Angular momentum and orientation effects in excitation-ionization collisions

Abstract

We present theoretical fully differential cross sections (FDCS) for electron-impact excitation-ionization of helium in which the final state He+ ion is oriented in a particular direction. Specifically, we study the process for He+ ions in the 2p0 state. Using our 4-body distorted wave model, we show a strong dependence of the FDCS on the ion’s orientation and trace some unexpected structures in the FDCS to the L = 2 term in the partial wave expansion for the ionized electron. A comparison is drawn to the ionization of oriented Mg (3p0) atoms, and unlike that process, we find that for excitation-ionization angular momentum must be transferred from either the projectile or the target atom.

Anisotropy in excitation-ionization double-differential cross sections

“…In the bottom row, the incident electron energy is fixed at 200 eV and the scattering angle is 5°. These results show a clear dependence of FDCS on orientation of the ion, consistent with previous work [27][28]. In the case of orientation along the beam direction or along the momentum transfer direction, almost no forward ejection is observed, but a strong backward recoil peak is seen.…”

“…Experimentally, the EI process is very challenging due to small cross sections. Prior work using our 4-Body Distorted Wave (4DW) model has shown clear orientation effects in FDCS for the EI process, with some of the structures being traced to specific angular momentum components of the ejected electron and the target atom wave function [27][28].…”

Partial wave analysis of oriented collisions

“…The theoretical models used here are the three-body and fourbody distorted wave models (3DW and 4DW, respectively). Full explanations of these calculations can be found in [5,[8][9][10], and we include only the pertinent details here. In both models, all continuum electrons are treated as distorted waves, and the post-collision Coulomb interaction (PCI) between the final state continuum electrons is included exactly.…”

“…They also provide information about the structure of the target atoms, and represent a stringent test of current theoretical models. In most cases, the initial state orientation is determined [1][2][3][4], however, a recent study has examined a collision process in which the final state ion is oriented [5]. In the case of oriented targets in the initial state, most of the work has been focused on oriented molecules.…”

“…Recent work has also been done for the excitationionization (EI) process in which the final state ion is left in an oriented 2p state [5]. From a theoretical point of view, the FDCS for the oriented EI process and the ionization of oriented targets can be shown to have the same functional form [3,5]. This implies that there could be some similarities between the two processes.…”