Radiationless transitions, photosynthetic

The next two chapters are devoted to ultrafast radiationless transitions. In Chapter 5, the generalized linear response theory is used to treat the non-equilibrium dynamics of molecular systems. This method, based on the density matrix method, can also be used to calculate the transient spectroscopic signals that are often monitored experimentally. As an application of the method, the authors present the study of the interfadal photo-induced electron transfer in dye-sensitized solar cell as observed by transient absorption spectroscopy. Chapter 6 uses the density matrix method to discuss important processes that occur in the bacterial photosynthetic reaction center, which has congested electronic structure within 200-1500cm 1 and weak interactions between these electronic states. Therefore, this biological system is an ideal system to examine theoretical models (memory effect, coherence effect, vibrational relaxation, etc.) and techniques (generalized linear response theory, Forster-Dexter theory, Marcus theory, internal conversion theory, etc.) for treating ultrafast radiationless transition phenomena. 

A microscopic theory for describing ultrafast radiationless transitions in particular for, photo-induced ultrafast radiationless transitions is presented. For this purpose, one example system that well represents the ultrafast radiationless transaction problem is considered. More specifically, bacterial photosynthetic reaction centers (RCs) are investigated for their ultrafast electronic-excitation energy transfer (EET) processes and ultrafast electron transfer (ET) processes. Several applications of the density matrix method are presented for emphasizing that the density matrix method can not only treat the dynamics due to the radiationless transitions but also deal with the population and coherence dynamics. Several rate constants of the radiationless transitions and the analytic estimation methods of those rate... 

Ultrafast radiationless transitions are often observed through ultrashort pump-probe time-resolved measurements. In this section, a theoretical formula of the pump-probe time-resolved stimulated emission spectra is briefly introduced and the relationship between the dynamics calculation and the pump-probe spectra are presented. For this purpose, the dynamics of a simple model system with vibrationally non-equilibrated is discussed. For a real application of the theoretical treatment given in this chapter, ultrafast charge transfer taking place in photosynthetic RCs is studied. 

The presented theoretical approach to large molecular systems such as bacterial photosynthetic RCs can provide microscopic details of ultrafast radiationless transition taking place faster than 100 fsec. In particular, this approach establishes a standard model for treating such ultrafast processes of RCs. It is possible to analyze and provide similar details for wild-type RCs or other mutant RCs for example, for wild-type RCs of Rb. sphaeroides the electronic coupling of radiationless transition from the B band to the higher excitonic band and that from the higher excitonic band to the lower one are found to be 105.5 and 123 cm-1. For R26.Phe-a mutant RCs, the former coupling is 105 cm-1 and the latter is 123.7 cm-1. 

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