# Atomic Site-Sensitive Processes in Slow Ar9+-Ar2 Collisions

W. Iskandar, J. Matsumoto, A. Leredde, X. Fléchard, S. Guillous, C.L. Zhou, Jimmy Rangama, A. Méry, B. Gervais, D Hennecart, et al.

► To cite this version:

W. Iskandar, J. Matsumoto, A. Leredde, X. Fléchard, S. Guillous, et al. Atomic Site-Sensitive Processes in Slow Ar9+-Ar2 Collisions. XXIX International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC2015), Jul 2015, Tolède, Spain. pp.032106, 10.1088/1742-6596/635/3/032106. in2p3-01294692

HAL Id: in2p3-01294692

https://in2p3.hal.science/in2p3-01294692v1

Submitted on 13 May 2025

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<figure><img src="image_1.png" /></figure>

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# Atomic Site-Sensitive Processes in Slow Ar<sup>9+</sup>-Ar<sub>2</sub> Collisions

To cite this article: W Iskandar *et al* 2015 *J. Phys.: Conf. Ser.* **635** 032106

View the [article online](#) for updates and enhancements.

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<figure><img src="image_2.png" /></figure>
Atomic Site-Sensitive Processes in Slow Ar⁹⁺-Ar₂ Collisions

W. Iskandar*<fnref n="1" />, J. Matsumoto§, A. Leredde†, X. Fléchard‡<fnref n="2" />, S. Guillous*, C. L. Zhou*, J. Rangama*, A. Méry*, B. Gervais*, D. Hennecart*, H. Shiromaru§ and A. Cassimi*

* CIMAP, CEA-CNRS-ENSICAEN-Université de Caen, BP 5133, F-14070 Caen cedex 5, France

§ Departement of Chemistry, Tokyo Metropolitan University, Hachiouji-shi, Tokyo 192-0397, Japan

† Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

‡ LPC Caen, ENSICAEN, Université de Caen, CNRS/IN2P3, 14050 Caen Cedex 04, France

**Synopsis** Electron capture processes for low energy Ar⁹⁺ ions colliding on argon dimer targets are investigated, focusing attention on charge sharing between the two argon atoms as a function of the molecular orientation and the impact parameter. Confrontation between experimental results and MC-COBM simulations have been investigated to provide access to atomic site sensitivity of the different fragmentation processes by measuring the angular correlation between the scattered projectile and the recoiling fragments.

Investigating an elementary reaction involving highly charged ions and molecules is a challenging subject: intramolecular charge redistribution after the collision may usually prevent the access to the primary process. Taking advantage of the low electron mobility in a dimer target, we present here a comparison between experimental and theoretical results giving access to the atomic site sensitivity for the collision system Ar⁹⁺-Ar₂ at low energy regime.

The projectile scattering angle $\phi_{proj}$ is expected to be closely related to the impact parameter vector $\vec{b}$ in the molecular frame. For molecule orientations transverse to the projectile beam, the molecular frame is defined unambiguously by the angle $\phi_{Ar^{A+}}$, where $Ar^{A+}$ is the most charged fragment (fig.1(d)). This angle is obtained by measuring in coincidence the position and time of flight of both ionic fragments. The transverse momentum transferred to the dimer center-of-mass during the collision is also inferred from these data sets, giving direct access to projectile scattering angle $\phi_{proj}$. Then, the correlation between these two angles can be obtained by calculating the angular distributions in $\phi_{diff}=\phi_{Ar^{A+}}-\phi_{proj}$.

A preference for charge-asymmetric dissociation channels is observed [1], with a strong correlation between the projectile scattering angle and the molecular ion orientation as shown in the figure 1. This measurements provide clear evidence that projectiles distinguish each atom in the dimer and that electron capture from the closest atom to the projectile is favored. Monte Carlo simulations combined with Classical Over-the-Barrier Model (MC-COBM), by considering the Ar₂ dimer target as two independent Ar atoms fixed in space, are successfully compared

to the data (fig.1). They give a new insight into the dynamics of the collision by providing, for the different fragmentation channels, the two-dimensional probability maps $p(\vec{b})$ [2].

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Angular distributions in $\phi_{diff}$ for the double, triple and quadruple electron capture and associated fragmentation channels. Experimental results (black dots) are compared to the MC-COBM calculations (red lines).</figcaption></figure>

References

[1] J. Matsumoto et al. 2010, Phys. Rev. Lett. 105 263202

[2] W. Iskandar et al. 2014, Phys. Rev. Lett. 113 143201

<footnote n="1">¹E-mail: iskandar@ganil.fr</footnote>

<footnote n="2">²E-mail: flechard@lpccaen.in2p3.fr</footnote>