Donald Secrest
Emeritus Professor of Chemistry
Professor Secrest received a B.S. from the University of Akron in 1955 and a Ph.D. in chemistry from the University of Wisconsin in 1961.
Research
As experimental dynamics studies are providing ever more detailed information by means of state selected and molecular beam techniques, spectra of van der Waal's molecules and LASER state selection, the theoretical study of molecular and atomic collision processes is becoming increasingly important. In addition, satellite data have recently raised questions about collision processes, particularly in dark interstellar clouds, taking place under conditions which cannot be met in the laboratory. It has been proposed that many molecules observed in such clouds are formed by ion molecule reactions at extremely low energies. At present only a theoretical solution to these problems is possible.
Solution of quantum mechanical, scattering problems involves the solution of many-dimensional partial differential equations of a particularly complex form. This type of equation has not been thoroughly studied by the mathematicians, and much basic work along this line remains to be done. A straightforward approach, even on the largest computers available, is doomed to failure. Clever numerical techniques must be developed and powerful mathematical techniques, such as the use of Green's operators must be employed.
For very large systems one must resort to approximate methods. It is difficult to make valid approximations without some detailed understanding of the dynamic phenomenon. Thus, it is of great importance that exact solutions be obtained for model problems exhibiting different characteristics of interest. These exact results are used in testing the powerful new approximations we are developing that will allow the solution of complex collision problems quickly and easily.
Some problems under investigation include rotational and vibrational energy transfer of both polyatomic and diatomic molecules in molecular beam scattering, predissociation states and scattering resonances associated with "bound states" in the continuum and orbiting encounters.
In conjunction with our work on resonances we have begun studying the dynamics of van der Waal's molecules. These molecules have weak and flexible bonds and their dynamics are poorly treated by the techniques used to study more conventional molecules. Both molecules in highly excited states, and van der Waal's molecules exhibit delocalized behavior which appears to be associated with classical ergodic motion. Such states have been related to chaos. We are in early stages of investigating this phenomenon.
We have developed a form of the Hamiltonian for polyatomic molecules which can be easily and accurately solved for molecules which undergo large displacements during vibrations. This technique is ideal for treating highly excited states of molecules and is well suited to studying van der Waal's molecules. This approach is also useful for studying collisions in which the molecule is greatly distorted.
The availability of massively parallel computers are making it possible to study chemical reactions with the hope of being able to make useful predictive calculations for the first time. We have started to develop techniques appropriate to these machines.
Molecular beam experiments are a potentially powerful means of studying molecular interactions and chemical reactions, but their full promise will only be realized when we are able to understand the scattering phenomenon theoretically. This theory is still in its infancy and there are may simple problems which have not yet been attacked.
Publications
"Reorientation Differential Cross Sections at High Energy," Michael Sisak and Don Secrest, J. Chem. Phys., 96, 230 (1992).
"Numerical Evaluation of Kratzer Oscillator Matrix Elements," Robert E. Tuzun and Don Secrest, Comp. Phys. Comm., 70, 362 (1992).
"Accurate Computation of Individual and Tables of 3-j and 6-j Symbols," Robert E. Tuzun, Paul Burkhardt and Don Secrest, Comp. Phys. Com., 112, 112 (1998).
