Internal vibrational redistribution, IVR

It is very likely that the metal-insulator transition, the unusual catalytic properties, the unusual degree of chemical reactivity, and perhaps even some of the ultramagnetic properties of metal clusters are all linked intimately with the dynamic, vibronic processes inherent in these systems. Consequently, the combination of pump-probe spectroscopy on the femtosecond time scale with theoretical calculations of wavepacket propagation on just this scale offers a tantalizing way to address this class of problems [5]. Here we describe the application of these methods to several kinds of metal clusters with applications to some specific, typical systems first, to the simplest examples of unperturbed dimers then, to trimers, in which internal vibrational redistribution (IVR) starts to play a central role and finally, to larger clusters, where dissociative processes become dominant. 

The word relaxation is here used in the sense of a system whose internal thermal equilibrinm has been destroyed by a flash returning to a state of equilibrium, but not necessarily to the same state as before the perturbation. For an account of internal vibrational redistribution (IVR) in ground-state molecules, see Ref. [49,e]. 

Fig. 3. Excitation of vibrational modes due to different reaction channels. Concerted double proton transfer leads to a symmetric stretching vibration and symmetry breaking single proton transfer to an antisymmetric bending motion. Damping of the vibrational motion by internal vibrational redistribution is indicated by IVR .

Fig. 1.2. Schematic illustration of electronic (a) and vibrational (b) predissociation. In the first case, the molecule undergoes a radiationless transition (rt) from the binding to the repulsive state and subsequently decays. In the second case, the photon creates a quasi-bound state in the potential well which decays either by tunneling (tn) or by internal energy redistribution (IVR).

The language of bright state and dark state is central to population quantum beats and also to the related polyatomic molecule radiationless decay processes (Bixon and Jortner, 1968 Rhodes, 1983), Intramolecular Vibrational Redistribution (IVR) (Parmenter, 1983 Nesbitt and Field, 1996 Wong and Gruebele, 1999 Keske and Pate, 2000), Inter-System Crossing (ISC), and Internal Conversion (IC), discussed in Section 9.4.15. 

This two-state quantum beat example is identical to the doorway mediated non-radiative decay problem frequently encountered in polyatomic molecule Intramolecular Vibrational Redistribution (IVR), Inter-System Crossing (ISC), Internal Conversion (IC), and compound anticrossings. There is a single, narrow bright state. It couples to a single, broad, and dark doorway state. The width of the doorway state is determined by the rate of its Fermi Golden Rule decay into a quasi-continuum of dark states. 

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