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C. William
McCurdy, Daniel A. Horner, Zhiyong Zhang, and Wim Vanroose, Lawrence Berkeley
National Laboratory
Thomas N. Rescigno, Lawrence Berkeley National Laboratory and Lawrence
Livermore National Laboratory
Mark Baertschy, University of Colorado/JILA
William Isaacs, Lawrence Livermore National Laboratory
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Wave functions for a quantum mechanical system of a charged particle
colliding with a model two-electron atom. The vertical axis represents
the coordinate of the incident particle, and the other two axes are
the electron coordinates. In the left panel, the collision energy
is lower than the breakup threshold; in the center panel, the collision
energy is sufficient to singly ionize the atom; in the right panel,
the energy is sufficient to cause complete fragmentation of the system. |
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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.
Computational
Approach
Our approach builds on the algebraic variational formalisms (the
Complex Kohn variational method) we have been developing to study electron-atom
and electron-molecule scattering. These approaches have now been extended
to include complex optical potential interactions, a scattered-wave/flux
operator formalism, and a variety of techniques based on analyticity.
These techniques allow us 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
We developed a new procedure for computing impact ionization amplitudes
that provides a direct and efficient route to computing the breakup cross
sections. This provided an independent check on the fundamental correctness
of the results we had initially obtained by numerically extrapolating
the quantum mechanical flux. The computational approach we have developed
has, to date, provided the only complete solution to the quantum mechanical
three-body Coulomb at low collision energies.
We have also developed a new technique for computing the scattered wave
function that does not require the solution of large systems of complex
linear equations. This is accomplished by writing the scattered wave function
as the Fourier transform of a time-propagated initial state that can be
computed using a discrete variable representation in conjunction with
a split-operator method.
We have completed the second phase of our work on electron-CO2
scattering, exploring resonant vibrational excitation in the 4 eV energy
region and carrying out time-dependent wavepacket studies in three dimensions.
This study represents the first time that all aspects of an electron-polyatomic
collision, including not only the determination of the fixed-nuclear electronic
cross sections but also a treatment of the nuclear dynamics in multiple
dimensions, has been carried out entirely from first principles.
Significance
Electron collision processes are central to the problems of interest
to DOE, playing a key role in such diverse areas as fusion plasmas, plasma
etching and deposition, and waste remediation. Electron-molecule collisions
play a central role in the plasma processing of silicon chips, but the
understanding and modeling of these low-temperature plasmas 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.
In mixed radioactive waste, the primary radioactive decay events produce
showers of secondary electrons which initiate the chemistry that produces
a mixture very different from the one that was present originally. The
tools we are developing will make it possible to understand and model
this kind of electron-initiated chemistry. Central to understanding plasma
formation are the fundamental studies of electron and positron impact
ionization of simple atoms. We developed the first complete method for
treating electron-impact ionization of atoms from first principles, solving
a fundamental problem in atomic physics that had resisted solution for
more than 40 years. We are now working extensions of the computational
approach that will allow us to study systems containing more than two
electrons.
Publications
M. Baertschy, T. N. Rescigno, and C. W. McCurdy, "Accurate
amplitudes for electron impact ionization," Phys. Rev. A 64,
022709 (2001).
M. Baertschy, T. N. Rescigno, C. W. McCurdy, J. Colgan, and M. S. Pindzola,
"Ejected-energy differential cross sections for the near threshold
electron-impact ionization of hydrogen," Phys. Rev. A 63,
050701 (2001).
W. A. Isaacs, M. Baertschy, C. W. McCurdy, and T. N.Rescigno, "Doubly
differential cross sections for the electron impact ionization of hydrogen,"
Phys. Rev. A 63, 030704(R) (2001).
www.lbl.gov/cs/amo_theory/
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