Mutual neutralization and charge transfer at low collision energies: the power of imaging

by Xavier Urbain (Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain)

Wednesday 21 Aug 2013, 10:00 → 11:00 Europe/Stockholm

FA32

Low energy ion-molecule reactions are difficult to characterize because of the need to separate slow impinging ions from scattered molecular ions, and most often rely on neutral atom detection [1,2]. To cover a wide range of collision energies, we designed a dedicated beam deceleration stage which allows efficient transport of the projectile ions to the interaction zone. Dissociative charge transfer of the extracted molecular ions with a potassium target, combined with momentum imaging of their atomic fragments, yields vibrational branching ratios. Absolute cross sections for the reaction H+/H2 and vibrational distributions of H2+ products were measured in the range 10 to 5000 eV [3].

Mutual neutralization studies are generally limited to energies above a few eV, and do not specify the electronic state of the products, merely indicating a band of principal quantum numbers based on time-of-flight intervals [4]. We upgraded our merged beam set-up to reach meV collision energies, and incorporated three-dimensional product imaging. Besides providing clear coincidence signals, this technique gives unambiguous identification of the electronic states of the products. Knowing their angular distribution at the different collision energies allows absolute cross sections to be retrieved. Results for the H+/H- and He+/H- systems will be presented.

[1] G. Niedner, M. Noll, J. P. Toennies, and C. Schlier, J. Chem. Phys. 87, 2685 (1987).
[2] M. W. Gealy and B. I. Van Zyl, Phys. Rev. A 36, 3091(1987).
[3] L. F. Errea, L. Fernández, L. Méndez, B. Pons, I. Rabadán, and A. Riera, Phys. Rev. A 75, 032703 (2007); X. Urbain et al., submitted to PRL.
[4] M. Terao, S. Szücs, M. Cherkani, F. Brouillard, R. J. Allan, C. Harel, and A. Salin, Europhys. Lett. 1, 123 (1986).