Session S03: Heavy Particle and Ion-impact Collisions

Effects of Coulomb Anomalies on Heavy Particle and Ion-Impact Collisions

One of the main conditions of the standard scattering theory is that the asymptotically active potentials, both in the incoming and outgoing channels, must decay to zero faster than a Coulomb potential. Obviously, this essential "asymptotic condition" is not met in most of the scattering processes in Atomic and Molecular Physics.  

From a pioneering work by John D. Dollard (1964), two ways of reconstructing the scattering theory to include long-range potentials were developed, using alternative forms of asymptotic free states.  It was even rigorously demonstrated that both generalizations were equivalent and can be deduced from each other.

These latest studies were developed for the simplest case of two unstructured particles interacting via a Coulombian potential. On the other hand, their respective generalizations for the multichannel case, although internally coherent and physically plausible, lack a rigorous demonstration, to the point that their validity could be questioned.

In recent years, mainly due to the possibility of making measurements of fully differential cross sections for different collision processes, serious differences between experimental data and theoretical calculations began to be noticed, being in most cases difficult to discern the origins of such discrepancies.

Some of the causes of these differences include, for example, an incorrect treatment of the branch cut at the on-shell limit of the (unavoidable) intermediate off-shell states, an inadequate definition (or even elimination) of the secondary term in the Gell-Mann and Goldberger amplitude, or an overuse of Wick's argument.

In this communication we will show how to discern and even correct these errors by critically reviewing the conceptual foundations and standard assumptions of the most common theoretical approaches.     

Interatomic Coulombic decay in ion-dimer collisions

Free-electron production is an exceedingly rare process in low-energy ion-atom and ion-molecule collisions. This may change if the target monomer is replaced by a dimer (or a larger cluster) which allows for interatomic Coulombic decay (ICD), i.e., the transfer of enough excitation energy from one constituent to a neighbor so that an electron can be released from the latter.

ICD in neon dimers impacted by slow ions has recently been observed at CIMAP/GANIL and semi-quantitatively explained by a classical over-the-barrier model calculation [1]. Using an independent-atom independent-electron model and a quantum-mechanical description of the electron dynamics we were able to confirm the occurrence of ICD in 2.81 keV/amu O$^{3+}$-Ne$_2$ collisions after capture of an inner-valence ($2s$) electron. We also showed that Li$^{3+}$ ions are far less efficient in initiating ICD in neon dimers because the energy level structure of these projectiles makes $2s$ capture unlikely [2]. 

Conversely, more recent calculations suggest that low-energy He$^{2+}$ ions are \textit{very} efficient ICD initiators, i.e., give rise to substantial low-energy electron emission from neon dimers in a regime in which continuum electrons are virtually nonexistent in the corresponding ion-atom collision [3]. In this talk, I will explain the main features and the limitations of our model, provide an overview of the results obtained, and discuss their implications.

[1] W. Iskandar et al., Phys. Rev. Lett 114, 033201 (2015)

[2] D. Bhattacharya and T. Kirchner, Phys. Rev. A 102, 062816 (2020)

[3] T. Kirchner, J. Phys. B 54, 205201 (2021)   

On the quest for projectile (de)coherence in C6+/He collisions

20 years ago, single ionization of Helium induced by 100 MeV/u C⁶⁺ projectiles was investigated in a kinematically complete experiment. The experimentally obtained results were in strong contrast to state of the art theories at this time and even most recent calculations. While the electron angular distribution should exhibit two well separated lobes, the so-called binary- and recoil-lobe, the node between them was mostly filled. This launched an avalanche of controversial discussions, which are still ongoing today. The most heavily debated explanations are a) experimental issues/limited resolution and b) transversal coherence of the projectile, a concept introduced in 2011 by Schulz et al.

 

In order to solve the “C⁶⁺-mystery”, we used a state-of-the-art COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) Reaction-Microscope and redid the initial experiment in Cave D4 of GANIL. As insufficient momentum resolution might have been an issue in the original experiment, the spectrometer was built in a time-focusing geometry; the ion arm additional in a space-focussing geometry. On both ends of the spectrometer, hexagonal delay-line-detectors were used, with an overall spatial resolution <100 µm. The gas jet was precooled to 80 K. Separate calibrations for electrons and ions using a 25 keV ion source were performed. With this, the ion momenta were calibrated, focusing on discrete structures in momentum space as a result of single electron capture using He²⁺ and He⁺ projectiles. A momentum resolution of Δp<0.1 au for He⁺ ions was achieved. The electron arm was calibrated via autoionizing states of a Neon target (He²⁺ + Ne → He⁺ + Ne²⁺ + e^-), which create various, energetically well-defined continuum electrons.   

Close-coupling approach to differential ionization in ion-atom collisions

Ion-atom collisions take place in astrophysical and laboratory plasmas. Therefore, accurate knowledge of excitation, electron-capture and ionisation processes occurring in such collisions plays an important role in practical applications. A wave-packet convergent close-coupling approach is capable of providing benchmark data on the integrated and differential cross sections for all processes taking place in such collisions. The approach expands the scattering wavefunction using a two-center pseudostate basis. This allows one to account for all underlying processes, namely, direct scattering and ionisation, and electron capture into bound and continuum states of the projectile. The wave packets are used to discretize the continuum. The generated pseudostates are used in the expansion of the total scattering wave function. The approach gives the integrated, fully differential, as well as various doubly and singly differential cross sections for ionization of atomic targets. It has recently been applied to p-He collisions. Calculations of ionization cross sections differential in the electron emission energy, in the emission angle, as well as in the scattered-projectile angle have been performed in the intermediate energy region where coupling between various channels and electron-electron correlation effects are important. The results agree well with experimental data, where available. Moreover, our calculations reveal an interesting interplay between direct ionization and electron capture into the continuum. We show that the ionization cross section differential in the angle of the ejected electron is dominated by electron capture into the continuum for ejection into small angles, while ejection into large angles is purely due to direct ionization.