Session G5: Electron Collisions

Dissociative Dynamics of Electron Attachment to Carbon Dioxide

Three-dimensional momentum imaging is used to observe the dissociative dynamics of O$^-$ production from electron attachment to CO$_2$ at two resonances, a $^2\Pi_u$ shape resonance at 4.4 eV and a Feshbach resonance at 8.2 eV. At 8.2 eV, images of the attachment dynamics are contrasted with existing data on the angular distribution of the resonant dissociation, while the 4.4 eV resonance momentum shows a breakdown of the axial recoil approximation with an asymmetry favoring dissociation and ejection of the O$^-$ opposite the direction of the incoming electron's momentum vector. Measurements from two independent experiments are interpreted in light of new ab initio calculations to describe the electron attachment dynamics.   

Radiative electronic attachment to molecules of astrophysical interest

We have developed a first-principles approach to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest $Mol +e^- \to Mol^- + \hbar\omega (Mol^- = $C$_n$H$^-$, C$_n$N$^-$ ). The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion, obtained through the complex Kohn variational method. We present our benchmark calculation for the formation of the simplest observed ion CN$^-$ by REA, as well as our preliminary results for the formation of C$_3$N$^-$ and C$_4$H$^-$ by REA. We calculated a low rate of $10^{-15}$cm$^3/$s at 30 K for CN$^-$ and rates about 20 time larger for C$_3$N$^-$ and C$_4$H$^-$ due to larger transition dipole moments. These two latter ions possess dipole states and we have also considered their potential role in the process of radiative electronic attachment. Finally, our results suggest that the negative molecular ions, recently observed in the interstellar medium, can hardly be formed by the process of radiative electron attachment.   

Guiding of electrons through insulating PET nanocapillaries

Electron transmission through insulating capillaries has been actively studied following the discovery of the guiding of 3 keV highly charged ions (HCI) transmitting through such media [1]. Previously results were obtained for electrons incident on polyethylene terephthalate (PET) capillaries [2] and for straight [3] and tapered [4] glass capillaries. New interest has focused on electron energy loss and the temporal charge deposition dependence for incident energies 500 and 800 eV. In the present work a PET sample with a diameter of 100 nm and thickness of 12 $\mu $m was bombarded by 300, 500 and 800 eV electrons for several small tilt angles. The angular profiles of the transmitted electron spectra for each sample tilt angle were fit with Gaussian functions that were related to the corresponding observation angle. The results show strong evidence for guiding and for the time dependence of the charge deposition.\\[4pt] [1] N. Stolterfoht \textit{et al., }Phys. Rev. Lett. \textbf{88} 133201 (2002).\\[0pt] [2] S. Das\textit{ et al.,} Phys. Rev$. $A \textbf{76} 042716 (2007).\\[0pt] [3] B. S. Dassanayak \textit{et al}., Phys. Rev. A \textbf{81} 020701(R) (2010).\\[0pt] [4] S. J. Wickramarachchi., Nucl. Instrum. Meth. Phys. Res B \textbf{269} 1248 (2011).   

Electronic Excitation of Water and Methanol by Electron Impact

It is only in recent years that measured absolute differential cross sections (DCS) for electronic excitation of water have become available. The electronic spectrum of water is characterized by broad, overlapping dissociative bands superposed by weaker Rydberg bands and the requiste analysis of measured energy loss spectra needed to produce absolute DCS is therefore far from straightforward. Recent measurements [1] of absolute DCS show significant differences with an earlier experimental study [2], prompting us to undertake a new theoretical study of this problem. We will report the results a multi-channel variational study of electronic excitation of water using the complex Kohn method and show how the computed cross sections compare with earlier theoretical studies as well as experiment. We will also present results of a similar study on electron impact excitation of methanol which has also been the focus of a recent experimental study [1]. \\[4pt][1] M. Khakoo and L. Hargreaves (private communication). \\[0pt][2] M. Brunger et al. {\it Int. J. Mass Spec.} {\bf 281}, 80 (2008).   

Photoionization and Electron-Ion Recombination of Ne IV

The inverse processes of photoionization and electron-ion recombination of Ne IV are being studied. The unified method is used for self-consistent data for the inverse processes as needed for determination of ionization fractions in plasmas in photoinization equilibrium. Precise abundance of neon in the sun has remained in discrepant due to lack of accurate atomic data. Unified method implements the ab initio relativistic Breit-Pauli R-matrix method in the close coupling approximation. Photoionization cross sections and electron-ion recombination rates are calculated using a 20 level wave function expansion. Fine structure effect shows low energy resonant structures. Illustrative results for the inverse processes will be presented.   

Simplified model to describe the dissociative recombination of linear polyatomic ions of astrophysical interest

We will present theoretical results on the dissociative recombination (DR) of the linear polyatomic ions HCNH$^+$, HCO$^+$ and N$_2$H$^+$. Besides their astrophysical importance, they also share the characteristic that at low electronic impact energies their DR process happens {\it{via}} the indirect DR mechanism. We apply a general simplified model successfully implemented to treat the DR process of the highly symmetric non-linear molecules H$_3^+$, CH$_3^+$, H$_3$O$^+$ and NH$_4^+$ to calculated cross sections and DR rates for these ions. The model is based on multichannel quantum defect theory and accounts for all the main ingredients of indirect DR. New perspectives on dissociative recombination of HCO$^+$ will also be discussed, including the possible role of HOC$^+$ in storage ring experimental results.   

Topological phases in atoms and molecules in spin-polarized electron scattering

Observations of spin-polarized electron impact excitation of zinc atoms, ionization of helium atoms and dissociative excitation of molecules indicate a topological phase. The parallel transport of the spin vector gives rise to an effective `monopole' magnetic field and an apparent spin-orbit interaction. In excitation, the Stokes parameters of radiated photons show alignment and orientation. Excitation of a superposition of 2s and 2p states on atomic hydrogen in an external electric field show beats in Lyman alpha radiation as predicted for a circular vortex. Ionization of helium atoms show minima in (e,2e) angular and energy differential cross sections associated with a linear vortex (Macek, Feagin). Exchange dissociative excitation of methane shows radiated photons from a Jahn-Teller ``avoided crossing'' of potential energy curves (Mead and Truhlar). The observations are consistent with fundamental principles that a gauge--invariant quantity is potentially a physical observable and the topology of a ring indicates a magnetic-flux line enclosed by the ring is equivalent to a vortex line.   

New tools for relativistic collision calculations

In a varying and uncertain relativistic transition region of velocity the methods available for treating collisions change completely, not only because of the v/c change but in the need to go from a 3-space to a space-time geometry. For problems with 2 or more particles this seems to require 2 or more independent time variables, a complication unseen in the real world. In an alternative form of special relativity I have shown that Lorentz covariance is completely obeyed if one refers a single time variable to a cosmic proper time and uses a Hubble expanding position frame of a negative curvature hypersphere for the 3 position coordinates of each of the n particles. The cosmic dimensions of course change nothing on a local scale except to clarify the validity of using a single time. This change alone clarifies and simplifies treatment of the transition connecting nonrelativistic and relativistic domains. The variables most useful for such a treatment and the form of the v/c correction terms that are encountered will be exhibited.