Internal vibrational redistribution processes

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 short time scale dynamics have been studied by means of femtosecond fluorescence upconversion. For all dendrimers these measurements revealed size-independent kinetic processes related to an internal vibrational redistribution, a vibrational/solvent relaxation. Singlet-singlet annihilation, only present in the multichromophoric compounds, was established by an excitation energy-dependent study. It has been shown that this type of process contributes to a larger extent in the para-substituted dendrimers compared to the meta-substituted ones. These differences between the meta- and para-substituted dendrimers... 

Although for the molecules in Fig. 4 gissociative attachment processes are expected to be fast (<<10 sec), for C2C1 at 0.0 eV (and for a number of other autodissociating long-lived parent negative ions) both the autodissociation and the autodetachment processes are slow, as a result of vibrational redistribution of internal energy ( ). 

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. 

The width of an absorption band for an individual vibronic transition depends on how long the excited molecule remains in the state created by the excitation. According to Eq. (2.70), the spectrum for excitation to a state that decays exponentially with time should be a Lorentzian function of frequency. The shorter the lifetime of the excited state, the broader the Lorentzian (Fig. 2.12). A variety of processes can cause an excited molecule to evolve with time, and thus can broaden the absorption line. The molecule might, for example, decay to another vibrational state by redistributing energy among its internal vibrational modes or by releasing... 

In studies of molecular dynamics, lasers of very short pulse lengths allow investigation by laser-induced fluorescence of chemical processes that occur in a picosecond time frame. This time period is much less than the lifetimes of any transient species that could last long enough to yield a measurable vibrational spectrum. Such measurements go beyond simple detection and characterization of transient species. They yield details never before available of the time behavior of species in fast reactions, such as temporal and spatial redistribution of initially localized energy in excited molecules. Laser-induced fluorescence characterizes the molecular species that have formed, their internal energy distributions, and their lifetimes. 

Statistical theories treat the decomposition of the reaction complex of ion-molecule interactions in an analogous manner to that employed for unimolecular decomposition reactions.466 One approach is that taken by the quasiequilibrium theory (QET).467 Its basic assumptions are (1) the rate of dissociation of the ion is slow relative to the rate of redistribution of energy among the internal degrees of freedom, both electronic and vibrational, of the ion and (2) each dissociation process may be described as a motion along a reaction coordinate separable from all other internal... 

Vibrational Predissociation, in this section we discuss the case of a transition from a predissociative state to the photofragment state that occurs on a single adiabatic pes. Such processes cannot occur for diatomic molecules, but they can be observed for polyatomic systems. The transition is caused by intramolecular energy transfer, that is, by internal redistribution of vibrational energy. 

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