Speaker
Description
Anions play important roles in the chemistry of various astrophysical environments ranging from planetary and stellar atmospheres to interstellar clouds [1-3]. A key reaction for the ionization balance in those media is the Mutual Neutralization (MN) of atomic or molecular anions and cations [3]: A+ + B− → A + B.
MN studies with atomic ions have so far mainly been limited to collision energies down to a few eV, which is higher than the range of energies relevant for cold astrophysical environments. Moreover, these experiments could not resolve the electronic, angular momentum, and spin states of the neutral products [4].
We recently upgraded the merged beam setup in Louvain-la-Neuve to reach 5 meV collision energies, and incorporated three-dimensional momentum imaging using two position sensitive detectors located 3.25 meters downstream from the 7 cm long region where the beams overlap. Besides providing clear coincidence signals, this technique gives unambiguous identification of LS-terms of the products via the measurement of the Kinetic Energy Release (KER). The KER-resolution (50 meV FWHM at 1 eV) is mainly limited by the finite length of the interaction region. Measuring the momentum distribution of both products yields the total and the energy and angular differential cross sections. The absolute cross section scale is defined by the simultaneous measurement of associative ionization A+ + B− → AB+ + e− cross sections by means of a 180∘ bending magnet, a 30∘ electrostatic deflector, and a channeltron detector.
We present absolute MN cross sections for N+ colliding with O− in the 0.005-10 eV energy range and KERspectra at subthermal energies for C+ on P−, C−, Si−, O− and S−. The latter leads to identifications of the capture states (n, L, and S) of the products. While several of the A+ + B− systems that we have studied give results well accounted for by means of simple multi-channel Landau-Zener calculations, there are striking exceptions. For the C+ + O− and C+ + P− systems, there are final states populated that require much stronger couplings to the initial channel than expected.
The present study will also serve to benchmark merged ion beams studies at the double electrostatic storage ring DESIREE now in operation at Stockholm University [13]. With DESIREE it will be possible to study MN between molecular ions with very low internal energies (down to 10 K).
References
[1] D. Smith and P. Spanel, Mass Spectrom. Rev. 14, 255 (1995).
[2] P. Chaizy, H. Rème, J. A. Sauvaud, C. d’Uston, R. P. Lin, D. E. Larson, D. L. Mitchell, K. A. Anderson, C.W. Carlson, A. Korth, and D. A. Mendis, Nature 349, 393 (1991).
[3] M. Larsson, W. D. Geppert, and G. Nyman, Rep. Prog. Phys. 75, 066901 (2012).
[4] M. Terao, S. Szücs, M. Cherkani, F. Brouillard, R. J. Allan, C. Harel, and A. Salin, Europhys. Lett. 1, 123 (1986).
[5] R. D. Thomas, H. T. Schmidt, G. Andler, M. Björkhage, M. Blom, L. Brännholm, E. Bäckström, H. Danared, S. Das, N. Haag, et al., Rev. Sci. Instrum. 82, 065112 (2011).
