Time-resolved absorption measurement

The transient absorption setup used in our experiments is a versatile tool providing both the necessary time resolution and the tunability to investigate different compounds. The pump-probe spectrometer is based on two noncollinearly phase matched optical parametric amplifiers (NOPAs) (see Fig. 11.3) [21, 22]. The NOPAs are pumped by a regenerative Ti sapphire laser amplifier delivering approximately 100 fs NIR pulses at around 800 nm with a repetition rate of 1 kHz. 

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Kochi and co-workers studied photoinduced Diels-Alder cycloadditions via direct photoexcitation of anthracene as a diene with maleic anhydride and various maleimides as dienophiles. Here, fluorescence-quenching experiments, time-resolved absorption measurements, and the effect of solvent polarity provide striking evidence for an ion-radical pair to be the decisive intermediate [83],... 

Figure 1. Experimental system used in the time resolved absorption measurements. (EL=excimer laser, KrF, 248nm DG=delay generator OMA=optical multichannel analyser MC=monochromator and gated diode array detector C=cell X=xenon flash lamp L=lenses )...

Most compounds exhibiting ESIPT experience subsequently a rather fast IC (see Section 11.1), which brings the molecule back to the electronic ground state. As discussed above, the ESIPT proceeds in all these molecules in a very similar manner. On the contrary, the time-resolved absorption measurements reveal that the lifetime of the first electronically excited state depends strongly on the molecule. 

Upon photoexcitation of the film of tmns-polyacetylene, it gives rise to two induced-absorption bands in the gap state, i.e., a high-energy (HE) band at 1.4 eV and a low-energy band at 0.5 eV Based on time-resolved absorption measurements in the femtosecond to millisecond time regime, as well as EPR (electron paramagnetic resonance) measurements, it is now well established that the elementary excitations which give rise to the HE band are neutral... 

The single crystals were placed in a cryostat and were kept at cryogenic temperature for the photo-induced and time-resolved absorption measurements. Both pump and probe beams polarized along the b-crystal axis were exposed onto the afc-plane of the crystals. As regards the photoinduced absorption measurements, we used an Ar -laser (Lexel, Model 95) and a CW dye laser (Spectra-Physics 375) for excitation. A probe beam was derived from a 40 W tungsten halogen lamp, dispersed by a monochromator (JASCO, CT-25C) in the spectral range of... 

Thus, time-resolved absorption measurements are often useful to initially characterize the kinetic characteristics of a reaction, but other spectroscopic methods may also be useful in probing more subtle or structure-specific mechanistic features. In the many cases in which one would like to obtain more information about the structural features of intermediates... 

Circular dichroism (CD), the differential absorption of left versus right cireularly polarized light, is the polarization spectroseopy perhaps best suited to detecting the presence of asymmetry in the structure or environment of moleeular ehromophores. Various time-resolved CD (TRCD) methods have been developed to take advantage of this sensitivity and obtain more detailed structural information about kinetic processes than is found from ordinary time-resolved absorption measurements [33]. Some examples of the processes studied with TRCD methods are the effeets of electronie excitation on the structure of chiral inorganic complexes, the ehanges in a-helical secondary... 

Figure 3, Representative time-resolved absorption measurement of a Cu(thd)2 aliquot passing the scanning Spectrafocus UV detector.

Most recently, the earlier generation of femtosecond time-resolved absorption measurements has been superseded by an extensive set of new experiments on Rb.sphaeroides (up to... 

The time-resolved absorption measurements were performed on Q -reconstituted reaction centers. The data obtained at the probing wavelength of 665 nm reveal the consequences of the mutations for the binding of the quinone and for the electron transfer kinetics in the reaction centers. 

Because the titrations indicated that it was possible to extract and reconstitute in all of the RCs, time-resolved absorption measurements of Lq were made on isolated RCs which either contained the native UQio oi which were depleted of UQ q reconstituted with exogenous menaquinone (MKq). Fig. 2 shows the difference spectrum (P" minus PHy ) measured at 40 25 ps after excitation at 600 nm. This spectrum of native wild-type RCs is representative of the various RC preparations presented here, since the essential features remain the same after quinone extraction or replacement of Trp 250 Xhe electron transfer rate Rq was measured by monitoring the recovery of the absorption band of at 545 nm (Reaction 1), which is bleached when is generated by the excitation pulse. Quinone-depleted wild-type RCs reconstituted with MKq have akQof(170ps)"lat 85K (Fig. 3, crosses). This is close to thevalueof... 

This example shows how time resolved absorption measurements result to be very informative on the photoreactivity of the supramolecular dyad and clearly... 

The phenomenology of ps-time resolved absorption measurements on RCs in external electric fields can be summarized as follows ... 

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