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RESEARCH: Reprints & preprints | Projects & Opportunities | |||||
Projects currently underway in my research group include
If you'd like more information about my program, please come see me. Quantum many-body effects in electron scattering One of the most fascinating challenges for those who study electron-impact
collisions at energies below a few tens of eV is the inclusion of intrinsically
quantum mechanical, many-body effects due to instantaneous Coulomb interaction
between the projectile and bound electrons. These effects, called correlation,
aren't included in theories based on the independent particle model, a
bulwark of modern scattering theory. They're hard to include rigorously,
because no matter how big a supercomputer you have, someone will find
a molecule so complicated that your computer can't handle it. Our alternative
strategy is to adapt techniques and insights of bound-state quantum physics
to develop new approaches to including correlation and understanding physically
the resulting scattering quantities. For example, we're extending the
principles of density functional theory, which thus far has been applied
to bound states, to low-energy electron collisions with atoms and molecules.
In another tack, we've adapted a distributed charge model originally developed
in nuclear physics to develop a computationally simple model potential
for electron-molecule scattering. Finally, we're investigating recent
controversial assertions that bound-bound correlation, heretofore neglected
in electron-molecule collisions, may significantly affect these processes.
Collaborators: Dr. Andy Feldt (OU), Mr. Jef Wagner (OU),
Prof. Weiguo Sun and Dr. Hao Feng (Sichuan University). Dissociative attachment in electron-molecule scattering In the simplest collisions, particles scatter without changing their
character. Examples include elastic scattering from an atom and rotational
excitation of a molecule. Even more interesting are rearrangement collisions,
in which the particles after scattering aren't the same as those before.
For example, in dissociative attachment: an incident electron causes
a molecule to break apart into its constituent atoms, then attaches to
one of those atoms to result in an atom and a negative ion. This process
requires a major transfer of energy between the scattering electron's
kinetic energy and the vibration dynamics of the target. Alas, this energy
transfer is ``non-adabatic'' in that it can't be treated accurately using
the standard Born-Oppenheimer separation of electronic and nuclear motion.
This nasty feature poses significant challenges to both computation and
understanding of such collisions. We are tackling this problem using an
approximate scattering theory that includes this vital energy transfer
in a way that allows one to work with "fixed nuclei" molecule
geometries, thereby retaining the simplifications of the Born-Oppenheimer
approach. Collaborators: Dr. Djamal Rabli (OU), Dr. Robert K. Nesbet
( IBM), Prof. Gregory Parker (OU). Time-dependent studies of nonadiabatic electronic transitions in molecules One thing that makes photon-induced transition processes in molecules
so interesting is the interplay between nonadiabatic electron dynamics
and the redistribution of vibrational energy that occurs as the molecule
forms or breaks a bond. By "nonadiabatic electron dynamics"
we refer to the breakdown of the Born-Oppenheimer approximation, a cornerstone
of molecular physics. This projet is a theoretical/experimental collaboration
intended to investigate this interplay in molecular photodissociation.
Our ultimate goal is to understand mechanisms for energy relaxation in
molecules in highly excited electronic states. We are working on computational
and visual investigations of photodissociation in the sodium dimer, research
that lays the foundation for our long-term goal: study of a series of
energetic nitrogen containing compounds, including nitroalkanes, nitramines,
and amides. Collaborators: Ms. Melanie Carter (OU), Profs.
James Schaffer and Gregory A. Parker (OU). The Great Hydrogen Controversy (electron transport theory in molecular hydrogen) One marker that a field has "come of age" is a high level of
agreement on the simplest, most fundamental problems. Atomic collision
physicists attained such a benchmark in the 1970s for electron-atom scattering
with results on e-He collisions. The counterpart for electron-molecule
scattering is electron scattering from molecular hydrogen. The news is
not good. In spite of two decades of concentrated research by theoreticians
and experimentalists throughout the world, including a long-standing collaboration
between members of our group and experimentalists at the Australian National
University, a highly significant discrepancy remains at energies below
a couple of eV among cross sections for the electron induced excitation
of the first vibrational state of this molecule. We have recently shifted
the focus of our efforts to straighten out this mess to challenge certain
assumptions that have undergirded transport analysis of swarm data for
molecules from the inception of this research field in the early '60s.
Collaborators: Profs. Rob Robson (ANU) and Ron White (James Cook University,
Australia). R-matrix theory for device physics As a tactic for solving the Schroedinger equations for collisions involving
charged particles, atoms, and molecules, the R-matrix method has proved
its mettle. Originally developed in nuclear physics, this method is based
on a physically appealing partition of space into different regions depending
on the nature of the interaction between the projectile and the target.
Mathematically speaking, it's just an extension of wave function continuity
conditions, familiar from undergraduate quantum physics, to the rich complexity
of atomic and molecular systems. Surprisingly, many features of this method
make it an appealing way to study electron transport in condensed matter
devices. Extending R-matrix theory to such problems is by no means trivial:
for example, realistic devices have geometries radically different from
the familiar spherical geometries used in application to atoms and molecules.
We're currently incorporating the experience of collision physicists in
atomic and molecular physics into this new field of application. Collaborators:
Prof. Kieran Mullen (OU) and Ms. Thushari Jayasekera (OU). A least-squares S-matrix method for extrapolating measured angular distributions Experimental physicists are expert at measuring electron-molecule differential
cross sections to exceptional accuracy in crossed-beam experiments. From
these data, they need to determine integral cross sections. Alas, this
process introduces significant error beyond those inherent in their original
measurements. The reason seems mundane: the apparatus they use won't let
them take data over whole range of scattering angles 0 to 180. To experimentalists
who see a few percent error in their differential cross sections explode
into errors of 20% or more in their integral cross sections, this is bad
news. To help, we're developing a procedure that combines simple statistical
optimization procedures with physical properties of the scattering matrix
that derive from the Schroedinger equation. This procedure will yield
integral cross sections whose error is not appreciably later than that
of the raw angular distributions experimentalists work so hard to measure..
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morrison@mail.nhn.ou.edu Department of Physics & Astronomy University of Oklahoma Site created December 16, 1999 Copyright © 2005 by Michael A. Morrison No natural laws were violated during the creation of this site. |
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