|RESEARCH: Reprints & preprints | Projects & Opportunities|
Projects currently underway in my research group include
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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).
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).
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).
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).
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).
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..