EGAS41 - Swansea University

41 st EGAS PL 4 Gdańsk 2009
Dissociative recombination of small molecular ions:
The spectroscopic frontier
Xavier Urbain
PAMO, Département de Physique, Université catholique de Louvain, chemin du cyclotron 2,
B-1348 Louvain-la-Neuve, Belgium
E-mail: xavier.urbain@uclouvain.be
Although atomic dielectronic recombination and molecular dissociative recombination
share the same mechanism, i.e. resonant electron capture followed by stabilization of
the complex, the vibrational degree of freedom offers to the latter a much higher rate,
and at the same time lifts the requirement of precise energy matching. The resonant state
being generally accessible over a broad Franck-Condon region, structureless cross sections
ensue, hiding the spectroscopic details of the molecule. We will demonstrate how highresolution
electron scattering on rovibrationally controlled molecular ions, when coupled
to fragment imaging, offers a direct access to the structure and dynamics of the molecule.
Anisotropies recorded by three-dimensional fragment imaging point to the symmetries
of the autoionizing Rydberg resonances [1]. The detailed evolution of the branching ratios
among the various dissociative channels sheds light on the long-range interactions,
performing a dynamical spectroscopy similar to what may be obtained by VUV photodissociation
experiments.
Such precision measurements are now routinely performed at the TSR facility of the MPI
for Nuclear Physics of Heidelberg (MPIK) where an ultracold electron target (500 µeV
transverse energy resolution), sophisticated ion sources (e.g. REMPI laser-ion source,
multipole trap with buffer gas cooling) and multiparticle time- and position-sensitive detectors
have been implemented. This complex arrangement has revealed the uttermost
sensitivity of low energy dissociative recombination to the rovibrational state, and the
active cooling experienced by molecular ions when merged with cold electrons [2]. The
reheating by black-body radiation competes with superelastic electron cooling, and may
be circumvented in a cryogenic environment, as offered by the Cryogenic Storage Ring
(CSR) under development at MPIK [3]. The inner vaccuum chamber of this electrostatic
storage ring will be cooled to liquid helium temperature, allowing for complete rotational
relaxation of most infrared active molecular ions. The long storage times (thousands of
seconds) will offer the possibility of laser manipulation of the ions, while merged electron
and atomic beams will be used to study the reactivity of such ultracold molecular ions,
bringing them closer to outer space conditions.
References
[1] S. Novotny et al., Phys. Rev. Lett. 100, 193201 (2008)
[2] H. Buhr et al., Phys. Rev. A 77, 032719 (2008)
[3] R. von Hahn, et al., in EPAC ’08 European Particle Accelerator Conference Proceedings,
394-396 (2008) http://accelconf.web.cern.ch/accelconf/e08/papers/mopc137.pdf
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