Interpretation of Pump-Probe Experiments

The transmission difference spectrum AT/T, i.e., the normalized difference between T for the excited state and T for the ground state contains only three types of signal, as sketched in Fig. 2.4  

PB is the reduction of optical density in the region of ground state absorption and corresponds to positive AT (increased transmission). It stems from 

Finally, PA is associated with negative AT and is due to optical transitions from newly occupied states (following photoexcitation) to higher-lying levels, such as 2 — 3 or 4 — 5. PA bands axe very often asymmetric towards high energy due to underlying vibrational structure which is seldom resolved [87]. 

In conjugated polymers and oligomers, the behavior of photoexcited states obeys general rules of molecular photophysics, such as the Kasha [88] and Vavilov [77] rules. These are empirical observations, rather than exact statements or laws. However, they provide a useful basis for discussion. The Kasha rule states that fluorescence occurs from the lower-lying excited state, independently of the excitation energy. The Vavilov rule states that the fluo- 

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