# Anion emission from H2O molecules colliding with positive O+ ions at keV energies: the role of dissociative excitation

J-y Chesnel, Z. Juhasz, E. Lattouf, J Tanis, B. A. Huber, E. Bene, S Kovács, P. Herczku, A. Méry, Jean-Christophe Poully, et al.

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J-y Chesnel, Z. Juhasz, E. Lattouf, J Tanis, B. A. Huber, et al. Anion emission from H2O molecules colliding with positive O+ ions at keV energies: the role of dissociative excitation. Journal of Physics: Conference Series, 2015, XXIX International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC2015) 22–28 July 2015, Toledo, Spain, 635 (3), pp.032108. 10.1088/1742-6596/635/3/032108. [hal-02894120](https://hal.science/hal-02894120v1)

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# Anion emission from H₂O molecules colliding with positive O⁺ ions at keVenergies: the role of dissociative excitation

To cite this article: J-Y Chesnel et al 2015 *J. Phys.: Conf. Ser.* **635** 032108

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

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# Anion emission from H₂O molecules colliding with positive O⁺ ions at keV energies: the role of dissociative excitation

J.-Y. Chesnel*<fnref n="1" />, Z. Juhász†, E. Lattouf*, J. A. Tanis*§, B. A. Huber*, E. Bene†, S. T. S. Kovács†, P. Herczku†, A. Méry*, J.-C. Poully*, J. Rangama*, and B. Sulik†

**Synopsis** It is shown that negative ions can be ejected from gas-phase water molecules when bombarded with positive ions at keV energies. Emission of both H⁻ and heavier (O⁻ and OH⁻) anions is observed. Electronic excitation and ionization processes are shown to play a decisive role in cation-induced anion formation.

We show that negative ions can be ejected from free water molecules by the impact of positive ions. The experiment was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) in Caen, France. The ion beam crossed an effusive gas jet of H₂O. The collision-induced ionic fragments were selected according to their kinetic energy by means of an electrostatic analyzer. To avoid electron contamination of the anion spectra, the electrons and anions were magnetically separated so that only anions could reach the detector.

Figure 1 shows experimental double-differential cross sections (DDCS) for anion emission in 6.6-keV ¹⁶O⁺ + H₂O collisions. The main component of each spectrum is a broad, slowly decreasing structure. There, the relative contribution of H⁻ anions was derived from complementary time-of-flight (TOF) measurements. At forward angles (less than 90°) pronounced peaks are observed at higher energies. These peaks are due to recoil anions formed in hard binary collisions occurring at small impact parameters [1-3], while the broad contribution results from soft many-body processes.

To further interpret our results, numerical simulations of the trajectories of the different fragments were performed. The method is similar to the one used in [4]. The calculated DDCS for H⁻ emission are shown in Fig. 1 (dotted curves). At low energies the calculated DDCS underestimate appreciably the experimental ones since in most of the simulated collisions at large impact parameters (> 1 a.u.) the kinetic energy transferred to the H atom is lower than the dissociation energy of an OH bond of H₂O in its ground state. However, electronic processes such as dissociative excitation and ionization are likely to lead to kinetic energy release (KER). By introducing into the model the same

KER distribution as in [4], the simulated cross sections (open circles in Fig. 1) agree fairly well with the TOF data points (full circles) and reproduce satisfactorily the experimental H⁻ recoil peak. According to the model, without electronic excitation or ionization, the total cross section for H⁻ emission would be about 3 times smaller.

This work was supported by the Hungarian National Science Foundation OTKA (Grant No. K109440), the French-Hungarian Cooperation Program PHC Balaton (No. 27860ZL/TéT_11-2-2012-0028), and the French-Hungarian CNRS-MTA Cooperation (ANION project).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Full curve: Experimental DDCS for anion emission in 6.6-keV ¹⁶O⁺ + H₂O collisions; Full circles: H⁻ contribution from TOF measurements. Open circles: Simulated DDCS for H⁻ emission at different angles with KER; Dotted curve: Same without KER; Each spectrum is multiplied by the indicated factor.</figcaption></figure>

## References

[1] Z. Juhász et al 2012 *J. Phys. Conf. Ser.* **388** 102051

[2] Z. Juhász et al 2013 *Phys. Rev. A* **87** 032718

[3] Z. Juhász et al 2014 *J. Phys. Conf. Ser.* **488** 102024

[4] E. Lattouf et al 2014 *Phys. Rev. A* **89** 062721

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<footnote>*Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), 6 Bd Mar. Juin, F-14000 Caen, France</footnote>

<footnote>†Institute for Nuclear Research, Hungarian Academy of Sciences (MTA Atomki), H-4001 Debrecen, Hungary</footnote>

<footnote>§Department of Physics, Western Michigan University, Kalamazoo, Michigan 49008 USA</footnote>

<footnote n="1">¹E-mail: jean-yves.chesnel@ensicaen.fr</footnote>