Roberto Marquardt

Universite de Marne-la-Vallee, 5 Bd Descartes, 77454, Champs-sur-Marne, FRANCE

Martin Quack and Ioannis Thanopulos

Laboratorium fuer Physikalische Chemie ETH-Zurich (Zentrum), CH-8092 Zurich, SWITZERLAND

 We investigate the time dependent quantum dynamics of the CH chromophore by calculation of the wave packet evolution during and after coherent excitation of the bending modes of the methane isotopomers CHD₃, CHD₂T and CHDT₂ [1]. For this purpose realistic potential energy surface [2] and electric dipole moment functions [3] are used, which have been derived previously from ab initio calculations and high resolution spectroscopic information. The results are obtained in two steps: firstly, the vibrational spectrum and eigenfunctions for the field-free molecular Hamiltonian are calculated with a discrete variable representation (DVR) technique; then the time dependent Schroedinger equation for the molecule in interaction with the external electromagnetic field is solved in the basis of the molecular eigenstates within the quasiresonant approximation (QRA) [4]. Different excitation pathways depending on the bending direction in an internal coordinate frame, the role of semiclassical and delocalized intramolecular vibrational redistribution [5] on these processes and the possibility of controlling the dynamics by localization of the wave packet motion in subspaces of the relevant configuration space are discussed. The excitation of a bending mode is further used to generate dynamical chirality [6], which can be quantified by the enantiomeric excess. The free evolution of the wave packet after generation of a chiral molecular structure corresponds to a stereomutation reaction on the femtosecond time scale superimposed by a racemization reaction, which is understood to arise from quantum delocalization effects due to intramolecular vibrational redistribution.

[1] R. Marquardt, M. Quack, and I. Thanopulos, J. Phys. Chem., in press (2000).
[2] R. Marquardt and M. Quack, J. Chem. Phys., 109, 10628-10643 (1998).
[3] H. Hollenstein, R. Marquardt, M. Quack, and M. A. Suhm, J. Chem. Phys., 101, 3588-3602 (1994).
[4] M. Quack. Chapter "Multiphoton excitation". In "Encyclopedia of Computational Chemistry",  P. v. Rague Schleyer, N. Allinger, T.  Clark, J. Gasteiger, P. A. Kollman, H. F. Schaefer III, and P. R. Schreiner, editors, Vol 3, pages 1775-1791. John Wiley and Sons, (1998).
[5] M. Quack, J. Mol. Struct., 292, 171-195 (1993).
[6] M. Quack, Angew. Chem. Int. Ed. Engl., 28, 571-586 (1989).