A MULTIDIMENSIONAL LANDAU-ZENER DESCRIPTION OF CHEMICAL-REACTION DYNAMICS AND VIBRATIONAL COHERENCE

We discuss the observation of coherent nuclear oscillations in chemica l and biological systems subsequent to excitation with an ultrashort l aser pulse. The experimentally observed coherent signals arising from rapid, photon initiated, chemical reactions are considered separately from vibrational coherences directly excited by the (electric) field i nterventions of the Light pulse. In order to treat the chemical reacti on, a modified Landau-Zener theory is developed that is quasi-classica l along the traditional reaction coordinate but fully quantum mechanic al in all other degrees of freedom. In this work, we explore explicitl y the effects of electronic state coupling, electron-nuclear coupling, reaction velocity, vibrational damping and temperature on the product electronic state population development and on the coherent oscillati ons of the nuclear coordinates. Recent experimental studies of the lig and photolysis reaction in heme proteins are presented as examples. (C ) 1997 American Institute of Physics.