Ultrafast radiationless transitions

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. 

In this chapter, the ultrafast radiationless transition processes are treated theoretically. The method employed is based on the density matrix method, and specifically, a generalized linear response theory is developed by applying the projection operator technique on the Liouville equation so that non-equilibrium cases can be handled properly. The ultrafast molecular... 

Density Matrix Treatments of Ultrafast Radiationless Transitions... 

Density Matrix Treatments of Ultrafast Radiationless Transitions 129 can be shown by noticing that... 

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... 

In Eq. (4), one can see that there are two terms that depend on the space-phase factor. Eq. (5) clearly shows this situation coherence is generated between the two molecules. This coherent excitation can be called as inter-molecular coherence. This type of coherent excitation can be seen in femtosecond pump-probe non-linear coherent optical scattering spectroscopy. This chapter shall only focus on ultrafast radiationless transition as well as the dynamics of the intramolecular coherence and its relaxation. 

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. 

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