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Fundamental processes in ion collisions

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  • Involved people : L. Adoui, A. Cassimi, J.-Y. Chesnel, A. Domaracka, F. Frémont, B. A. Huber, A. Méry, J.-C. Poully, J. Rangama, P. Rousseau, V. Vizcaino (permanent researchers) ; W. Iskandar, V. Kumar, A. P. Mika, N. Sens (PhD students) ; A. Agnihotri, M. Ryzska (postdoc)
  • Contracts : CNRS-MTA, PHC Balaton, PICS Hungary, ANR-JCJC IMAGERI

The AMA team has a long-standing experience in investigating fundamental processes in ion collisions. Since 2010, different imaging experiments (COLTRIMS) have been focused on rare gas dimers in collisions with slow and fast projectile ions in collaboration with different groups (LPC France, TMU Japan, IKF Germany), leading to several breakthroughs [1,2]. During the last years, advantage has been taken from the local expertise in imaging experiments to investigate the role of a neighbor ion in the fragmentation dynamics of covalent diatomic molecules. This work is described in the first subsection below. The second subsection is devoted to fundamental aspects of the ion-induced fragmentation of small molecules. An extensive experimental investigation allowed the identification of the main processes underlying the emission of both anion and cation species from neutral water and methane molecules bombarded by positively charged ions. The last subsection reports on a new generation of investigations focusing on fundamental processes occuring in collisions between slow ions and nanoparticules.

Role of a neighbor ion in the fragmentation dynamics of covalent diatomic molecules

For about 10 years, we used a COLTRIMS setup to explore the relaxation dynamics of small atomic or molecular clusters after ion irradiation. After a fruitful investigation of rare gas dimers (Ar2 and Ne2) [1], the team has more recently focused on dimers of diatomic molecules ((N2)2 and (CO)2). Two distinct aspects can be studied when colliding such systems with slow highly charged ions. The first one is to determine the initial three-dimensional structure of the neutral dimer by using the Coulomb Explosion Imaging technique by the coincident detection of the emitted fragment ions. Dissociation channels where the target fully or partially breaks up in atomic fragments are sorted from the experimental data and the initial geometry is then indirectly inferred from a comparison to classical trajectory calculations of the three (or four) repulsing ions. Thanks to this method, the structure of both (N2)2 and (CO)2 dimers has been partially elucidated and has shown that both molecules are oriented perpendicualr to the dimer axis. The second aspect is the influence of a strong external electric field on molecular dissociation. Here, capture channels where two or more electrons are captured on one molecular site and only one electron is removed on the other molecule are carefully selected. Following such a multiple capture process, the dissociation of the multiply ionized molecular site occurs in the vacinity of the other singly charged molecular ion. Our setup allows to finely investigate these three-body dissociation channels and some results are detailed in [2].
The Coulomb Explosion Imaging technique will be further extented to access the geometry of molecular trimers. Here, the mesurement requires the detection of at least four ionic fragments and microchannel plates with increased detection efficiency are mandatory. The performance of such a detector is currently under progress. Additionnaly, we plan to investigate mixed clusters such as ArN2 or Ar(CO) to systematically study the influence of the charge state of the neighbor ion -intensity of the external electric field- on molecular fragmentation.
[1] W. Iskandar et al., Phys. Rev. Lett. 114, 033201 (2015)
[2] A. Méry et al., Phys. Rev. Lett. 118, 233402 (2017)

Anion and cation emission from small molecules after collisions with positively charged ions

Collisions between positive ions and neutral molecular targets are essential processes in several areas of physics, biology, and chemistry. Negative-ion formation in such collisions is scarcely studied. However, in the early 2010’s we found that this phenomenon is a general process in ion-molecule collisions at impact energies of a few keV [3]. Before, works about anion formation in collisional molecular fragmentation were limited to specific collisions involving hydrogen molecular species. Also, the studies were restricted to small emission angles with respect of the beam direction. In contrast, we observed the emission of negative H– ions in a wide angular range after slow collisions between positive OH+ ions and neutral targets of argon and acetone [3]. We discovered the formation of anions via a hard binary-encounter process between an H center of one collision partner and a heavy center of the other collision partner. A striking result was that as much as 1% of all H fragments created via this process are negatively charged. The observation of such a process for producing anions opened the way for a new generation of studies involving various molecules that are relevant for the research areas for which anions are of importance. During the last five years, we extended our initial work to methane and water molecules [4] which are relevant for life science and astrophysics. We found that the hard two-body process leads not only to H– emission, but also to the emission of much heavier anions (C–, O–).
Besides the hard two-body process, soft many-body processes also contribute to anion formation. However, separate identification of anions produced in soft collisions is a challenging task, since electron emission largely overlaps the signal of slow anions. We thus developed an original method to achieve for the first time the measurement of pure double-differential anion spectra in the entire emission-energy range. Our major goal is the measurement of absolute double-differential cross sections (DDCS) for anion and cation emission in both the entire emission-energy and angular ranges in order to provide the full picture of the fragmentation dynamics leading to ionic fragment emission. As electronic excitation/ionization processes may influence both the energy and angular distributions of the emitted fragments, we perform a detailed comparison of the anion and cation DDCS, with the aim of revealing specific features as a function of the final charge of the emitted fragments, such as the interplay between nucleus-nucleus interactions and electronic excitation in anion and cation formation.

Full curves : DDCS for anion and cation emission in 6.6-keV 16O+ + H2O collisions at the observation angles of 60° (top) and 75° (bottom). The points (red circles) at lower energies show the relative contributions due to hydrogen fragments (H+ or H–).

Our most recent data provide new insights into the fragmentation dynamics, as well as into the role of electronic excitation. In the case of anion and cation formation in 6.6-keV O+ + H2O collisions [4], they show that :
(i) Both the kinetic-energy and angular distributions of the anion fragments are very similar to those of the ejected cations (Fig. 1).
(ii) The main component of the energy distribution of the fragments is broad and slowly decreasing with energy (Fig. 1). These fragment ions originate from soft many-body processes. At forward angles (< 90°), the pronounced peaks observed at higher energies are due to recoil fragments formed in hard binary collisions occurring at small impact parameters.
(iii) The fact that the double-differential cross sections for H+ and H– formation are nearly proportional at all angles and over the entire emission energy range shows that the relative populations of the different charge states of the hydrogen fragments do not depend significantly on the emission angle, the impact parameter or the momentum transferred between the collision partners. This finding suggests that the charge state distribution of the hydrogen fragments is akin to a statistical distribution, independently of whether these fragments are formed via binary or many-body processes.
(iv) Simulations within a Monte-Carlo four-body scattering model suggest that electronic excitation and/or ionization processes play a crucial role in both H+ and H– formation.
(v) Calculations in the framework of a statistical thermodynamic model shows that the overwhelming majority of the produced H+ ions stems from transfer ionization processes occurring at impact parameters smaller than 1 a.u.
[3] J.-Y. Chesnel et al., Phys Rev A 91, 060701(R) (2015) and references therein
[4] J.-Y. Chesnel et al., J. Phys. 875, 102013 (2017) ; Z. Juhász et al., Phys. Rev. A 100, 032713 (2019) and references therein

Electron emission

Electron emission following ion collisions with atoms or molecules has been studied for several decades to better understand fundamental ionization mechanisms (soft collisions, binary encounter…) [6] but also for its wide range of applications including radiation cancer therapy. In particular, electron emission might explain the efficiency of nanoparticles (NP) when used as radiosensitizers, to enhance tumor destruction.
Indeed, the use of radiosensitizers, including high-Z nanoparticles (Ag, Au, Gd), was proposed to enhance the effects of ionizing radiation in ion beam therapy [7]. The mechanism responsible for such an efficiency enhancement is assumed to be the important release of electrons from the nanoparticles (NPs) triggered either by the primary ion beam or by secondary charged particles created along the track (see Figure 1).

Electron emission from NPs

These electrons may then interact directly with biomolecules. Even at low energy (below ionization threshold), electrons can induce molecular damage via Dissociative Electron Attachment. They can also produce highly reactive species such as hydroxyl radicals (OH•) from the surrounding water molecules. However, the abundance and energy of these electrons have never been measured.
Within the IMAGERI project, we propose to quantify this low energy electron emission by providing absolute cross sections in order to better understand the intrinsic properties of NPs under irradiation and the physical processes leading to the efficiency enhancement of radiation damage. We will measure and compare the energy distribution of the emitted electrons for various size (from nm to few tenths of nm) and elements (Ag, Au, Pt and Gd) of the NPs. Furthermore, typical ion beam kinetic energies of the Bragg-peak (MeV) and of secondary particles (keV) will be investigated. Such absolute cross sections can be integrated to Monte Carlo simulation codes that are developed to simulate the trajectory of particles through matter. Moreover, our data will be used as benchmark measurements for the validation of theoretical calculations.
[6] Stolterfohlt N, DuBois RD, Rivarola RD, 1997 Springer-Verlag, Berlin Heidelberg
[7] Porcel E et al. 2014 Nanomedicine Nanotechnol. Biol. Med. 10, 1601–1608