Session DT23: Electron Collisions

Toward a frame-work for calculating comprehensive electron collision data sets for low-temperature plasma modeling

Modeling low-temperature non-equilibrium plasmas with Monte Carlo particle-in-cell and Boltzmann solver codes requires comprehensive sets of collision cross sections. However, these cross sections are particularly difficult to calculate for low-temperature plasmas, where near-neutral atoms and molecules, and excited state species are abundant. As a result, in general, comprehensive sets of cross sections do not exist in the literature.

 

Recently we have developed a universal frame-work for quickly calculating electron-impact excitation and ionization cross sections of ground and excited state diatomic molecules and diatomic molecular ions. In conjunction with the Los Alamos suite of atomic physics codes, this new capability is moving us toward a comprehensive description of the collisional processes required for modeling low-temperature plasmas. Here we present a range of comparisons of the new results with state-of-the-art theory, and investigate the effect of including comprehensive collision data in plasma kinetic simulations.   

Scattering Mechanisms in Ionization of Helium by Electron Vortex Projectiles

For electron-impact ionization of helium into the azimuthal plane with symmetric energy sharing, the triple differential cross sections (TDCSs) show qualitative features that can be used to distinguish single and double scattering mechanisms.  For traditional, non-vortex projectiles, emission of electrons into the azimuthal plane requires a double scattering mechanism, and this mechanism becomes more important with increasing projectile energy.  At low projectile energies, the dominant ionization mechanism is a single binary collision.  In this work, we examine scattering mechanisms for electron vortex projectiles through the calculation of TDCSs using the distorted wave Born approximation.  Our results predict that the ionization mechanisms for vortex projectiles are similar to those of non-vortex projectiles.  However, some key differences are observed.  For vortex projectiles, a double scattering mechanism is not required for electron emission into the azimuthal plane, although the shape of the TDCS is still significantly influenced by double scattering, particularly at higher projectile energies.  At low projectile energies, ionization proceeds primarily through a single scattering mechanism, although unlike non-vortex projectiles, double scattering is also important.  Our results lead to several predictions that can be experimentally tested.   

Electron Scattering Cross Sections and Calculated Transport Data for a New Class of Cooling Gases

Non-equilibrium nature of plasmas needed for numerous applications involving thermally sensitive targets has increased the need for complete sets of cross section data.  Modeling of such plasmas, especially kinetic modeling, requires complete sets of data normalized by the swarm technique.  At the same time one has to be aware of the limitations of the swarm technique such as non-uniqueness and dependence on the range of energies covered in the swarm experiment.  In this presentation we shall present newly determined sets of cross sections for some of the mores standard gases such as C₂H₆O (dimethyl ether-DME) as well as the newly synthesised gases belonging to hydrofluoroolefins (HFOs), perfluoroketones (PFKs) and perfluoronitriles (PFNs) (our presentation will include (like C₃HF₅ (HFC-1225ye) and C₃H₂F₄ (HFO-1234ze)) .  These new gases have to satisfy the requirements for their application as well as to be short lived in the atmosphere so that their footprint on the global warming is minimized and thus may be used as replacement for several more polluting gases.  The cross sections were obtained on the basis of the available data and the newly measured data by the group of Jaime de Urquijo as well as the group of Christian Franck. As these are pulsed Townsend (PT) experiments we shall also address the interpretation of the results of PT experiments under the influence of non-conservative collisions.

In collaboration with: J. De Urquijo, T. Hammer, M. Ise, J. Atic, S. Marjanovic, D. Maric, G. Malovic, S. Dujko, O. Šašic, S. Dupljanin   

Collisions with atoms and molecules calculated with the Convergent Close-Coupling method

There has been immense progress in the field of (anti)electron scattering on atoms and molecules in the last few decades. Though historically excitation and ionization processes have been, and continue to be, categorized as different subfields, it has been shown that computational methods which aim to fully solve the underlying Schroedinger equation yield accurate results for both. One of these methods is the Convergent Close-Coupling (CCC) method. It expands the total wavefunction of the collision system in a truncated complete basis and convergence in the results of interest is obtained by simply increasing the basis size. The CCC method first solved the e-H collision excitation problem in the early 1990s, and then showed how to obtain fully differential ionization results around ten years later. More recently our attention has turned to the e-H₂ collision system. It is fair to say that the CCC method for this molecular target system is where CCC code was for the atomic targets some 30 years ago. In the presentation we will review the many applications of the CCC method to collisions involving atomic and molecular targets. We will also discuss the availability of the atomic CCC code via the AMP Gateway (https://ampgateway.org).   

Low energy electron scattering from molecules - measuring cross sections and investigating collision dynamics

Electron scattering provides for a basic tool for studying the structure and dynamics of quantum interactions quantum physics appertaining to electron impact on atomic and molecular targets. A selected experimental system consisting of an impinging collimated electron beam on a target beam can provide such information.

We will present recent experimental results dealing with of the interaction of electrons with fundamental molecular targets. Here measurements of electron impact scattering cross-sections on dissociative electron capture, angular distribution of scattering and ionization of selected particles, it is possible to shed light on many interesting problems about the structure dependence and collision dynamics of electron-molecule molecular systems relevant to the electron impact process of interest. 

 

We will discuss several experimental setups, which provide independent measurements of the dissociative electron attachment and, high-resolution differential electron scattering cross-sections, involving conventional electrostatic and a newly developed time-of-flight spectrometer in our laboratory. Here we have set up crossed beam apparatus of electron-gas simple organic target molecules e.g. double-bond polyatomics such as C₃H₄O₃ (pyruvic acid), C₅H₄O₃ (furoic acid) and triple bond molecules: triple bond polyatomics such as CH₃CN (acetonitrile). Our incident electron energy range is of 0-20 eV a region which covers the formation of negative ion resonances which dominate the electron scattering. This is undertaken to provide data that is needed for quantum mechanical models of the electron-molecule collision system,  that are used in turn to model environments with free electrons, e.g. natural or man-made plasmas.   

Electronic and vibrational close-coupling method for resonant electron-molecule scattering

In the field of electron-molecule scattering, direct and resonant scattering processes are treated using entirely separate theories. Close-coupling techniques such as molecular convergent close-coupling (MCCC) or R-matrix have had considerable success in studies of direct scattering, by applying the Born-Oppenheimer approximation to allow the electronic and nuclear motions to be decoupled. On the other hand, studies of resonant scattering typically decouple the direct and resonant scattering channels, solving the nuclear dynamics problem for the resonant processes with electronic resonance energies and widths taken from separate electronic scattering methods. We have now extended the MCCC method to include both electronic and vibrational states in the close-coupling expansion, allowing calculations to be performed in which direct and resonant scattering are coupled and treated on the same footing. We have applied the new technique to electron scattering on H2, and studied the resonance structures present in both elastic and excitation cross sections.