C. William McCurdy, Thomas N. Rescigno, and Daniel A. Horner, Lawrence Berkeley National Laboratory
William A. Isaacs, Lawrence Livermore National Laboratory
Mark Baertschy, JILA, University of Colorado

Wave function for a model 2D simulation of positron impact on atomic hydrogen. A 20 eV positron is incident along the r1 axis. The radial waves in the sector r1 > r2 correspond to ionization, leaving a bare proton plus a free electron and a free positron, which carries off most of the energy. The concentration of density along the r1 = r2 axis corresponds to rearrangement events that produce bound positronium.

Research Objectives
This project seeks to develop theoretical and computational methods for treating electron collision processes that are currently beyond the grasp of first-principles methods. We are developing methods for studying electron-atom and electron-molecule collisions at energies above that required to ionize the target, for calculating detailed electron impact ionization probabilities for simple atoms and molecules, and for treating low-energy electron collisions with polyatomic molecules, complex molecular clusters, and molecules bound to surfaces and interfaces.

Computational Approach
We are developing a new generation of electron-atom and electron-molecule scattering codes. These codes now include complex optical potential interactions, a scattered-wave/flux operator formalism, and a variety of techniques based on analyticity. Our new FEM/DVR discretization method combines finite elements with a discrete variable representation based on the use of Gauss-Lobatto quadrature, thus offering the best features of both methods (sparse matrices and simplicity of representation). We have also been examining more efficient new integral methods for computing breakup cross sections. These developments will allow us to extend our ionization studies to two-electron targets and to treat a broad range of problems, from low-energy electron-molecule collisions using elaborate variational wave functions, to direct solutions of the Schrödinger equation for simple atomic targets that provide detailed ionization cross sections.

Accomplishments
The methods we developed have been shown capable of treating all details of electron impact ionization of atoms, including energy sharing differential cross sections and triply differential cross sections (energy and solid angles for both ejected electrons).

We have finished the first computational study of low-energy electron-CO₂ scattering that successfully reproduced the two features that dominate the low-energy cross sections, namely, the dramatic rise in the elastic cross section below 2.0 eV and the resonance peak centered near 3.8 eV. We initiated the second phase of work on this system, which is to further explore the effects of nuclear motion on the low-energy scattering cross sections. We have found that both symmetric stretch and bending motion are crucial in determining accurate resonant vibrational excitation cross sections and in understanding the nature of the low-energy virtual state enhancement of the elastic and momentum transfer cross section. We have begun to carry out multi-dimensional time-dependent wavepacket calculations using the results of our electronic fixed-nuclei electron scattering calculations to quantify the nuclear dynamics and to compute the vibrational excitation cross sections.

Significance
Electron collision processes play a key role in such diverse areas as fusion plasmas, plasma etching of silicon chips, and mixed radioactive waste remediation. Understanding of these processes is severely hampered by the lack of a database of electron-molecule collision cross sections. This project will significantly add to that base of knowledge.

Publications
T. N. Rescigno, M. Baertschy, W. A. Isaacs, and C. W. McCurdy, "Collisional breakup in a quantum system of three charged particles," Science 286, 2474 (1999).

T. N. Rescigno, D. A. Byrum, W. A. Isaacs, and C. W. McCurdy, "Theoretical studies of low-energy electron-CO₂ scattering: Total, elastic and differential cross sections," Phys. Rev. A 60, 2186 (1999).

C. W. McCurdy and T. N. Rescigno, "Practical calculations of quantum breakup cross sections," Phys. Rev. A 62, 32712 (2000).

http://jolt.lbl.gov