Internal vibrational redistribution

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 .

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... 

Nordholm S 1989 Photoisomerization of stilbene—a theoretical study of deuteration shifts and limited internal vibrational redistribution Chem. Phys. 137 109-20... 

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. 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).

By inelastic resonances we mean resonances which decay via energy redistribution between the internal vibrational-rotational modes or a transition from a quasi-bound to a continuum state. Elastic resonances, on the other hand, decay via tunneling through a potential barrier without the necessity of internal transitions. 

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 ( ). 

Because of their sensitivity to small redistributions of electron density, the computation of the intensities of vibrational modes has proven to be more demanding than the frequencies. Table 3.5 reports calculations of the intensities at the SCF and correlated levels. The intensity of the internal vibration of the proton-acceptor molecule is changed only little by the perturbation, but that of the donor undergoes a large increase by a factor of three or four. The latter intensification is characteristic of H-bonds and will be seen repeatedly. 

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