Picosecond Dynamics of Intramolecular Vibrational-Energy Redistribution (IVR) in Jet-Cooled Molecules

Theoretical and experimental results concerning the dynamical manifestations of intramolecular vibrational-energy redistribution (IVR) in the temporally resolved fluorescence of jet-cooled molecules are presented. A first concern is with the derivation of the characteristics of the beat-modulated fluorescence decays which arise from multilevel vibrational coupling within a molecule. Relations connecting quantum beat frequencies, phases, and modulation depths to coupling parameters are presented. Likely sources of deviations of experimental results from theoretical predictions are considered. And, the direct link between IVR and time-resolved fluorescence experiments is discussed with emphasis placed on the physical interpretation of vibrational quantum beats and the nature of IVR as a function of vibrational energy in a molecule. Secondly, the results of picosecond time-resolved measurements of IVR on jet-cooled anthracene at different excess energies are reported and analyzed. The nature of IVR as a function of vibrational energy, the relevant timescales for the process, and the details of pertinent vibrational couplings are determined. Thirdly, experimental and theoretical results pertaining to the role of molecular rotations in IVR are presented. Finally, experimental results on jet-cooled t-stilbene are reported and analyzed with the aim of assessing the generality of our theoretical treatment and the anthracene results.