between protons and alkali atoms shifted towards fusion research (see e.g. [181,182]). From a theoretical perspective, an appealing feature of ion-alkali-atom collision systems is the shell structure of the alkalis, i.e., a single valence electron outside closed inner shells. It suggests the applicability of quasi-one-electron models in which the dynamics of the loosely bound Abstract In this paper a detailed study of keV H + +Na collisions is presented. The MOTRIMS experiments confirm that capture of the outer-shell 3s electron dominates at low energy. But for higher energies they present the first direct evidence of charge transfer being dominated by capture of a 2p inner-shell electron instead of outer-shell capture. With this observation one-electron capture can be seen as a result of two distinct processes. At low energies, E < 10 keV/amu, it is dominated by outer-shell capture into H(n = 2), while at high energies, E > 40 keV/amu, inner-shell capture from the 2p-shell into H(n= 1) is the main process, i.e. already at energies lower than expected from the "velocity matching" argument. In the Na + recoil spectra two inner-shell capture processes could be identified, namely ISC leaving the outer-shell electron in the 3s state or exciting it to 3p. The relative intensities of these processes revealed the prominent role of multi-electron dynamics in low energy inner-shell capture and a transition to ISC without active outer-shell participation occurring at ~ 1.4 vorb of the outer-shell electron. Inner-shell capture leading to Na + recoils has larger cross sections than that ofNa 2+ production, the latter being dominated by transfer ionization. Good overall agreement between our MOTRIMS data and the TC-BGM calculations has been found.
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