Dr. M. Ben-Nun
Department of Chemistry
University of Illinois at U-C
Urbana-Champaign, IL

Thursday, September 18, 1997
2:00 pm (CT)
3269 Beckman Institute

Abstract

The majority of dynamical studies in the literature assume that the dynamics takes place on the ground electronic state alone. If the ground electronic state is well separated from low-lying excited states, this assumption is completely justified. However, this is often not the case. Evidence is mounting that conical intersections and avoided crossings are ubiquitous in polyatomic molecules, particularly so in the excited state manifold. This has profound implications for photochemistry, where the interesting dynamics begins on an electronically excited state. The recently introduced Full Multiple Spawning (FMS) method for molecular dynamics beyond the Born-Oppenheimer approximation will be presented. The method retains the computational simplicity and familiarity of classical mechanics while incorporating enough quantum mechanics to provide a clear meaning to the concept of coupled electronic states. The quenching of a Na (3p 2P) atom by H2 due to a conical intersection of two potential energy surfaces will be used as a computational example since it illustrates many of the features of the method. As seen by H2, the p orbital of the electronically excited Na atom is like a quadrupole, which during the collsiion may, or may not, align with the molecular axis. Quantum mechanically, variations in the alignment of the orbital represent changes in the electronic state of the system, so that dynamical methods which allow for such interstate transitions must be used.

Tea and coffee will be served in R3151 Beckman Institute at 2:15pm.