INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION AND UNIMOLECULAR REACTION -CONCEPTS AND NEW RESULTS ON THE FEMTOSECOND DYNAMICS AND STATISTICS IN CHBRCLF

We present the conceptual background for intramolecular vibrational (r otational) redistribution (IVR) in polyatomic molecules and its analys is from high resolution spectra. We illustrate some of the basic conce pts relating to the use of effective and real molecular hamiltonians f or IVR with examples drawn from our previous spectroscopic investigati ons of the chiral molecule CHFClBr. The importance of the initial stat e for the dynamics is shown, demonstrating that simple intuitive conce pts of survival probabilities for some arbitrary chromophore (''zero o rder bright'') state are insufficient for a quantitative understanding of IVR. An approximate (C-s-like) symmetry prevents relaxation toward s quasi-equilibrium in highly excited C-1-asymmetrical CHFClBr. The co herent infrared multiphoton excitation of CHFClBr at the CH-stretching frequency leads to coherent vibrational states with a very wide energ y distribution corresponding to short time oscillatory behaviour of th e CH-stretching motion with a period of 11-12 fs. On longer time scale s between 50 and 500 fs substantial vibrational motion is found in the CH-bending coordinates. For modest excitation energies both the initi al vibrational angular momentum quantum number (l = 0) and a' symmetry of the wavepacket are well conserved for 1-2 ps. At higher excitation s and longer times (2 - 4 ps) these approximate symmetries are lost. T hese results are briefly discussed in relation to a general classifica tion of IVR mechanisms and in relation to reactive processes in CHFClB r.