EGAS41 - Swansea University

41 st EGAS CP 36 Gdańsk 2009
Electron stereodynamics in charge-asymmetric Coulomb
explosion of N 2 molecules with slow highly charged ions
T. Ohyama-Yamaguchi 1,∗ , A. Ichimura 2
1 Tokyo Metropolitan College of Industrial Technology,
Shinagawa, Tokyo 140-0011, Japan
2 Institute of Space and Astronautical Science (JAXA),
Sagamihara, Kanagawa 229-8510, Japan
∗ Corresponding author: yamaguti@tokyo-tmct.ac.jp
This investigation is motivated by a triple coincidence measurement [1] with the momentum
imaging technique for collisions of slow (10 — 200 eV/amu) Kr 8+ ions with
N 2 molecules. In a coplanar geometry are detected a dissociating ion pair (N Q+ , N Q′+ )
in transverse to the beam axis (⃗v) and a scattered ion in forward left and right directions.
A remarkable result thereby was unequal strengths of charge-asymmetric fragmentation
between the near and far sites relative to the projectile trajectory. For a given
nonequivalent charge pair Q ≠ Q ′ , the asymmetry parameter is obtained as A(Q, Q ′ ) =
(P > −P < )/(P > +P < ) through P > and P < , where P > (P < ) denotes a coincident population
for Q far > Q near (Q far < Q near ), with Q far and Q near being the far and near fragment
charges. The experiment indicated a striking positive asymmetry A; the far site is populated
more by a higher charge than by a lower charge. This result was interpreted [1] to
come from target electron polarization by the projectile charge during a collision.
In previous works [2], we have analyzed the observations by developing three-center
Coulombic over-the-barrier models. Although leading to a correct sign of A, the polarization
effect was found negligibly small with a two-step picture, i.e., ‘electron removal
followed by Coulomb explosion’. Hence, we extended the model so as to describe an
inseparable process, i.e., formation of a three-center quasi-molecule and its decay into
three (one scattering and two dissociating) moving atomic ions. It was suggested that
large asymmetry arose from rapid bond elongation during a collision for small impact
parameters as b < 5 au, where all 10 valence electrons form the quasi-molecule.
In the present contribution, we further analyze the charge-pair distributions by applying
the model above. The electron localization probabilities are estimated at configurations
of three ion cores determined for respective electrons (rank t = 1 ∼ 10) with
respective impact parameter vectors ⃗ b (⊥ ⃗v). We thereby calculate the parameter A of
asymmetric population and compare it with the experimental result.
References
[1] M. Ehrich, U. Werner, H.O. Lutz, T. Kaneyasu, K. Ishii, K. Okuno, U. Saalmann,
Phys. Rev. A 65, 030702(R) (2002); T. Kaneyasu, T. Azuma, K. Okuno, J. Phys. B: At.
Mol. Opt. Phys 38, 1341 (2005)
[2] T. Ohyama-Yamaguchi, A. Ichimura, J. Phys.: Conf. Series 58, 247 (2007); ibid, to
be published in J. Phys.: Conf. Series (2009).
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