Intrinsic decay

Global compartmental analysis can be used to recover association and dissociation rate constants in some specific cases when the lifetimes are much shorter than the lifetimes for the association and dissociation processes. An example is the study for the binding dynamics of 2-naphthol (34, Scheme 14) with / -CD.207 Such an analysis is possible only if the observed lifetimes change with CD concentration and at least one of the decay parameters is known independently, in this case the lifetime of the singlet excited state of 33 (5.3 ns). From the analysis the association and dissociation rate constants, as well as intrinsic decay rate constants and iodide quenching rate constants, were recovered. The association and dissociation rate constants were found to be 2.5 x 109M-1 s 1 and 520 s 1, respectively.207... 

From a practical point of view the consequences of TOF dispersion are important only for short intrinsic fluorescence decay times of to < 1 nsec. Figure 8.15 shows an example with to = 50 psec and realistic optical constants of the substrate. The intensity maximum in Fb(t) is formed at At 30 psec after (5-excitation. After this maximum, the fluorescence decays with an effective lifetime of r ff = 100 psec that increases after long times to t > > 500 psec. The long-lived tail disappears as soon as there is some fluorescence reabsorption, and for Ke = K there is practically no difference to the intrinsic decay curve (curve 3 in Figure 8.15). 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

The intramolecular mechanism, illustrated on the left-hand side of Figure 6.8, is based on four separate operations [52]. (a) Destabilization of the stable translational isomer light excitation of the photoactive unit P (step 1) is followed by the transfer of an electron from the excited state to the Al station, which is encircled by the macrocycle (step 2) with the consequent deactivation of this station such a photoinduced electron-transfer process has to compete with the intrinsic decay of P (step 3). (b) Ring displacement the ring moves from the reduced station Ah to A2 (step 4), a step that has to compete with the back electron-transfer process from Ah (still encircled by the macrocycle) to the oxidized photoactive unit P+ (step 5). This is the most difficult requirement to meet in the intramolecular mechanism, (c) Electronic reset a back electron-transfer process from the free reduced station Ah to P+ (step 6) restores the electron-acceptor power to the Al station, (d) Nuclear reset as a consequence of the electronic reset, back movement of the ring from A2 to Al takes place (step 7). 

The interesting thing is that the maximum intensity of the FID at the top of the echo is still less that that at the start of the FID not all of the coherence is recovered by refocusing in the second half of the spin echo. The part that is lost is the intrinsic decay, the loss of coherence due to pure T2 relaxation, a fundamental relaxation process. The spin echo simply gets back the losses due to inhomogeneity of the magnetic field (T losses). This gives us a method to measure 7 We could repeat the spin-echo experiment a number of times with different echo delays (t values) and start the acquisition of the FID at the top of the echo ... 

Destabilization of the stable translational isomer. Light excitation of the photoactive unit P2+ is followed by transfer of an electron from the excited state to the EA22+ station, which is encircled by the ring (step 1), with the consequent deactivation of this station such a photoinduced electron-transfer process must compete with the intrinsic decay of the excited state of P2+. 

The critical distance is the distance at which the probabihty of energy transfer is equal to the probability of intrinsic decay... 

Figure 3. Energy level diagram and restrictions on kinetic parameters for the series of chromo-phores C, C2, C3. k C ) represents the intrinsic decay rates for C , regardless the radiative or nonradiative nature of the processes k and k2 are energy transfer rate constants.

Thus, the critical interaction distance is the sensitizer-activator separation for which the transfer rate is equal to the intrinsic decay rate. 

Although, formally, the integral in Eq. (2.9) is over the range [0,00], the domain of integration may be shortened via three mechanisms.4 First, the effective lifetime of the wavepacket on the excited-state potential energy surface is limited by radiative decay rate and/or the collisional deactivation rate of the excited electronic state these effects can be represented by a phenomenological lifetime, T 1. Second has an intrinsic decay that... 

The sensitizer (xanthone in water) triplet is reasonably long lived (20 ls) and its intrinsic decay is negligible in the presence of benoxaprofen (acceptor). 

Although muons are thought to have the same properties as electrons, as is indeed the case for negative muons, p , which behave as heavy electrons , the chemical properties of positive muons should rather be regarded as light protons . The pt tive muon can pick up an electron from a substance and form a neutral partick called muonium (p e , chemical symbol Mu). TTie atomic paran ters of Mu are very clore to those of H atom excqtt that the mass is 9 times smaller aiKi it is not stable because of the intrinsic decay nature of p. Mu can be regarded as a radioactive isotope of H atom (Table 2). 

The o-Ps lifetime T3 is the reciprocal of the total decay rate A.3, which is the sum of the pickoff decay rate and the intrinsic decay rate Xi ... 

The demodulatioo factois of the two states diqilay similar propesties. I om Eq. [18.20], one finds that the demodulatioo of the relaxed stale (nix) is the product of the demodulation of the unrelaxed state (ntf,) and that demodulation doe to the intrinsic decay of the R state done (mm). That is. 

Some applications have specific requirements on response time. If the incident X-ray beam is attenuated by the object to be imaged, the emission of the scintillator will be delayed. In this case, image quality will be degraded and image resolution will be reduced. It has been found that two mechanisms are responsible for such delayed emission. One is called primary speed, which is the intrinsic decay time of the emitting site or activator, and the other is known as afterglow, which is the delayed excitation and emission of activators, due to the trapping of electrons or holes by lattice defects and their delayed thermal release. 

The strategy devised in order to obtain the photoinduced shuttling movement of R between the two stations Ai + and A2 + is based on a four stroke synchronized sequence of electron transfer and molecular rearrangement processes, as illustrated in Figure 27(b).Light excitation of the photoactive unit P + (process 1) is followed by the transfer of an electron from this unit to Ai + (process 2) which competes with the intrinsic decay of the P + excited state (process 3). After the reduction of Ai +, with the consequent deactivation of this station, the ring moves (process 4) by 1.3 nm to encircle A2 +, a step that is in competition with the back electron transfer from Ai+ (still encircled by R) to the oxidized unit P + (process 5). Eventually, a back electron transfer from the free reduced station Ai + to the oxidized unit P + (process 6) restores the electron-acceptor power to this radical cationic station. As... 

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