Bene, Erika

Chesnel, Jean-Yves

Frémont, François

Herczku, Péter

Huber, Bernd

Juhász, Zoltán

Kovács, Sándor T. S.

Lattouf, Elie

Méry, Alain

Poully, Jean-Christophe

Rangama, Jimmy

Rousseau, Patrick

Sulik, Béla

Journal of Physics Conference Series

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A collision process responsible for widespread formation of H anions
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2014 J. Phys.: Conf. Ser. 488 102024
(http://iopscience.iop.org/1742-6596/488/10/102024)
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A collision process responsible for widespread formation of H- anions  
Zoltán Juhász*1, Béla Sulik*, Sándor T. S. Kovács*, Elie Lattouf†, Jimmy Rangama†, Erika Bene*, 
Bernd Huber†, François Frémont†, Alain Méry†, Jean-Christophe Poully†, Patrick Rousseau†,  
Péter Herczku*, and Jean-Yves Chesnel† 
*MTA Institute for Nuclear Research (Atomki), Bem tér 18/c, H-4026 Debrecen, Hungary 
†Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), Unité Mixte CEA-CNRS-EnsiCaen-
Université de Caen Basse-Normandie, UMR 6252, Bd Maréchal Juin, F-14000 Caen, France 
Synopsis The hydride ion H– is widely understood to play a major role in many processes important for physics, 
cosmology, astrophysics and chemistry. Here we report that H– ions can be formed by collision-induced frag-
mentation of molecules containing hydrogen and heavier nuclei. We observed H– emission from OH+ + Ar and 
OH+ + acetone collisions at the energy of 7 keV. Our data reveal a process of electron-grabbing by fast hydrogen 
fragments emitted in hard collisions with large momentum transfer between atomic cores. 
The formation of negative ions in various environ-
ments has been a subject of great interest over the 
past decades. Even a small concentration of anions 
has an appreciable effect on the physical and 
chemical properties of their environment. 
In this work we observed H– formation in colli-
sions of OH+ projectiles with Ar atoms and acetone 
molecules (CH3-CO-CH3). The experiments were 
conducted at the 14.5 GHz ECR ion source of the 
ARIBE facility, at GANIL in Caen, France. The 
molecular OH+ beam with incident energy of 7 keV 
crossed an effusive gas jet of either argon atoms or 
acetone molecules. Electrons and negative ions 
produced in the collision were measured by an elec-
trostatic spectrometer. The spectra were taken from 
2° to 150° with respect to the beam direction.  
The most remarkable feature of the spectra was 
the appearance of well-defined peaks [1]. These 
structures could not be attributed to electron emis-
sion. A kinematic analysis has shown that one of 
the peaks is due to the emission of H– ions moving 
with nearly the velocity of the OH+ projectile ion. 
The other peak corresponds to H– emission from the 
acetone target in direct two-body collisions with the 
oxygen center of the projectile. Moreover, we com-
pared the measured cross sections with a classical 
two-body potential-scattering calculation.  
Figure 1 shows the measured cross sections for 
H– emission in OH+ + Ar collisions. Our new data 
recorded at small observation angles are indicated 
by full symbols, while our earlier data at larger an-
gles are shown by open circle symbols. It is a nota-
ble feature that the H– production cross section is 
proportional to the calculated H + Ar elastic scatter-
ing cross section. This fact can be explained by a 
two step process: (i) A large momentum transfer 
collision liberates a proton. (ii) The receding proton 
 
grabs two electrons in a double capture process with 
a practically observation-angle-independent prob-
ability. The agreement of the theoretical curve with 
earlier experimental data for H2++Ar collisions at 
the same velocity supports this picture and its gen-
eralization [2]. These earlier data are shown in 
Fig. 1 by open square symbols. Since the process is 
likely in any collisions of H-containing molecules 
at few-keV energies, it is important for both its gen-
erality and its fundamental understanding.  
This work was supported by the Transnational 
Access Program ITS-LEIF, granted by the Euro-
pean Project HPRI-CT-2005-026015, the Hungar-
ian OTKA Foundation (Grant N° K73703) and the 
French-Hungarian Cooperation Program PHC Bala-
ton (N° 27860ZL / TÉT_11-2-2012-028) 
0 30 60 90 120 150 180
10-20
10-18
10-16
10-14
10-12
0 2 4 6 8 10
10-18
10-16
10-14
 
 d
 /
 d
 
 (c
m
2 / 
sr
)
Observation angle (degree)
0.012 x H-Ar
 
 
 
 
Figure 1. Measured cross sections for H– ejection from 
the OH+ projectile as a function of the observation angle 
for an Ar target. The curves represent 2-body elastic 
scattering cross sections multiplied by a factor of 0.012. 
Squares: data of Ref. [2] for H2+ impact on Ar.  
References 
[1] Z Juhász at al 2012 J Phys: Conf Ser 388 102051, 
     and in press in Phys. Rev. A (2013). 
[2] F B Alarcón et al 2006 Int. J. Mass Spectr. 248 21 
 
1 E-mail: zjuhasz@atomki.hu 
 
XXVIII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2013) IOP Publishing
Journal of Physics: Conference Series 488 (2014) 102024 doi:10.1088/1742-6596/488/10/102024
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A collision process responsible for widespread formation of H- anions

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A collision process responsible for widespread formation of H- anions

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