Session C10: Collisions in Complex Systems

Resonant collisions in many-body and complex systems

Resonant exchange is a general process playing a key role in many-body and complex systems, such as in spin, charge, or excitation diffusion. The underlying process is described by the resonant exchange cross section σ_exc. A prime example is the diffusion of an ion A⁺ in its parent neutral gas A. In fact, the charge actually behaves as a hole (h) at ultralow temperatures, hopping from atom to atom instead of staying on its heavy center (the ion). We have predicted a faster diffusion coefficient for the hole (D_h) than if the charge was diffusing via collision (D_coll). We model charge hopping in the dynamics of an ultracold ^6/7Li⁺ ion immersed within an ultracold gas of ^6/7Li atoms at micro-Kelvin temperatures and show that the charge hopping and collisional diffusion compete, giving unique results leading to charge trapping in regions of high atomic density gradient. We also explore resonant collision involving much larger projectile and target, namely C₆₀ scattering with C⁺₆₀. We find that in such complex molecules, internal degrees of freedom play an important role, and that the formation of “temporary bridges” between the two molecular systems must be accounted for to explain experimental measurements.   

When ionizing radiation elicits unforeseen processes: dynamics beyond the coulomb explosion.

When ionizing radiation interacts with molecules or clusters in the gas phase, it induces their ionization and excitation. This phenomenon is observable regardless the type of ionizing radiation used, including collisions with multiply charged ions, energetic synchrotron radiation, or intense laser fields. The direct consequence of this interaction is the formation of molecular species with high degrees of excitation and ionization, leading to fragmentation into two or more positively charged moieties which repeal each other, the so-called Coulomb explosion. However, alongside this process, a variety of other phenomena occur in competition; these include intramolecular charge transfer [1,2], hydrogen migration [3,4], roaming of a neutral fragment [6,7] or even intermolecular reactivity in a cluster forming larger species [8,9]. These reactions are triggered by ionization and excitation and can only be observed if they occur within the femtosecond timescale characteristic of the Coulomb explosion. In this communication I will present a detailed study of the dynamics of these unexpected processes. Specifically, I will show how, through the integration of state-of-the-art experiments, where ions are detected in coincidence, with simulations based on quantum chemistry, we can infer a comprehensive understanding of such reactions and track their dynamics, thereby unambiguously characterizing competing channels.   

Universal Behavior of Low Energy Potential Scattering for General Dispersions

Scattering behavior in the low energy limit has been well studied for quadratic dispersion relations. However, in recent years, systems that exhibit higher order (>2) dispersions have

become experimentally realizable in synthetic quantum systems, including multilayer graphene and atomic arrays, and scattering for these dispersions is not as well understood. In a recent

work, we discovered classes of novel universal scattering behaviors for general dispersions, characterized by the divergent density of states, although these universality classes were only

determined for emitter scattering. In this research, we further study this universality by classifying the behavior for general potential scattering at low energies.   

Controlling chemi-ionization of individual conformers

In this work, we study the chemi-ionization reaction of hydroquinone with metastable Ne atoms in a novel cross-beam experiment. Our results, supported by quantum chemistry and classical trajectory calculations, indicate that the collision-induced alignment due to the long-range dipole-induced dipole interaction influences reaction pathways for different conformers. However, this alignment is hindered by the rotation of the molecule. We demonstrate how the interplay between molecular geometry, chemical steering forces, and rotational dynamics govern the outcome of reactions and illustrate the capability of advanced molecule-control techniques to unravel these effects. We show detailed insights into how the complex interplay between molecular geometry, geometry-dependent intermolecular interactions, and molecular rotation influences the outcome of chemical processes.   

Electron transfer, ionization, and direct excitation in collisions between protons and highly charged hydrogenic ions from N6+ to S15+

With the first Born approximation, Oppenheimer in 1928 was the first to treat Electron transfer in p-H(1s) collisions.  Beginning in the 1960's, collisions have also been considered using various non-perturbative, coupled-state approaches, as well as second-order perturbation theory. In view of the high asymmetry of the collisional systems considered here, the 

chosen bases consist of many functions (specifically, many Sturmian functions) centered on the target nucleus, but just a single $1s$ function centered on the proton.  The extent to which simple scaling rules with target nuclear charge $Z$ are valid---rules from perturbation theory---is being examined for ionization and electron transfer, and particularly for direct excitation, at intermediate energies near where the cross sections peak, as well as at higher energies.  The present work extends that for $Z\le6$.

T. G. Winter, Phys. Rev. A 87, 032704 (2013)   

Looking for Reaction Complexes in Trapped Ion Reactions of Xe+ and O2+ with C3H4 Isomers

Ion-molecule reactions play an important role in a variety of environments such as the interstellar medium and the upper ionosphere. Significant progress has been made in understanding the mechanisms that control such reactions, but there is still much to discover. I will present recent work expanding on prior studies of the C₃H₄ isomers allene (H₂C₃H₂) and propyne (H₃C₃H), which have shown isomeric dependence on their reactivities. Specifically, I will discuss our results from reactions of Xe⁺ and O₂⁺ cations with these isomers in a cold ion-trap environment. These ions have nearly identical ionization potentials and thus offer nearly isoenergetic conditions for reactions due to the low level of kinetic energy available. Experimental measurements and theoretical calculations show that the propensity of forming a reaction complex depends not only on the isomeric structure of the neutral C₃H_4, but also on its ionic reaction partner. This expands on what was previously understood about reactions in these systems and offers a more thorough picture of ion-molecule reaction mechanisms._­­­