Session SF5: Electron Collisions with Atoms and Molecules II

Imaging of the dissociation dynamics of polyatomic molecules following low-energy electron resonant attachment

We will present a study that combines experimental data along with theoretical analysis of dissociative electron attachment to carbon dioxide, methanol and uracil. In these studies we demonstrated that an understanding of anion dissociation dynamics beyond simple one-dimensional models is crucial in interpreting the measured angular distributions. Although, for example, several possible dissociation mechanisms involving conical intersections have been identified for the lowest resonance and discussed in the case of CO$_2$, the most likely scenario points to an initial linear asymmetric stretch motion to geometries where the autodetachment probability is small, followed by bending motion around a conical intersection. We also investigated the dynamics of DEA to methanol for the low-energy Feshbach resonance at 6.5 eV. The angular distributions of the recoiling fragments were found to deviate significantly from the axial recoil approximation that was used previously to accurately describe the dynamics in the analogous 2B1 resonance in water. Observation of the dynamics of dissociative electron attachment (DEA) in biomolecules has recently become possible by momentum imaging of the fragments. Guided by electronic structure and scattering we observed key aspects of the dynamics of ring-breaking dissociation of the transient anion formed upon DEA to the nucleobase uracil.   

Comprehensive Study of 3-Body and 4-Body Models of Single Ionization of Helium

For decades the frozen core approximation has been successfully used to model 4-Body collisions as 3-Body processes. In recent years, full 4-Body models have been used to calculate fully differential cross sections (FDCS) for single ionization of helium, and these 4-Body models show discrepancies with the 3-Body models. We have identified four possible sources of the discrepancies, which are: the initial state helium wave function, the final state He$^{+}$ wave function, the final state potential for the outgoing electrons, and the perturbation. To identify which of these four sources causes in the differences in FDCS, we have performed a comprehensive study of 3-body and 4-body models for a wide range of incident projectile energies, ionized electron energies, and scattering angles.   

Accurate calculations of the relativistic rise in electron-impact excitation cross sections for highly charged ions

Exact relativistic plane-wave Born (RPWB) matrix elements of the M{\o}ller interaction are incorporated in the ``analytic Born subtraction technique'' and employed in the Relativistic Convergent Close-Coupling (RCCC) method. Application to the calculation of high-energy electron-impact excitation cross sections of highly charged hydrogenlike ions demonstrates the ``Bethe rise'', an effect that is manifest in Bethe's original 1932 work~[1] on relativistic high-energy, electron-impact excitation. The result represents an improvement over Bethe's relativistic high-energy theory developed in the 1930's in that (i) both target and projectile electrons are represented relativistically with Dirac spinor wavefunctions and (ii) the dipole approximation plus additional assumptions are not employed in the RPWB scattering amplitude of the M{\o}ller interaction. We show that as the Z of the target increases, the onset of the rise occurs at lower, absolute projectile energies. The onset occurs at significantly lower energies, when expressed in threshold units, as Z increases, which could have important consequences for the collisional-radiative modeling of high-Z plasmas.\\[4pt] [1] H. Bethe, Z. Phys. 76, 293 (1932).   

Sturmian approach to the study of photoionization of atoms and molecules

In this presentation we study the photoionization of atoms and molecules using ultrashort laser pulses, solving the time-independent Schr\"{o}dinger equation (TISE) in a first order perturbation theory. The interaction laser-matter is described with the dipolar operator in the velocity gauge. Generalized Sturmian functions [1] are used to solve the driven equation for a scattering wave function which includes all the information about the ionization problem. For the atomic case, we study the photoionization of He atom using the Hermann-Skillman potential together with the one-active electron approximation. For molecular systems (CH$_{4}$ in this work), we use first a spherically symmetric potential U$_{i}$(r) [2], and then a more realistic potential that includes all the nuclei and other electrons interaction, as in [2]. For each molecular orbital we use Moccia's wave functions [3], solve the TISE with an initial molecular orbital i of the ground state and extract the corresponding photoionization cross sections. For both atomic and molecular systems we compare our results with previous calculations and available experimental data. \\[4pt] [1] D. M. Mitnik et al, Comp. Phys. Comm. 182, 1145 (2011); Gasaneo et al, submitted to Adv. Quantum Chem (2013)\\[0pt] [2] L Fernandez-Menchero and S Otranto, Phys. Rev. A 82 022712 (2010)\\[0pt] [3] R Moccia, J. Chem. Phys. 40 2164. (1964)