Transient absorption decay

At 77 K in EPA, lifetimes of 60 10 ms for 1-nitro- and 242 10 ms for 2-nitronaphthalene have been obtained from analysis of the transient absorption decay curves >. Differences to phosphorescence lifetimes under the same condi-... 

Using laser flash photolysis with a frequency-quadrupled neodynium laser, Stevens and al 161b) measured the lifetime of the triplet state of fluoro- and pentafluoro-benzene in the gas phase along with the energy transfer efficiencies to cis-2-butane and oxygen. The triplet transient absorption decay was found to be predominantly first order with a... 

Recently, it was shown that transient absorption decay for hematite nanoparticles was very fast, 70% of the transient absorption disappeared within 8 ps and no measurable transient absorption remained beyond 100 ps [43]. This represented a much faster decay than many other semiconductors, which is consistent with the observed poor charge transfer properties in hematite. It should be mentioned that this decay was independent of the excitation power, which suggests alternative relaxation mechanisms compared to those observed for Ti02 and ZnO for instance [43]. Since the relaxation was independent of pump power, probe wavelength, pH and surface treatment the fast decay was interpreted to be due to intrinsic mid-bandgap states and trap states rather than surface defects. This is in agreement with earlier investigations [44]. 

FIGURE 17 Dependence of transient absorption decay at 600 nm on platinum loading for Pt-loaded Ti02 powder, with 390 nm excitation. (Reproduced with permission from Furube et al. (2001a).)... 

As an example, photochemical excitation of donor-acceptor complexes may be considered. Irradiaiion into the CT band of the anthracene-tetracyano-ethylene complex leads directly to the radical ion pair, the components of which are identifiable from their UV-visible spectra. The transient absorptions decay in 60 ps after excitation, as the radical ion pairs undergo rapid back electron transfer to afford the original donor-acceptor complex (Hilinski et al., 1984). With tetranitromethane as acceptor, however, an addition product is obtained in both high quantum and chemical yield. This is due to the fact that the tetranitromethane radical anion undergoes spontaneous fragmentation lo a NO, radical and a trinitromethyl anion, which is not able to reduce the anthracene radical cation (Masnovi et al., 1985) ... 

Figure 8. Typical transient absorption decays at 970 nm of the carotenoid radical cations of C-P-Q triads 2 and 4-6. The decays were measured in dichloromethane solution following excitation with a 590-nm laser pulse. The relative AA values reflect the relative quantum yields of C -P-Q .

FIGURE 5 - Transient absorption decay from a microcrystalline sample of 1,5-diphenyl-3-styryi-2-pyrazoline, using pump and probe wavelengths of 295 and 590 nm, respectively... 

Figure 3. Transient-absorption decay kinetics for a 4 pm thick transparent mesoporous film sensitized with AR25. The solid line corresponds to the fitting to a stretched exponential function AO. D. = exp[-(t/T) ]. The excitation wavelength was = 535 nm and the probe wavelength was Ap, = 800 nm.

Figure 7a presents the four traces needed to obtain a corrected transient absorption decay at a specific wavelength baseline, top-line, emission, and absorption. By taking into account the entire wavelength range where the excited species absorbs, and also by a suitable choice of the time scale, triplet-triplet time resolved absorption spectra can be obtained, such as the one presented in Figure 7b for benzophenone crystals. These spectra provide both spectroscopic and kinetic information regarding the powdered opaque sample. It is obvious that these spectra will enable one to study the occurrence of a chemical reaction on surfaces, pro-... 

The ideal excitation source would provide a pulse of light that drops instantaneously from a high value to zero. Usually, however, the source has its own intensity profile, and the decay of very short-lived transients may be difficult to measure unless the effects of source decay are separated from the effects of transient absorption decay through a technique known as... 

The transient absorption decay in these PS2 reaction centre particles is biexponential at 526nm. This is most clearly shown under anaerobic conditions where the 1ms P680 triplet decay is preceded by a microsecond component. The transient absorption spectrum of this microsecond component is shown in figure 1b. The peak at (526 4)nm is characteristic of the spectrum for the carotenoid triplet state. This carotenoid triplet state has a lifetime of (10 1) is under anaerobic conditions, which shortens to (4 1))xs under aerobic conditions. The yield of this state is 2-3% and is not significantly reduced by photodamage. 

FIGURE 18.7 Normalized transient absorption decay profiles at 900nm after 400nm excitation for (a) Cat sensitized SNP TiOj ( ) in microemulsion and (b) Cat sensitized NP Ti02 (O) in water. The solid lines are best fits to the data. Inset Same kinetic decay trace is plotted in a shorter timescale. 

Immediately following excitation of 17 in DMF with a low intensity 30-ps laser pulse at 532 nm the observed transient differential absorption spectrum was considered to be characteristic of a gold(III) porphyrin triplet excited state [17]. The transient absorption decayed via biexponential kinetics with lifetimes of (170 20) ps and (2.0 0.4) ns, even at very low laser intensities. Absorption spectra recorded at different times during the decay process indicated that the triplet state was reduced to the gold(III) porphyrin neutral radical. With increasing laser intensity, the fractional contribution of the shorter-lived component increased and, whereas the longer lifetime... 

According to Yu et al, the temporal profile of the transient absorption of ZnTPP (Xjx = 397 nm and probe wavelength ranges from 571 to 702 nm) displays at most two decay times (1.5 ps and 12 ps) in benzene [124], This differs markedly from Enescu and coworkers measurement of the transient absorption decay of ZnTPP in ethanol = 404 nm and Ap e = 430 and 470 nm), which exhibits an ultrafast component of 150 fs and a slower component of 2 ps [126], The former was assigned to IVR within the Sj state and the latter to fluorescence from the relaxed Soret state. The contribution of the Sj state to the 150 fs time-constant requires further investigation. 

The 2 lifetime of water-soluble ZnTPPS (TPPS meso-tetrasulfonatophenyl porphyrin) was previously determined to be 1.3-1.4 ps [120,129], However, the Sj lifetime for water-soluble ZnTMPyP(4) (TMPyP(4) tetrakis(N-methyl-4-pyridyl)-porphyrin) is much shorter (< 100 fs) [126]. Its transient absorption reveals that following rapid Sj-Sj internal conversion, IVR occurs within the Sj state in ca. 150 fs. A 3 ps time constant present in the transient absorption decay was attributed to either an excited state conformational change or EVR. 

Transient absorption decay profiles in IR region of a gold/Ti02 film as a function of the Ti02 nanoparticle diameter. 

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