Nanosecond lifetimes

Ion lifetimes from nanoseconds down to microseconds have been determined in experiments involving ionization within the confines of a molecular beam and decomposition within a strong electric field [381, 667, 672]. Using electron impact ionization, rates of decomposition have been determined as a function of time over the range 3 ns to 1 p s for ions formed from butane, heptane, perdeuteromethane, benzene, carbon dioxide and benzonitrile. 

Rate coefficients k(E) have been obtained in this way for decompositions of benzene, thiophene and benzonitrile over an energy range of some electron volts. These were the first direct experimental determinations of rate coefficients, k(E), for ionic decompositions. Moreover, a range of energies was accessible, since the times at which rates were measured extended over the 3 orders of magnitude, which contrasts with the later PIPECO experiments in which rates have been measured only in the microsecond time-frame. 

Ion lifetimes of the order of nanoseconds have also been measured in experiments which confined the detectable ionization to the region of a filament stretched along the axis of a cylinder [468, 469, 470, 815]. Electron impact ionization was employed and decomposition occurred within the strong field between filament and cylinder. Ethylene, ethane and hexane were among molecules studied. 

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The other coimnon way of measuring nanosecond lifetimes is the time-correlated single-photon counting... 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

The effect of solvation on uracil and thymine photophysics has been studied by Gustavvson and coworkers, who have studied uracil with four explicit water molecules and PCM to study distorted geometries [92,93,149], The conical intersection connecting Si to the ground state that was found in the gas phase is also present in solution. The barrier connecting the Si minimum to the conical intersection is lower in solution, however, causing much shorter lifetimes. So the nanosecond lifetime which is observed in the gas phase is not observed in solution but a picosecond lifetime is observed. 

E. Gaviola First direct measurement of nanosecond lifetimes by phase fluorometry (instrument built in Pringsheirrfs laboratory)... 

Long r s (hundreds of nanoseconds to >100 /tsec), which make r measurements much simpler and less expensive compared to the low-nanosecond lifetimes of most organic fluorophores. 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

A digressing result concerning the Trp fluorescence decay has been reported by Sarkar and Song [109] for 114/118-kDa phytochrome the decay at 293 K was found to be monoexponential with a nanosecond lifetime in the case of Pr, and biexponential with lifetimes around 2 and 5 ns in the case of Pfr. Since the degraded phytochrome possesses only eight Trp residues [110],... 

The enrichment of the concentration of the polar solvent component in the cage and, therefore, the relative amount of the red shift of the fluorescence band is a function of viscosity, since the diffusion-controlled reaction time must be smaller than the excited-state lifetime. This lifetime limitation of the red shift is even more severe if the higher value of the excited-state dipole moment is not a property of the initial Franck-Condon state but of the final state of an adiabatic reaction. Nevertheless, the additional red shift has been observed for the fluorescence of TICT biradical excited states due to their nanosecond lifetime together with a quenching effect of the total fluorescence since the A to 50 transition is weak (symmetry forbidden) (Fig. 2.25). 

Another recent study makes use of the participation of the T2 state in the S - TISC process in anthracene [31]. 1,3-Octadiene was used to intercept some of the T2 states before they relaxed to Tx and the decrease in 7, yield was used to estimate the T2 lifetime. Further, this study compensated for the effects of static and time-dependent quenching that comes into play at the relatively large quencher concentrations that are required when quenching sub-nanosecond-lifetime transients. The lifetimes obtained (given in Table 5) were significantly less than previously estimated from other quenching studies and are in line with the lifetimes implied from the T-T fluorescence quantum yields discussed above. 

The time resolution of the electronics in a single photon counting system can be better than 50 ps. A problem arises because of the inherent dispersion in electron transit times in the photomultiplier used to detect fluorescence, which are typically 0.1—0.5 ns. Although this does not preclude measurements of sub-nanosecond lifetimes, the lifetimes must be deconvoluted from the decay profile by mathematical methods [50, 51]. The effects of the laser pulsewidth and the instrument resolution combine to give an overall system response, L(f). This can be determined experimentally by observing the profile of scattered light from the excitation source. If the true fluorescence profile is given by F(f) then the... 

Figure 2.12. Photograph of silicon nanocluster-mesoporous silica composite film under UV irradiation at 77 K, showing bright visible nanosecond lifetime luminescence. Reproduced with permission from [90],...

It has also been demonstrated that mesoporous materials are viable candidates for optical devices [90]. Silicon nanoclusters were formed inside optically transparent, free-standing, oriented mesoporous silica film by chemical vapor deposition (CVD) of disilane within the spatial confines of the channels. The resulting silicon-silica nanocomposite displayed bright visible photoluminescence and nanosecond lifetimes (Fig. 2.12). The presence of partially polymerized silica channel walls and the retention of the surfactant template within the channels afforded very mild 100-140°C CVD conditions for the formation of... 

A similar pair of nonionic/ionic TICT species are ADMA (nanosecond lifetime [43]) and A-Rh (picosecond lifetime [12]). 

A paper has appeared which provides a procedure for determining quantum yields of non-luminescent excited states and photoproducts having sub-nanosecond lifetimes. The method has been applied to inorganic complexes and photosynthetic models. 

In summary, the pyridine ylide method has told us quite a lot about 1,2-hydrogen migration reactions of invisible carbenes. We learned that simple aUcyl and dialkyl carbenes are true intermediates with nanosecond lifetimes. The pyridine ylide method revealed that the rate of rearrangement of alkylcarbenes are influenced by the cationic stabilizing power of X k. increases as X = FI < CH ... 

The reasons for these different types of behaviors can be discussed in terms of the photophysical mechanisms schematized in Fig. 23. It can be seen that the free-base and zinc-porphyrin have excited singlet states with nanosecond lifetimes, that deactivate via both intersystem crossing to the triplet state (90-95% efficiency) and fluorescent emission (5-10% efficiency). 

Gated detection can be accomplished in two ways. One method is to turn on or gate the g n of the detector for a short period during the intensity decay. Surprisingly, thb can be accomplished on a timescale adequate for measurement of nanosecond lifetimes. Alternatively, the detector can be on during the entire decay, and the electric cal pulse measured with a sampling oscilloscope. Such devices can sample electrical signals with a resolution of tens of picoseconds. 

One may question why the exchange interaction is important in quendring of lanthanides, but not in energy transfer from more usual fluorophores with nanosecond lifetimes. For nanosecond-lifetime fluoropbores. the rates of transfer ate much faster due to the dependence on in. Hence, energy transfer occurs as soon as the donors and accqitors are within the Farslet distance. The longer lifetimes of the lanthanides tesuh in slower transfer rales and... 

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