Ultrafast electronic-excitation energy

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... 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

The photochemical ligand substitution reaction of la was investigated by ultrafast TR-IR spectroscopy (Fig. 16) 51). An acetonitrile solution of la was irradiated by a 266-nm laser pulse ( 150 fs pulse width). A broad IR absorption band which was attributed to the reaction products in higher vibrational excited states was produced within 1 ps after the laser flash. The broad band sharpened and a vqo peak at 1828 cm of the reaction product was observed in the 50- to 100-ps duration. This time scale is much shorter than the decay of the lowest MLCT excited state (right-hand side of Fig. 16). The TR-IR results indicate that this photochemical reaction proceeds from higher vibrational states or high-energy electronic excited states instead of the lower vibrational excited states of MLCT and thermal accessible states from MLCT such as the LF state. 

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