Absorption spectra evolution with time

The transient Ag3 + ion have an intense absorption spectrum with two maxima, at 310 and at 265 nm. Its second-order decay leads to the cluster Ag4 +. Under total reduction conditions the neutral dimer Ag2 is observed at 275 and 310 nm. The optical transitions of low-nuclearity silver oligomers, the rate constants, and the extinction coefficients are derived from adjustment between experimental (Fig. 2, bottom) and calculated absorption spectrum evolution. An even number of atoms favors the high stability of the magic hydrated clusters Ag4 + (275 nm), Agg + and possibly Agi4 + (Fig. 2, top). After a longer time, the plasmon... 

The first subnanosecond experiments on the eh yield were performed at Toronto (Hunt et al., 1973 Wolff et al., 1973). These were followed by the subnanosecond work of Jonah et al. (1976) and the subpicosecond works of Migus et al. (1987) and of Lu et al. (1989). Summarizing, we may note the following (1) the initial (-100 ps) yield of the hydrated electron is 4.6 0.2, which, together with the yield of 0.8 for dry neutralization, gives the total ionization yield in liquid water as 5.4 (2) there is -17% decay of the eh yield at 3 ns, of which about half occurs at 700 ps and (3) there is a relatively fast decay of the yield between 1 and 10 ns. Of these, items (1) and (3) are consistent with the Schwarz form of the diffusion model, but item (2) is not. In the time scale of 0.1-10 ns, the experimental yield is consistently greater than the calculated value. The subpicosecond experiments corroborated this finding and determined the evolution of the absorption spectrum of the trapped electron as well. 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

Most recently, Mizuno et al. presented a femtosecond version (250 fs time resolution, 160 cm spectral resolution) of the RR experiment to probe the O-H band of the electron as it hydrates following 2 X 4.66 eV photon excitation. Mizuno et al. conclude that the precursor of the hydrated electron that undergoes continuous blue shift on the time scale of 1-2 ps also yields a downshifted O-H stretch signal whose resonance enhancement follows the efficiency of Raman excitation as the absorption spectrum of the s-like state shifts to the blue (thus indirectly confirming its identity as a hot s-like state). The comparison of anti-Stokes and Stokes Raman intensities indicates that the local temperature rise is < 100 K at 250 fs. This estimate agrees with the estimates based on the evolution of the spectral envelope during the thermalization, using the dependence of the absorption maximum of thermalized electron on the bath temperature. 

Such reduction reactions have been observed directly by pulse radiolysis for several metal ions. Most ofthe reduction steps have been observed and their rate constants determined. Figure 1 presents the example of Ag reduction observed by pulse radiolysis coupled with time-resolved spectrophotometry. The evolution of the optical absorption spectrum in the successive fast steps is recorded just before and after the short electron pulse delivering the irradiation dose, as in a movie filming the fast cascade of reactions initiated... 

Pulse radiolytic studies of the kinetics of formation of clusters containing two different metals are more readily accessible, for the reasons given above, when both ions may be reduced by a monoelectronic process. This can be achieved with mixed solutions of the monovalent ions Ag and Au, in the form of KAg(CN)2 and KAu(CN)2. " The evolution of the optical absorption spectrum with time was followed specifically at 400 and 520 nm, which correspond to the maxima of the surface plasmon bands of the monometallic silver and gold clusters, respectively. The early steps of the mechanism are rapid reductions of Ag and Au into atoms... 

As shown in figure 4.9, the time evolution P f) shows three distinct time dependencies, each characterized by either 7, F, or e (1) The Lorentzian-like envelope, with width F for the complete absorption spectrum transforms into an exponential decay with rate constant F/ft, which is for the short-time decay of PXt)- (2) The set of resonances, separated on average by an energy e, transform into a set of oscillations (i.e., recurrences) whose periods are approximately elh. (3) The envelope of each individual resonance also transforms into an exponential-like decay, characterized by the rate 7/ft, which corresponds to leakage from the sparse i) — /) subspace into the quasi-continuum ). The recurrences described above in (2) are damped out by this slow decay. 

The sub-picosecond PIA spectrum and its time evolution after photoexcitation are altered significantly upon addition of C60 to MEH-PPV and BCHA-PPV, consistent with photoinduced electron transfer [56]. As shown in Figures 8.17(b) and 8.18(b), a broad absorption band in the PIA spectrum centered near 1.45 eV is observed in both BCHA-PPV/Ceo (1%) and MEH-PPV/Ceo ( %) In both cases, the spectra are... 

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