Molecular Dynamics Beyond the Adiabatic Approximation: New Experiments and Theory

Abstract

At the heart of the quantum mechanical description of molecular structure and dynamics lies the adiabatic or Born-Oppenheimer approximation (1 , 2). Starting with an assumption of separability of time scales for nuclear and electronic motion, a familiar picture emerges of nuclei subject to well­ defined forces corresponding to the potential energy surface for a particular electronic state. Although the well-established success of these ideas in the areas of molecular spectroscopy (3-5) and reaction dynamics (6-9) is likely to secure the adiabatic approximation as a continuing foundation of molecular science, the range of dynamical processes that lies within its scope is far from complete. Recent experimental and theoretical advances in particular are beginning to yield a coherent understanding of several phenomena that, far from requiring minor corrections to the adiabatic approximation for their explanation, by their very nature exist entirely outside its framework. Examples of importance in diverse areas of chemistry and physics range from the dynamics of radiationless decay (1015) and nonadiabatic processes in chemical reactions (16-19) to the spectroscopy of excited (20-23) and ionized (24-26) states of isolated molecules, from single collision electronic energy transfer (27--41) to exciton (42) and soliton (43) dynamics and spin-lattice relaxation (44). Specialized reviews on these topics are available.