# Comparison between anion and cation emission from CH_4 molecules colliding with 10.5-keV C⁺ ions: fragment-energy aspects

J.-Y. Chesnel, Z. Juhasz, E. Lattouf, B. Huber, S. T. S. Kovács, P. Herczku, E. Bene, V. Vizcaino, A. Méry, J.-C. Poully, et al.

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

► To cite this version:

J.-Y. Chesnel, Z. Juhasz, E. Lattouf, B. Huber, S. T. S. Kovács, et al., Comparison between anion and cation emission from CH_4 molecules colliding with 10.5-keV C⁺ ions: fragment-energy aspects. Journal of Physics: Conference Series, 2017, 875, pp.102013. 10.1088/1742-6596/875/11/102013. [hal-02894110](https://hal.science/hal-02894110)

HAL Id: hal-02894110

https://hal.science/hal-02894110

Submitted on 29 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Comparison between anion and cation emission from CH_4 molecules colliding with 10.5-keV C⁺ ions: fragment-energy aspects

J.-Y. Chesnel, Z. Juhasz, E. Lattouf, B. Huber, S. T. S. Kovács, P. Herczku, E. Bene, V. Vizcaino, A. Méry, J.-C. Poully, et al.

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

► To cite this version:

J.-Y. Chesnel, Z. Juhasz, E. Lattouf, B. Huber, S. T. S. Kovács, et al., Comparison between anion and cation emission from CH_4 molecules colliding with 10.5-keV C⁺ ions: fragment-energy aspects. Journal of Physics: Conference Series, 2017, 875, pp.102013. 10.1088/1742-6596/875/11/102013. [hal-02894110](https://hal.science/hal-02894110v1)

HAL Id: hal-02894110

https://hal.science/hal-02894110v1

Submitted on 29 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Comparison between anion and cation emission from CH₄ molecules colliding with 10.5-keV C⁺ ions: fragment-energy aspects

J.-Y. Chesnel*<fnref n="1" />, Z. Juhász†<fnref n="2" />, E. Lattouf*, B. A. Huber*, S. T. S. Kovács†, P. Herczku†, E. Bene†, V. Vizcaino*, A. Méry*, J.-C. Poully*, J. Rangama*, J. A. Tanis*·§, and B. Sulik†

**Synopsis** It is shown that both positive and negative ions can be ejected from gas-phase methane molecules when bombarded with positive ions at keV energies. The kinetic energy distribution of the negatively charged fragments shows strong similarities with that of the ejected cations. However, in a hard binary collision between the C⁺ projectile and an H atom, the formation of H⁻ ions is more endothermic than that of H⁺ ions.

We show that both cations (H⁺, C⁺, CHₓ⁺) and anions (H⁻, C⁻) are emitted from methane molecules when bombarded by positive ions at an impact energy of a few keV. The experiment was performed at GANIL in Caen, France. A 10.5-keV C⁺ ion beam crossed an effusive gas jet of CH₄. 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 [1].

Figure 1 shows the energy distribution of the anions and cations ejected at the forward angle of 70° (with respect to the beam direction) in 10.5-keV C⁺ + CH₄ collisions. The anion yield is relatively large, more than 3% of the cation yield, and the total cross section for anion formation is estimated to be higher than 10⁻¹⁸ cm<fnref n="2" />. The main component of each spectrum is a broad structure, slowly decreasing with energy. This component results from soft many-body processes involving large impact parameters. On the other hand, at angles below 90°, pronounced peaks are observed at higher emission energies (Fig. 1). As recently observed for other collision systems [1-3], these peaks are due to recoil ions formed in hard binary collisions occurring at small impact parameters.

The kinetic energy distribution of the ejected anions shows strong similarities with that of the cations. This finding sug-

gests a statistical picture in which the final charge state distribution of the emitted H centers barely depends on how closely the atomic centers approach each other during the collision. However, the binary peak due to H⁻ ions is centered at a lower energy than the peak due to H⁺ ions with an energy difference of 20–30 eV in the 40°–70° angular range. This result shows that the formation of H⁻ ions in hard binary collisions is more endothermic than that of H⁺ fragments.

This work was supported by the Hungarian National Science Foundation OTKA (Grant No. K109440) and the French-Hungarian CNRS-MTA Cooperation (ANION project).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Energy distributions of anions and cations emitted at 70° in 10.5-keV C⁺ + CH₄ collisions.</figcaption></figure>

## References

[1] J-Y Chesnel et al 2015 Phys Rev A **91** 060701(R)

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

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

<figure><img src="image_4.png" /></figure>

<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: ean-yves.chesnel@ensicaen.fr</footnote>

<footnote n="2">²E-mail: zjuhasz@atomki.hu</footnote>
Comparison between anion and cation emission from CH₄ molecules colliding with 10.5-keV C⁺ ions: fragment-energy aspects

J.-Y. Chesnel*<fnref n="1" />, Z. Juhász†<fnref n="2" />, E. Lattouf*, B. A. Huber*, S. T. S. Kovács†, P. Herczku†, E. Bene†, V. Vizcaino*, A. Méry*, J.-C. Poully*, J. Rangama*, J. A. Tanis*·§, and B. Sulik†

**Synopsis** It is shown that both positive and negative ions can be ejected from gas-phase methane molecules when bombarded with positive ions at keV energies. The kinetic energy distribution of the negatively charged fragments shows strong similarities with that of the ejected cations. However, in a hard binary collision between the C⁺ projectile and an H atom, the formation of H⁻ ions is more endothermic than that of H⁺ ions.

We show that both cations (H⁺, C⁺, CHₓ⁺) and anions (H⁻, C⁻) are emitted from methane molecules when bombarded by positive ions at an impact energy of a few keV. The experiment was performed at GANIL in Caen, France. A 10.5-keV C⁺ ion beam crossed an effusive gas jet of CH₄. 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 [1].

Figure 1 shows the energy distribution of the anions and cations ejected at the forward angle of 70° (with respect to the beam direction) in 10.5-keV C⁺ + CH₄ collisions. The anion yield is relatively large, more than 3% of the cation yield, and the total cross section for anion formation is estimated to be higher than 10⁻¹⁸ cm<fnref n="2" />. The main component of each spectrum is a broad structure, slowly decreasing with energy. This component results from soft many-body processes involving large impact parameters. On the other hand, at angles below 90°, pronounced peaks are observed at higher emission energies (Fig. 1). As recently observed for other collision systems [1-3], these peaks are due to recoil ions formed in hard binary collisions occurring at small impact parameters.

The kinetic energy distribution of the ejected anions shows strong similarities with that of the cations. This finding sug-

gests a statistical picture in which the final charge state distribution of the emitted H centers barely depends on how closely the atomic centers approach each other during the collision. However, the binary peak due to H⁻ ions is centered at a lower energy than the peak due to H⁺ ions with an energy difference of 20–30 eV in the 40°–70° angular range. This result shows that the formation of H⁻ ions in hard binary collisions is more endothermic than that of H⁺ fragments.

This work was supported by the Hungarian National Science Foundation OTKA (Grant No. K109440) and the French-Hungarian CNRS-MTA Cooperation (ANION project).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Energy distributions of anions and cations emitted at 70° in 10.5-keV C⁺ + CH₄ collisions.</figcaption></figure>

References

[1] J-Y Chesnel et al 2015 Phys Rev A **91** 060701(R)

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

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

<figure><img src="image_4.png" /></figure>

<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: ean-yves.chesnel@ensicaen.fr</footnote>

<footnote n="2">²E-mail: zjuhasz@atomki.hu</footnote>