Session RR43: Ion Atom and Heavy Particle Collisions

On the Quest for Projectile Coherence Effects in Fast 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 experimental results, were in strong contrast to state of the art theories at this time and even most recent calculations. While the electron momentum distribution should exhibit two distinct lobes, the so-called binary- and recoil-lobe, the node between them was mostly filled. This launched controversial discussions, which are still ongoing today. The most heavily debates explanations are a) experimental issues/limited resolution and b) transversal coherence of the projectile, introduced by Schulz and coworkers.

 

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, ion arm of the spectrometer was build in a time- and space-focussing geometry in order to reduce the diminishing influence of the extended target size. The electron arm was build in a time-focussing geometry. On both ends of the spectrometer, hexagonal delay-line-detectors were used, which have an overall non-linearity <100 µm.

 

Also the gas jet was precooled to 80 K and for both, electrons and ions, separate calibrations, using an additional 25 keV ion source were used. With this we achieved a He⁺ momentum resolution of Δp<0.1 au was achieved. The electron arm of the spectrometer was calibrated via autoionizing states of a Neon, which create numerous energetically sharp isotropically emitted electrons. Results of the experiment will be presented and discussed.   

The role of s-wave scattering in atom-ion resonant charge transfer near room temperature

Resonant exchange is a general process playing a key role in many-body dynamics and transport phenomena, such as spin, charge, or excitation diffusion. A particular example is given by the collision between an ion and its parent neutral atom. The underlying process is described by the resonant exchange cross section. We show that the s-wave scattering, generally thought to contribute mainly in the ultracold (or Wigner) regime, dictates the overall cross section and corresponding rate coefficient over a broad range of energies. We derive an analytical expression and explain its applicability high above the Wigner regime, and demonstrate its relationship to the classical capture (Langevin) cross section. We apply it to resonant charge transfer for several isotopes of Yb, and show that fitting higher energy data allows to obtain the cross section and rate over a large range of energies down to the ultracold regime.   

Langevin Dynamics Modeling of Gas-Phase Ion Recombination with Dilute Ion Concentration

Ion recombination in the gas phase plays an important role in the stability of plasmas and their chemistry that is closely related to various applications in combustion and materials processing. Due to the lack of robust theoretical models of three body ion recombination, we present a Langevin dynamics-based ion recombination rate constant model that is applicable for a broad pressure and temperature range combination considering the effect of ionic structure of polyatomic ions. Ion recombination is modeled within the framework of classical physics and thus does not explicitly model the quantum nature of electron transfer kinetics. We hypothesized that electron transfer takes place with near certainty when two atoms that are part of the recombining ions have a nuclear separation less than or equal to σ - distance σ between two atoms at which the potential energy (due to van der Waals or polarization interaction) between the ions is zero. The comparison with experimental data for several ion pairs reveals an agreement of ±50% for most of the tested ion pairs at high pressure. At low pressure, good agreement is observed as well between polyatomic ion pairs.   

Closed-Form Equilibrium Solution to Boltzmann Equation for Charged Particles in Electric Field

For threshold-level electrostatic discharge at atmospheric pressure, in the initial stages of the spark, ambient temperatures are moderate, and the rate at which electrons lose energy due to collisions with heavy species is such that the electron temperature (their kinetic energy due to random motion) is constrained. The direct acceleration of electrons by the electric field in a mean-free path can be comparable to or greater than the thermal velocity.

The electron velocity distribution function is significantly skewed due to this direct acceleration. With some assumptions about how collisions drive the distribution, the Boltzmann equation can be used to solve for equilibrium distribution functions. Assuming a constant relaxation rate to the Maxwell-Boltzmann distribution, a closed form solution (Krook distribution) can be found in the literature. We present a more realistic, closed form (albeit complicated) solution assuming the relaxation rate is a sum of a constant rate plus a rate proportional to the velocity in the direction of the electric field. The drift velocity (first moment of the distribution) has the correct limit in both the low electric field (high temperature/constant mobility) limit, and the high electric field (low temperature/ballistic) limit.

LA-UR-21-25744   

Interference effects in fully differential cusp electron production cross sections for p + He collisions

We have measured fully momentum analyzed He⁺ recoil ions and scattered projectiles in coincidence for 75 keV p + He collisions.  From the data, we obtained fully differential ionization cross sections (FDCS) for electrons with an energy of 43.9 eV ejected into the scattering plane.  This energy corresponds to an electron speed close to the projectile speed (velocity matching).  The measurements were performed for a small collimator slit - target distance corresponding to a relatively small transverse projectile coherence length of about 1.0 a.u.

Previously, we reported FDCS for several ejected electron energies in the velocity matching regime, measured for a relatively large transverse projectile coherence length.  At large projectile scattering angles, apart from the well-established “binary peak”, another well-separated peak structure in the forward direction was observed.  This was explained by interference between the first- and higher-order amplitudes.  At the matching velocity, one higher-order process which is known to be particulalrly important is post-collision interaction (PCI), leading to the so-called cusp peak in the electred electron energy spectra.  If this explanation is correct, the double-peak structure should disappear for a coherence length approaching 0.  Indeed, the new data, taken for small coherence length, show a less pronounced double peak structure.   

High Temperature Nonequilibrium Effects on Atomic and Molecular Polarizability

At high temperatures aero-optical distortions and knowledge of noneqilibrium effects are of critical importance in flow diagnostics and for proper interpretation of schlieren and shadowgraph visualization techniques. To calculate the dynamic polarizability we used a semi-classical approach, when the total polarizability is a summation of virtual transitions from  the ground state to upper electronic, vibrational and rotational states. In this study we analyzed the atomic and molecular polarizability of nitrogen and oxygen and their dependence on the set of oscillator strengths and populations of vibrational and rotational states.