Bulletin of the American Physical Society
71st Annual Gaseous Electronics Conference
Volume 63, Number 10
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session HW1: Electron-Molecule Scattering |
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Chair: Klaus Bartschat, Drake University Room: Oregon Convention Center A103-A104 |
Wednesday, November 7, 2018 9:30AM - 10:00AM |
HW1.00001: Dissociative electron attachment dynamics in polyatomic gases Invited Speaker: Daniel Slaughter The resonant process of dissociative electron attachment can be described by a transient molecular anion intermediate, exhibiting highly non-Born-Oppenheimer behavior in the coupling of electronic motion into internuclear motion within the molecule, followed by dissociation. The transient molecular anion is initially in the electronic continuum, therefore the resonance autodetachment lifetime must be included explicitly in an accurate theoretical description of this process. Detailed examination of the dynamics of the transient molecular anion requires multidimensional experimental techniques to analyze the outgoing fragments in detail. Anion fragment momentum imaging[1,2] captures the final-state momentum of the dissociated anion fragment and allows both the kinetic energy release and the angular distribution to be determined in the laboratory frame, which is defined by the incident electron momentum. Transforming this information into the molecular frame can be achieved with the support of ab initio electron scattering calculations to determine the electron attachment amplitude. This potent combination of experiment and theory allows the dynamics of dissociative electron attachment to be interrogated in detail. The results of recent investigations on polyatomic molecules will be presented, along with our latest results on the simplest organic acid, formic acid, performed in collaboration with C. S. Trevisan (Cal Maritime Academy), and A. Belkacem, C. W. McCurdy, T. N. Rescigno, and R. R. Lucchese (LBNL). References cited: [1] H. Adaniya, D. S. Slaughter, T. Osipov, Th. Weber, A. Belkacem (2012), “A momentum imaging microscope for dissociative electron attachment”, Rev. of Sci. Inst. 83 (2), 023106; [2] D. S. Slaughter, A. Belkacem, C. W. McCurdy, T. N. Rescigno, D. J. Haxton (2016), “Ion-momentum imaging of dissociative attachment of electrons to molecules”, J. Phys. B: Atomic, Molecular and Optical Physics 49 (22), 222001 [Preview Abstract] |
Wednesday, November 7, 2018 10:00AM - 10:30AM |
HW1.00002: Simplified approach to dissociation of polyatomic molecules by electron impact Invited Speaker: Samantha Fonseca Dissociative Recombination (DR) and Dissociative electron attachment (DEA) of molecules are important processes in various plasma environments. Despite several approaches developed for diatomic molecules, the theoretical description of electron-molecule scattering for polyatomic molecules is an extremely complex problem. For both DR and DEA we have used simplified models that highlight the essence of what drives the mechanisms. The DR process can be divided into direct and indirect DR. For indirect DR, which is mostly relevant at energies up to mili electron volts, the simplified approach models the vibrational states using normal modes and the non-adiabatic couplings between them are obtained simply by computing the scattering matrix elements in this vibrational space. Electronic structure calculations, as well as scattering calculations, were carried out entirely from ab initio principles using the MESA program combined with the complex Kohn variational method. At higher energies, up to few electron volts, direct DR becomes more prominent and its treatment starts with carrying out electron scattering calculations as a function of the three internal degrees of freedom to obtain the resonance energy surfaces and autoionizing resonance widths. Then, this data is used as input to form the Hamiltonian relevant to the nuclear dynamics. The multidimensional wave equation is solved using the Multi-Configuration Time-Dependent Hartree (MCTDH) technique. The simplified model has been applied to different systems along the years and the results compare surprisingly well with available experimental data. In this talk I will focus on the DR results obtained for H$_2$O$^+$, N$_2$H$^+$ and HCl$^+$. The DEA approach follows the treatment proposed by Bardsley (1968) developed for diatomic molecules. The formalism of resonant scattering has been adapted to polyatomic molecules and applied to the H$_2$CN molecule, which has six normal modes and is believed to be responsible for the formation of the CN$^-$ and H$^-$ ions and the HCN molecule in the interstellar space. The relevant electronic states are calculated from ab initio principles by combining electron scattering calculations to obtain resonance positions and autoionization widths with multi-reference configuration interaction calculations of the Rydberg states and the ion. [Preview Abstract] |
Wednesday, November 7, 2018 10:30AM - 10:45AM |
HW1.00003: Experimental and theoretical study of the energy and angular dependence of the triple differential cross sections for electron-impact ionization of aligned H2. Esam Ali, Enliang Wang, Xingyu Li, Xueguang Ren, Chuangang Ning, Xiangjun Chen, Alexander Dorn, Don Madison We have measured triple differential cross sections for 520 eV electron impact ionization of aligned H2 in the perpendicular plane. For ejected electron energies of 10 eV and 20 eV, we measured cross sections for all possible alignment angles. For two perpendicular alignment angles, we measured cross sections for ejected electron energies ranging between 10 eV and 30 eV. The experimental results will be compared with theoretical M3DW (molecular 3-body distorted wave) and MCDW (multicenter distorted-wave) models calculations. Overall, good agreement is found between theory and experiment. [Preview Abstract] |
Wednesday, November 7, 2018 10:45AM - 11:00AM |
HW1.00004: Multicenter distorted wave approach for electron-impact ionization of molecules. Don Madison, Esam Ali, Chuangang Ning In the M3DW (molecular 3-body distorted wave) approach that we have been using for examining electron-impact ionization of molecules, the continuum wavefunctions are calculated using a spherically symmetric distorting potential and consequently do not depend on the orientation of the molecule. Here we report a new version of this approach for which the distorted waves depend not only on the orientation of the molecule but also on the exact location of each nuclei. To test this new model, we compare our theoretical results with experimental cross sections measured by Alexander Dorn's group for 54 eV electron-impact ionization of aligned H2 in the perpendicular plane [\textit{Phys. Rev. Lett.} 109 123202, (2012)]. [Preview Abstract] |
Wednesday, November 7, 2018 11:00AM - 11:30AM |
HW1.00005: Towards Visualizing the Driving Principle of a Photochemical Reaction by Means of Time-Resolved Electron and Atomic Momentum Spectroscopies Invited Speaker: Masahiko Takahashi One of the goals in the field of chemical reaction dynamics may be to watch reactions in real time. Indeed, a variety of time-resolved spectroscopic techniques have been developed so far. Nevertheless, there still remains a challenge in exploring why the atoms are dancing in such a way. To tackle with the challenge, we have been developing a real-time spectroscopic complex, which consists of time-resolved versions of (e, 2e) electron momentum spectroscopy (TR-EMS) and atomic momentum spectroscopy (TR-AMS). TR-EMS is designed to measure in real time the momentum distributions of each electron, bound in a decaying system, with different binding energies. The observed change in electron motion represents the driving force behind chemical reaction. On the other hand, TR-AMS aims to measure the momentum distributions of each atom, involved in a decaying system, with different mass numbers, which tell about how and how much the change in atomic motion is brought about by the change in electron motion. In the contribution, present status and future prospect of this new real-time spectroscopic complex will be reported and discussed. [Preview Abstract] |
Wednesday, November 7, 2018 11:30AM - 11:45AM |
HW1.00006: Vibrationally-resolved electron-impact excitation of molecular hydrogen Liam Scarlett, Jonathan Tapley, Dmitry Fursa, Jeremy Savage, Igor Bray, Mark Zammit Molecular hydrogen and its isotopologues are present in a range of vibrationally excited states in fusion, atmospheric, and interstellar plasmas. Electron-impact excitation cross sections resolved in both final and initial vibrational levels of the target are required for modeling the properties of many low-temperature plasmas. Measurements of excitations in H$_2$ are typically limited to scattering on the ground vibrational state, and hence there is significant demand for accurate theoretical calculations of scattering on excited states. At low to intermediate energies, the currently recommended data are up to a factor of two higher than the available measurements for scattering on the ground vibrational state. Recent calculations performed using the convergent close-coupling (CCC) method have demonstrated the convergence of excitation cross sections with respect to the number of coupled channels, and yielded good agreement with experiment for scattering on the ground vibrational level of H$_2$. Here we extend the CCC method to provide a fully vibrationally-resolved description of e-H$_2$ scattering, providing results for excitation of all vibrational levels in several low lying singlet and triplet states, from all vibrational levels of the ground state. [Preview Abstract] |
Wednesday, November 7, 2018 11:45AM - 12:00PM |
HW1.00007: Photon-H$_2$ cross sections and hydrogen plasma equations of state Mark Zammit, James Colgan, Jeremy Savage, Dmitry Fursa, Igor Bray, Christopher Fontes, David Kilcrease, Peter Hakel, Jeffery Leiding, Eddy Timmermans Studies of molecular plasmas both in local thermodynamic equilibrium (LTE) and non-LTE require state-resolved (electronic, vibrational and rotationally resolved) transition cross sections or rate coefficients to calculate populations (for non-LTE plasmas), opacities and emissivities. Here we present state-resolved results of photodissociation and radiative association of H$_2$ and its isotopologues (D$_2$, T$_2$, HD, HT, and DT), and preliminary results of low-temperature hydrogen plasma equations of state and opacities. [Preview Abstract] |
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