Light wavelength transients

Sx, Ti -> Tx). Figures 3.2 and 3.3 illustrate the principle of flash spectroscopy/65 If the second light source is continuous, the change in optical density due to the transient species can be monitored as a function of time at a particular wavelength selected on a monochromator. This type of system is illustrated in Figure 3.4. 

Triplet quantum yields for eosin and some other dyes have been determinded by Soep et al. The system to be studied is exposed to a dye laser pulse (with tunable wavelength) and the transient absorb-tion changes in the system due to triplet population are recorded with the light from a dc xenon lamp. 

Figure 3. Transient absorption spectrum of a 2 x 10 M solution i in butyronitrile at 100 ps following a 0.3 mJ, 0.5 ps, 600 nm laser flash. Filters that reject stray excitation light cut out the 580-620 nm wavelength region, while the sharp cutoff at 440 nm is due to the intense absorption of the porphyrin Soret band at 419 nm.

The spectrum obtained for the transients produced in a-methyl-styrene, between the wavelengths 315 to 480 m/a, is shown in Figure 5. The arrows indicate the two maxima previously reported by Hirota (13) for this monomer. The lower end of the spectrum has been corrected for light scattered within the monochromator. Each point on these spectra represents the maximum absorption (all species) at zero time. There appears to be little difference in the spectra of the dry and the wet monomer. 

Fig. 1. (a) Differential absorbance spectra of native PYP, after excitation at 430 nm, at different pump-probe delays. The scattered pump light around 430 nm has been masked. Steady-state absorption and fluorescence spectra are represented with dotted lines, (b) Kinetics extracted from the transient spectra at selected wavelengths... 

I and II. At very low temperatures a transient form photorhodopsin with a wavelength maximum at 580 nm may precede bathorhodopsin.461b,501-502a Furthermore, nanosecond photolysis of rhodopsin has revealed a blue-shifted intermediate that follows bathorhodopsin within 40 ns and decays into lumirhodopsin.500,503,504 The overall result is the light-induced isomerization of the bound 11-czs-retinal to all-fraus-retinal (Eq. 23-38) and free opsin. Tire free opsin can then combine with a new molecule of 11-czs-retinal to complete the photochemical cycle. 

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