Ultrafast relaxation dimers

A new area of energy-transfer research involves the use of strongly-coupled donor-acceptor moieties. Such systems are capable of displaying extremely fast excitation delocalization over several pigments. Similar studies have considered ultrafast energy relaxation in closely-coupled porphyrin dimers. 

Pump-probe experiment is an efficient approach to detect the ultrafast processes of molecules, clusters, and dense media. The dynamics of population and coherence of the system can be theoretically described using density matrix method. In this chapter, for ultrafast processes, we choose to investigate the effect of conical intersection (Cl) on internal conversion (IC) and the theory and numerical calculations of intramolecular vibrational relaxation (IVR). Since the 1970s, the theories of vibrational relaxation have been widely studied [1-7], Until recently, the quantum chemical calculations of anharmonic coefficients of potential-energy surfaces (PESs) have become available [8-10]. In this chapter, we shall use the water dimer (H20)2 and aniline as examples to demonstrate how to apply the adiabatic approximation to calculate the rates of vibrational relaxation. 

The experiments presented here, require in some cases sophisticated theoretical approaches. The calculations of the wave packet propagation (Sect. 2.2.1) observed for excited dimers and trimers (Chap. 3) are of this special kind. The photodissociation phenomena (Chap. 4), however, can be treated by two rather easy approaches, as described in Sect. 2.2.2. The essentials of thefirst theoretical approaches to interpretion of the results on ultrafast structural relaxation (Chap. 5) obtained by the NeNePo technique are summarized in Sect. 2.2.3. 

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