Theoretical Studies of Electron-Polyatomic Scattering and Photodetachment of Molecular Anions Using AB Initio Methods.

The theoretical studies of low-energy electron scattering from the polyatomic molecules silane and ethane are performed using the complex Kohn variational method with an ab initio description of target response, and are the best theoretical results to date. The elastic differential, integral, and momentum transfer cross sections of electron -silane scattering were obtained at incident energies ranging from 0.2 to 20 eV. The integral cross section shows a Ramsauer-Townsend (RT) minimum at 0.3 eV and a broad d -wave shape resonance at 3.0 eV. The overall agreement between our polarized-Kohn results and the recent experiment measurements of Wan, Moore, and Tossel is excellent. The elastic integral, differential, and momentum transfer cross sections of electron-ethane scattering were calculated for both the staggered (D_{3d} symmetry) and eclipsed (D_{3h } symmetry) conformations of ethane. Our studies agree well with the most recent experimental data. For staggered ethane, our integral cross section shows a RT minimum at 0.18 eV. The minimum in our momentum transfer cross section occurs at 0.12 eV, which agrees with the swarm-derived data of McCorkle and co-workers. Our study also correctly shows the f-wave shape resonance observed in experiments. The cross sections we obtained for staggered and eclipsed ethane indicate that the RT minimum is not sensitive to the barrier to internal rotation between the two conformations of ethane, but that this small barrier generates changes in the region of the f-wave shape resonance. The photodetachment cross sections of F _sp{2}{-} are studied using the complex-basis-function (CBF) method at the static -exchange level. This study is the first application of the CBF method to molecular negative ions. A high energy resonance feature is found in the 3sigma _{u} channel of F_sp {2}{-}, which is evidently not associated with tunneling through an angular momentum barrier. This feature is checked by Born calculations and by varying the internuclear distance of F_sp{2 }{-}, showing that this feature is unlikely to be a computational artifact. This resonance remains but shifts in our configuration-interaction (CI) and multiconfiguration self-consistent field (MCSCF) studies for the 3sigma_{u} channel of F_sp{2}{-} . The total photodetachment cross section of F _sp{2}{-} obtained by summing the contributions of the 1pi_ {u}, 1pi_{g }, 3sigma_{g} , and 3sigma_{u} detachment channels is analysed.