EGAS41 - Swansea University

41 st EGAS CP 35 Gdańsk 2009
Electron stereodynamics in multiple ionization of rare gas
dimers by slow highly charged ions
A. Ichimura 1 , T. Ohyama-Yamaguchi 2
1 Institute of Space and Astronautical Science (JAXA),
Sagamihara, Kanagawa 229-8510, Japan
2 Tokyo Metropolitan College of Industrial Technology,
Shinagawa, Tokyo 140-0011, Japan
∗ Corresponding author: ichimura@isas.jaxa.jp
Much attention has been called to multiple ionization dynamics of molecules by impact
of highly charged ions. For diatomic molecules, the angular and charge state distributions
of Coulomb explosion fragments have been intensively measured for ‘fixed-in-space’
molecules with the momentum imaging technique. In contrast to covalent molecules, however,
rare gas dimers have been little investigated, where the dynamics is expected far
different due to electron localization character.
In the present work, we address multiple ionization (actually electron transfer) with
slow (v < 1 au) highly charged ions by modifying the three-center Coulombic over-thebarrier
model developed by the present authors [1]. In particular, we clarify electron
stereodynamics peculiar to rare gas dimers (e.g. Ne 2 ) and analyze how it manifests itself
in the orientation-dependent charge-pair distribution [2].
The outermost electrons at two atomic sites, B and C, in a target BC are assumed to
successively form a quasimolecule with the projectile ion A q+ , where the target ion-core
state is labeled by a pair of charge states, which evolves in respective steps as (q B −
1, q C ) → (q B , q C ), or as (q B , q C − 1) → (q B , q C ). It is assumed for simplicity that the
electron never returns to the target once trapped in the quasimolecule. Three ion cores
(q A = q, q B , q C ) induces a three-center Coulombic potential, which necessarily has two
saddle points. Hence an orientation-dependent critical distance for the quasimolecule
formation is determined by equating the electron energy localized at B or C with either
of the two saddle energies according to the configuration of three ion-cores. The critical
distances of all active electrons make a mosaic pattern of spacial regions for fragment
charges (r B , r C ) produced in a collision. This is in sharp contrast to a target of covalent
molecule (e.g. N 2 ), where emerges a regular shell pattern with the number r = r B + r C
of total removal electrons.
We have applied this model to collisions of A 8+ + Ne 2 with four active electrons
and calculated orientation-dependent partial cross sections σ(r B , r C ; θ) of the charge-pair
production. It is found that symmetric charge states as r B = r C (so that r = even) are
dominantly populated when the target dimer is oriented in a parallel direction to the
incident beam, while asymmetric charge states are dominant in a perpendicular direction.
Final states with |r B − r C | ≥ 2 are little produced.
References
[1] A. Ichimura, T. Ohyama-Yamaguchi, Physica Scripta T80, 190 (1999)
[2] T. Ohyama-Yamaguchi, A. Ichimura, Nucl. Instr. and Meth. T80, 620 (2003)
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