Fast-decaying

Lifetimes of 1 ps translate into linewidths of about 5 cm Thus, Ime-shape methods are ideally suited to measure very fast decay processes, in particular predissociation of excited species. An example is the... 

Excitation of an aqueous solution of poly(A/St/Phen) with a 355-nm, 22-ps laser pulse in the presence of MV2+ generated a transient absorption band peaking at about 600 nm due to MV + [120]. As shown in Fig. 16, the buildup of the 600-nm band completes immediately after the pulse excitation, indicating that the photoinduced ET from the singlet-excited Phen residue ( Phen ) to MV2 + occurs on a time scale comparable to or shorter than the duration of the laser pulse (ca. 22 ps) [120], Figure 16 also shows that a fast decay of the absorbance at 600 nm owing to the back ET from MV + to the Phen cation radical (Phen+ )... 

For APh-2, on the other hand, the forward ET from Phen to MV2+ was a little slower than that for the poly(A/St/Phen)-MV2+ system i.e., the intensity of the S <- Sj band for the Phen moiety at 510 nm still remained significant for 27 ps after the pulse excitation (Fig. 17) [120]. In striking contrast to the poly(A/St/Phen)-MV2 + system, the APh-2-MV2+ system showed an extremely fast decay in the transient absorbance at 600 nm over the picosecond regime and no subsequent slower decay. The transient absorbance almost completely decayed in 200 ps after the pulse. 

Singlet diradicals are usually extremely short-lived intermediates. For example, trimethylene (TM, 2) was observed to have a fast decay time of 120 fs by femtosecond spectroscopy [84, 85]. Since the localized 1,3-cyclopentanediyl diradical (62) was characterized by Buchwalter and Closs in 1975 [81, 82], experimental efforts have been made to prepare and characterize the persistent, localized singlet 1,3-diradicals. Some experimental achievements of the localized diradicals are collected in Fig. 25 and Table 3. It should be mentioned that the literature of experimental studies selected here is not exhaustive and more related references can be found in [83-115] and others. 

In order to directly probe the dynamics of CT between Et and ZG, and to understand how the intervening DNA base stack regulates CT rate constants and efficiencies, we examined this reaction on the femtosecond time scale [96]. These investigations revealed not only the unique ability of the DNA n-stack to mediate CT, but also the remarkable capacity of dynamical motions to modulate CT efficiency. Ultrafast CT between tethered, intercalated Et and ZG was observed with two time constants, 5 and 75 ps, both of which were essentially independent of distance over the 10-17 A examined. Significantly, both time constants correspond to CT reactions, as these fast decay components were not detected in analogous duplexes where the ZG was re-... 

Radioisotopes may occur in the earth naturally as primordial radioisotopes, formed when the planet was created, or be produced by natural or artificial processes. Most fast decaying primordial radioisotopes have long disappeared from the planet since the earth originated about 4.5 billion years ago, such isotopes have decayed and reached a final, stable form. The relatively few primordial radioisotopes still extant in the earth today, therefore, decay very slowly. Among these are potassium-40 and some isotopes of uranium, such as uranium-235 and uranium-238, which are of use for dating archaeologically related minerals and rocks (see Textboxes 15 and 16). 

Q-dependence of the plateau levels. The results for the model parameters are given in Table 3. The crossover time xe = 15 ns (T = 492 K) calculated with Eq. (41b) agrees well with the observed spectral shape in dividing the initial fast decay from the plateau-like behavior at longer times. Figure 25 (lower part) compares the experimental spectra for times longer than xe with the local... 

P-15 ZnO Zn ZnO (reducing atmosphere) Used originally in a flying spot scanner is applicable for any CRT requiring a highly visible screen and a fast decay. Green-emitting phosphor (515 nm). 

P-36 ZnCdS2 Ag.Ni ZnS, CdS, (Zno.7i5.Ago.285)S2, (Zn0933.Ni0.067)S2, MgCl2, NaCl, BaCl2 Fast decay screens for display. Yellow-green emission at 532 nm... 

Deep blue-emitting (centered at 415 nm) phosphor screen used wherever a very fast decay is needed, such as flying spot scanners... 

FIGURE 6.3 Decay of the hydrated electron yield with time compiled from various experiments. There is relatively little decay from 30 ps to -1 ns and a fast decay from 1 to 10 ns. These results were found difficult to reconcile with diffusion theory. The error bars indicate experimental uncertainties. 

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. 

The quantity k3 may be considered as an instrumental constant to be determined in a blank experiment—that is, without added solute. In this case, the current is given by I(t)/I(0) = (1 - vt/d) exp( - k3 t), from which k can be determined. With the solute added, the current initially decays exponentially (fast decay) from which is determined h + k2 + k3, while the ratio of the initial plateau to the initial current gives k2/(k] + k2 + k ). The detachment rate k2 is now obtained from the last two numbers, and then the attachment rate fe, is also obtained since k3 is already predetermined. In short, both attachment (kj and detachment (k2) rates are obtainable from the time dependence of the cell current following a brief pulse of ionizing radiation. 

Fig. 3 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of stilbene (S) and chloranil (Q) in dioxane. (a) The fast decay ( 20 ps) of the contact ion-radical pair S+ , Q generated by direct charge-transfer excitation (CT path), (b) The slow growth ( 1.6 ns) of the ion pair S+ Q due to the diffusional quenching of triplet chloranil (A path) as described in Scheme 13. Reproduced with permission from Ref. 55.

This instrument can be used in two modes the peak mode for fast-decaying pulses of light (<10s) and the repeat mode for slow kinetics (up to lOOmin). 

Ultrafast emission measurements are possible with the dendrimer metal nanocomposites. The gold and silver internal dendrimer nanocomposites showed a fast emission decay of approximately 0.5 ps, which was followed by a slower decay process. The fast decay emission is attributed to decay processes of the gold (or silver) metal nanoparticles. Ultrafast emission anisotropy measure-... 

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