BERRY'S PHASE AND TOPOLOGICAL EFFECTS IN MOLECULES.

Summary form only given, as follows. Berry's prediction of an additional phase acquired during adiabatic evolution of quantum systems has recently attracted a great deal of attention. In molecules, adiabaticity is very well fulfilled for electron states subject to time-varying internuclear coordinates, and molecules exhibiting Berry's phase gamma equals pi should be ubiquitous. The existence of Berry's phase has been directly confirmed in the enrgy level spectra of Na//3, recorded by resonant two-photon ionization of a jet-cooled metal cluster beam. A precise determination of the phase's value may be accomplished via picosecond laser excitation. A simple interpretation of systems exhibiting Berry's phase in the absence of magnetic fields is given, and it is shown that it belongs to the class of topological effects (for example, chirality and topological connectivity) in molecular systems.