Bulletin of the American Physical Society
69th Annual Gaseous Electronics Conference
Volume 61, Number 9
Monday–Friday, October 10–14, 2016; Bochum, Germany
Session VF1: Electron Collisions with Small MoleculesFocus
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Chair: Zoran Petrovic, University of Belgrade Room: 1 |
Friday, October 14, 2016 11:00AM - 11:30AM |
VF1.00001: Electronic excitation of molecular hydrogen by low-energy electrons Invited Speaker: Leigh Hargreaves Molecular hydrogen is the most abundant element in the universe, particularly in interstellar plasmas such as atmospheres of gas giant planets and stars. Electron collision data for hydrogen is critical to interpreting the spectroscopy of interstellar objects, as well as being of applied value for modelling technological plasmas. Hydrogen is also fundamentally interesting, as while highly accurate wave functions for this simple molecule are available, providing an accurate, ab initio, treatment the collision dynamics has proven challenging, on account of the need to have a complete description of channel coupling and polarization effects. To date, no single theoretical approach has been able to replicate experimental results across all transitions and incident energies, while the experimental database that is available is far from complete and not all available measurements are in satisfactory agreement. In this talk, we present differential and integral cross section measurements for electronic excitation cross sections for molecular hydrogen by low-energy electron impact. The data were measured at incident energies below 20eV, using a well-tested crossed beam apparatus and employing a moveable gas source approach to ensure that background contributions to the scattering are accurately accounted for. These measurements are compared with new theoretical results employing the convergent close coupling approach. [Preview Abstract] |
Friday, October 14, 2016 11:30AM - 11:45AM |
VF1.00002: CCC calculated integrated cross sections of electron-H$_2$ scattering Mark Zammit, Dmitry Fursa, Jeremy Savage, Igor Bray Recently we applied the molecular convergent close-coupling (CCC) method to electron scattering from molecular hydrogen H$_2$ [1]. Convergence of the major integrated cross sections has been explicitly demonstrated in the fixed-nuclei approximation by increasing the number of H$_2$ target states in the close-coupling expansion from 9 to 491. The calculations have been performed using a projectile partial wave expansion with maximum orbital angular momentum $L_{\rm max}=8$ and total orbital angular momentum projections $|M| \leq 8$. Coupling to the ionization continuum is modeled via a large pseudo state expansion, which we found is required to obtain reliable elastic and excitation cross sections. Here we present benchmark elastic, single-ionization, electronic excitation and total integrated cross sections over a broad energy range (0.1 to 300 eV) and compare with available experiment and previous calculations. [1] M. C. Zammit $et$ $al$. Phys. Rev. Lett. $\textbf{116}$, 233201 (2016). [Preview Abstract] |
Friday, October 14, 2016 11:45AM - 12:15PM |
VF1.00003: Theoretical studies on dissociative recombination of molecular ions Invited Speaker: Åsa Larson In dissociative recombination a molecular ion captures an electron forming a neutral state that dissociates into fragment. Due to the Coulomb attraction between the reactants, the cross section is typically large at low collision energies and the process is important for different types of plasmas. Here, it will be described how the reaction can be studied theoretically. The goal is to compute reaction cross sections and to determine what fragments are formed. The calculations are done in close collaboration with experiments. In the process, the electron may be captured directly into an electronic resonant state that then is dissociated into fragments. An alternative mechanism is driven by an electron capture into a ro-vibrationally excited Rydberg state that then is predissociated. The two mechanisms are competing and should be considered coherently. The study of dissociative recombination requires both the accurate treatment of the electron scattering processes, but must also include an accurate representation of the potential energy curves, both for electronically bound states and the resonant states. In addition, the couplings between these states, both the coupling between the resonant states and the scattering continuum (the autoionization width) and the non-adiabatic coupling between all states are needed to complete describe the cross section including the branching ratios into final states. These are obtained using structure calculations as well as scattering calculations, using the complex Kohn variational method. The electronic states are diabatized before the nuclear dynamics is studied quantum mechanically. The theoretical method will be illustrated with examples on dissociative recombination of small molecular ions such as HF$^+$, BeH$^+$, H$_2$O$^+$ and N$_2$H$^+$. [Preview Abstract] |
Friday, October 14, 2016 12:15PM - 12:30PM |
VF1.00004: Vibrational excitation in O$_{\mathrm{\mathbf{2}}}$\textbf{ and Cl}$_{\mathrm{\mathbf{2}}}$\textbf{ inductively-coupled plasmas and DC discharges} Jean-Paul Booth, Daniil Marinov, Mickael Foucher, Adriana Annusova, Vasco Guerra Low-energy electrons can interact with molecules via resonances to cause vibrational excitation with large cross-sections. Such processes can absorb significant energy from the plasma electrons, affecting the electron energy distribution and potentially (via vibration-translation (VT) energy transfer) causing substantial gas heating. The presence of vibrationally excited molecules may significant increase the rates of collisional processes, including electron dissociative attachment and electron impact dissociation into neutral atoms. However, the cross-sections of these processes are often poorly known since they are extremely difficult to measure directly, and reliable theoretical calculations are only now appearing for simple diatomic molecules. We have measured the vibrational distributions in discharges in pure O$_{\mathrm{2}}$ and pure Cl$_{\mathrm{2}}$, using high-sensitivity ultra-broadband ultraviolet absorption spectroscopy. In O$_{\mathrm{2}}$ plasmas significant vibrational excitation is observed, up to v''$=$18, with a tail temperature of around 8000K. In Cl$_{\mathrm{2}}$ excitation is only observed up to v''$=$3, and the distribution appears to be in local equilibrium with the gas translational temperature (up to 1500K). We are developing a detailed self-consistent 0D global model of these systems including vibrational excitation. [Preview Abstract] |
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