Intramolecular energy redistribution

Figure A3.13.1 illustrates our general understanding of intramolecular energy redistribution in isolated molecules and shows how these processes are related to intemiolecular processes, which may follow any of the mechanisms discussed in the previous section. The horizontal bars represent levels of nearly degenerate states of an isolated molecule.

A 3.13.6 STATISTICAL MECHANICAL MASTER EQUATION TREATMENT OF INTRAMOLECULAR ENERGY REDISTRIBUTION IN REACTIVE MOLECULES... 

The realization of SPODS via PL, that is, impulsive excitation and discrete temporal phase variations, benefits from high peak intensities inherent to short laser pulses. In view of multistate excitation scenarios, this enables highly efficient population transfer to the target states (see Section 6.3.3). Furthermore, PL can be implemented on very short timescales, which is desirable in order to outperform rapid intramolecular energy redistribution or decoherence processes. On the other hand, since PL is an impulsive scenario, it is sensitive to pulse parameters such as detuning and intensity [44]. A robust realization of SPODS is achieved by the use of adiabatic techniques. The underlying physical mechanism will be discussed next. 

Lifshitz, C. Intramolecular energy redistribution in polyatomic ions, J. Phys. Chem. 1983, 87, 2304-2313. 

The development of the picosecond-jet technique is presented. The applications of the technique to the studies of coherence (quantum beats), photodissociation, isomerization and partial solvation of molecules in supersonic-jet beams are detailed with emphasis on the role of intramolecular energy redistribution. Experimental evidence for intramolecular threshold effect for rates as a function of excess molecular energy is given and explained using simple theory for the redistribution of energy among certain modes. Comparison with R.R.K.M. calculation is also made to assess the nature of the statistical behaviour of the energy redistribution. 

Figure 2.12 illustrates the potential energy profile for a unimolecular radical decomposition. Bimolecular collisions with a bath gas can activate the molecule so that it contains enough energy to react. However, for reaction to occur this energy must be located in the bond to be broken and hence the dissociation is not instantaneous. During the intramolecular energy redistribution further collisions can occur with the bath gas which will deactivate the molecule. The simple treatment proposed by Linde-... 

Bemshtein, V. and Oref, 1. (1999) Intramolecular energy redistribution in Cgo following high energy collisions with Lt, Chem. Phys. Lett. 313. 52-56. 

T o end this section we note again that the assumption of fast intramolecular energy redistribution in the reactant well introduced the number n of molecular modes as an important parameter of the theory. In a broader context n is taken to be the number of strongly coupled molecular modes, and it is assumed that the reaction coordinate is part of this set. In liquid solvents n is exp>ected to be equal to the total number 3AT — 7 of modes. In the low-pressure gas phase n can be smaller, and the possible slow energy transfer between different... 

The title of this chapter seems to promise a general discussion of the nature of collision-induced intramolecular energy transfer in electronically excited polyatomic molecules. If interpreted as just stated, the title promises more than can be delivered at this time. It is only recently that advances in experimental technique have permitted the study of the pathways of intramolecular energy redistribution following collision, and the few results now available were neither anticipated nor can they yet be fully accounted for by the available theories of collision-induced energy transfer. This chapter describes a preliminary synthesis of the limited experimental and theoretical information in hand and discusses some of its implications. It will be seen that more questions are raised than are answered. 

The bond-selective control of a chemical reaction has been a longstanding goal of modern chemical physics. Early attempts using selective laser excitation were thwarted by fast intramolecular energy redistribution. Now, ultrafast laser pulses, optical pulse shaping, and feedback algorithms have been successfidly combined in a number of laboratories to control bond dissociation reactions in simple isolated molecules (see also Chapter 19). 

Particularly interesting, in view of the application to solar cells, is the competition between inter- and intramolecular processes. Thus, the dye RuN719 has been found to fluoresce with a very short lifetime in solution (<30 fs), non-changing when absorbed on substrates, whether electron injection occurred (titania) or it didn t (alumina). It was thus concluded that intramolecular energy redistribution occurred within 10 fs (Scheme 7.15). Thus, injection likewise had to occur on the < 10 fs timescale and involved non-thermalized states of the dye. Ultrafast processes are of great significance for dye-sensitized solar cells [32]. 

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