Transient absorption spectroscopy stimulated emission

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

All these studies with femtosecond pulses on the primary photochemical processes of rhodopsin were done by means of transient absorption (pump probe) spectroscopy [10]. However, absorption spectroscopy may not be the best way to probe the excited-state dynamics of rhodopsin, because other spectral features, such as ground-state depletion and product absorption, are possibly superimposed on the excited-state spectral features (absorption and stimulated emission) in the obtained data. Each spectral feature may even vary in the femtosecond time domain, which provides further difficulty in analyzing the data. In contrast, fluorescence spectroscopy focuses only on the excited-state processes, so that the excited-state dynamics can be observed more directly. 

The general features of transient absorption spectroscopy on Rps. viridis are similar to those found previously for Rb. sphaeroides [8, 9]. The decay of the excited electronic state P of the special pair is seen at 1050 nm (Fig. 1). Here stimulated emission (gain) occurs which decays at later delay times with a time constant of approximately 3.5 ps. At early times the absorption changes more rapidly suggesting the existence of a faster kinetic component (See systematic deviations of data points from the model curve). This component is evident at measurements in the Qx and Qy absorption bands of the monomeric bacteriochlorophylls (Fig. 2a and b). Here a fast process with a time constant of 0.65 0.3 ps occurs within a narrow spectral range. The other components observed have time constants of 3.5 ps and 200 ps. 

Experimentally, the singlet states of free-base and zinc-porphyrins can be conveniently monitored not only by fluorescence spectroscopy but also by picosecond transient absorption (Fig. 24). By comparison with the absorp-tion/emission spectra of Fig. 22, it can be seen that the transient spectra consist of a broad featureless positive absorption throughout the visible region, with superimposed bleaching of the groimd-state Q-bands and additional apparent bleaching corresponding to stimulated fluorescent emission. As such. 

---