University of Florida - 1982
The Pennsylvania State University - 1983-1986
Professor of Chemistry
Baylor University - 1986-
Current research interests are in the area of theoretical investigations of gas phase and gas-surface collisions. Of particular interest is the transfer of electrons and/or electronic and vibrational energy transfer during collisions including rearrangement collisions. At hyperthermal and thermal energies the exchange of electrons or electronic energy is usually governed by the mixing of electronic states through the nuclear gradient operator which is generally referred to as nonadiabatic coupling. These nonadiabatic reactions are usually characterized by avoided crossings of two or more adiabatic potential energy surfaces. Semi-classical techniques are used for the reaction dynamics leading to time dependent descriptions which help to understand the processes. Previous work in this area involves the collision of a proton with H2. The studies were done at a fixed total energy and the electron transfer probability was determined as a function of the initial vibrational state of H2.
It was found in agreement with other studies and experiments that an increase in vibrational energy, as opposed to kinetic energy, was more important in enhancing electron transfer. It was also found that the H2 vibrational motion was an important factor in rearrangement collisions. Future studies in this area include the scattering of excited Argon with H2. This system is interesting because not only is there transfer of electronic energy but a rearrangement also takes place. There is also an interesting isotope dependence (H2 vs D2 vs HD) for this system.
The construction of the potential energy surfaces and semi-classical dynamics should help to understand this system which would be a prototype for these types of reactions. Electron transfer at metal surfaces is an important phenomenon that occurs in many surface science experiments. Research in this area has been in developing a fundamental description of this process based on gas phase techniques. The emphasis is not in obtaining an accurate electronic description of the surface, but rather in constructing realistic gas-surface potential energy surfaces. Currently a reformulation of the Diatomics in Molecules procedure is being done that leads to a considerable simplification of the problem. With this simplification, interactions of more complex molecules such a diatomics and triatomics with a surface can be obtained.
Another objective is to incorporate classical surface interacting particle quantities such as an image potential, polarization and an induced dipole into the DIM procedure to give accurate long and intermediate range potentials. It is hoped that chemisorption energies can be predicted. Previous dynamical studies at hyperthermal energies involve the scattering of sodium from a tungsten surface which was represented as a cluster. It was found the electron transfer probability was large as the atom approached the surface and that the transfer took place in a localized region a few Angstroms from the surface. There was also a large dependence of the transfer probability on the collision energy. Inelastic collisions were also investigated. The work function dependence of sputtering of alkalis from surfaces has been studied and agreement with experiment was obtained.
Future research involves diatomics and triatomics interacting with surfaces. Prototype studies are planned for homonuclear molecules and molecular ions and heteronuclear ions and molecules. It would be of special interest to determine the effect of the combination of the ion charge and oscillating dipole on the charge transfer process. Both scattering and sputtering studies are planned for these systems. Studies involving interactions with semiconductors characterized by a band gap and electronic excitations near surfaces are also planned.