Laser flash photolysis absorbance change

In practice, the composite rate constant is corrected for the independently-measured rate constant for the probe reaction. The precision improves when the kinetic contribution from the kx path is as large as possible while still retaining an acceptable absorbance change provided by the probe reaction. This approach is used widely in laser-flash photolysis studies of radical reactions, which are described in detail in one of the following chapters. 

Laser flash photolysis time-resolved spectrophotometry, utilizing deazariboflavin-EDTA as a photochemical reductant, has been used with this system in order to characterize the initial step in the ET mechanism. Figure 3 shows examples of the type of data obtained in these studies. In the top panel, a transient is shown [54] that was obtained at 507 nm in 100 mM phosphate buffer, pH 7.0, containing 35 pM Fd, and in the middle panel, 10.3 pM FNR has been added to the solution prior to photolysis. This wavelength corresponds to an isosbestic point for the FAD cofactor of the reductase, and thus the absorbance change monitors the oxidation state of the [2Fe-2S] cluster of Fd (and also the formation and decay of the dRfH species). As is evident, immediately after the laser flash there is a rapid rise in absorbance due to dRfH formation. This is followed by a sharp absorbance decrease corresponding to Fd reduction and dRfH oxidation. The subsequent slow increase in absorption shown in the middle panel is a consequence of Fd reoxidation that is due to electron transfer to FNR. The latter is confirmed by measurement at 610 nm (bottom panel), a wavelength which monitors FAD neutral semiquinone formation the rate constant obtained from the 610 nm absorbance rise is the same as that obtained from the slow absorbance increase at 507 nm, consistent with this interpretation. 

In highly scattering materials, absorbance is not directly observable, and changes in diffnse reflectance after the laser flash are monitored instead. Wilkinson and Kelly have described details of the techniqne [157]. Hurrell et al. [151] used diffuse-reflectance laser-flash photolysis to stndy the behavior of several... 

Fig. 12. Transient IR difference spectra showing changes in absorbance (a) 5 / seconds, (b) 25 seconds, and (c) 1.25 mseconds after the UV flash photolysis of [CpFe(CO)2]2 in cyclohexane solution under 1 atm pressure of CO. Bands pointing upward represent an increase in absorbance (i.e., formation of a compound) and those pointing downward a decrease [i.e., depletion of starting material, (A)]. The bands are assigned as follows A, [CpFe(CO)2]2 B, CpFe(CO)2 and C, CpFe(p.-CO)3Fe(Cp). Points marked were recorded with a 12CO laser and those marked + with a 13CO laser. [Reproduced with permission from Moore et al. (61).]...

Fig. 15. Schematic representation of the time-resolved infrared (TRIR) flash photolysis apparatus used at Nottingham. The UV pulse laser generates transient species the continuous IR laser monitors the change in transmission at a particular IR frequency, producing a trace showing the IR absorbance as a function of time. The experiment is repeated at different IR frequencies so that a complete IR spectrum of the transient can be built up [reproduced with permission from (97), p. 103],...

In order to detect the intensity change of one mode in the presence of many others, the laser output has to be dispersed by a monochromator or an interferometer. The absorbing molecules may have many absorption lines within the broad-band gain profile of a multimode dye laser. Those laser modes which overlap with absorption lines are attenuated or even completely quenched. This results in "spectral holes" in the output spectrum of the laser and allows the sensitive simultaneous recording of the whole absorption spectrum within the laser bandwidth, if the laser output is photographically recorded behind a spectrograph. ATKINSON et al. [8.17] demonstrated this technique by monitoring with a flash lamp-pumped dye laser the time-dependent concentration of the short-lived radicals NH2 and HCO which had been formed by flash photolysis of NH. ... 

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