ION-ION COLLISIONS INVOLVING MOLECULAR TARGETS:
ELECTRON CAPTURE FROM H₂⁺ BY He²⁺ AND Ar²⁺

I Reiser¹, H Bräuning², A Diehl², A Theiß², E Sidky¹, M Stöckli¹, C L Cocke¹, E Salzborn²

1) J.R.MacDonald Lab, Kansas State University, Manhattan, Kansas, USA
2) Institut für Kernphysik, Strahlenzentrum, Justus-Liebig Universität, Giessen, Germany

We investigated electron capture in collisions of positive ions with H₂⁺, the simplest molecular target. Such experiments can be carried out only through ion- ion collisions. In particular, He²⁺ - H₂⁺ is a true one- electron molecular collision system, providing a more stringent test for theoretical models^1,2,3,4. The aim of this present study is to explore whether electron cap- ture from a molecule can be thought of as coherent capture from two isolated centers. So far, this rela- tively simple picture for an ion-molecule collision has been adapted and compared to H₂ targets^2,4. Our choice of a true one-electron molecular target enables to focus on the two-center aspect only. The first step towards this goal was to determine the absolute total electron capture cross sections for the reactions

The experiment has been carried out at the crossed-beam ion-ion collision facility in Giessen⁵. Both molecular and atomic ions were produced in ECR ion sources. After intersecting each other at an angle of 17.5°, both ion beams were charge- state analyzed through electrostatic analyzers. The atomic charge-exchange products were detected with a channeltron detector. The molecular fragments were detected on a position-sensitive MCP detector in order to verify the collection of the entire Coulomb-sphere of exploding H₂²⁺. The experimental results for the H₂⁺ + He²⁺ collision system are shown in Figg. 1.

We also calculated cross sections for reactions (1) and (2), following the one-electron part of a model proposed by Shingal and Lin². Within this model, atomic transition amplitudes for the collision between the projectile and each one of the hydrogenlike atoms are added coherently. The atomic capture amplitudes were obtained through a close-coupling calculation for a He²⁺ (Ar²⁺) - H(1s) (q_eff=1.25) collision and added coherently for two molecular centers separated by 2 a.u. , the equilibrium separation of H₂⁺. One obtains the electron capture cross section for a fixed alignment of the molecule with respect to the projectile velocity. In order to compare to the data, we averaged over all molecular alignments. The comparison of the calculated TCS with the data shows very good agreement.

The next step in this study will be to extend the measurements to cross sections as a function of alignment angle in order to observe interference of the molecular centers.

Figures:

Figure 1: Total cross sections for H₂⁺ + He²⁺ - H⁺ + H⁺ + He⁺
Symbols: Experiment, Line: Theory (s. text)

References:

1) L F Errea, J D Gorfinkiel, A Macias, L Mendez, and A Riera, J. Phys. B 30, 3855 (1997)
2) R. Shingal and C.D. Lin, Phys. Rev. A 40, 1302 (1989)
3) M. Kimura, Phys. Rev. A 32, 802 (1985)
4) Wolfgang Fritsch, Phys. Rev. A 46, 3910 (1992)
5) K Rinn, F Melchert and E Salzborn, J. Phys. B 18, 3783 (1985)

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

This work was also supported by the Deutsche Forschungsgemeinschaft (DFG) and the Alexander von Humboldt foundation (H.B.).

Submitted to ICPEAC 2001, July 2001 in Santa Fe, NM.

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