Excimer decay

Derive an expression for l/d>D vs. 1/[A] assuming first that A E is not important and then that fc E is an important mode of decay for the excimer ... 

Figure 39. Fluorescence decay curves of the excimer observed at 480 nm for the multilayers of L-PyrAla-C1 s prepared at different conditions.

Poly(methyl methacrylate) [PMMA] is an excellent polymer for studying photoresist dissolution because of its minimal swelling characteristic. For this work, PMMA molecules were labelled with phenanthrene (Phe) dye since its fluorescence is quenched by MEK. In addition, this dye has the advantage of forming few excimers (23-241 which results in self-quenching. Thus, the reduction in fluorescence intensity of PMMA-Phe is virtually solely due to MEK quenching. Consequently, the permeation of MEK into a PMMA film can be monitored from fluorescence intensity decay. 

For EPy-doped PMMA film, a 308 nm excimer laser (Lumonics TE 430T-2, 6ns) was used as as exposure source. We used a tine-correlated single photon counting systen (18) for measuring fluorescence spectra and rise as well as decay curves of a snail ablated area. The excitation was a frequency-doubled laser pulse (295 nm, lOps) generated from a synchronously punped cavity-dumped dye laser (Spectra Physics 375B) and a CW mode-locked YAG laser (Spectra Physics 3000). Decay curves under a fluorescence microscope were measured by the same systen as used before (19). 

Fluorescence Rise and Decay Curves. Both monomer and excimer fluorescence decay curves of the unirradiated film are nonexponential and the excimer fluorescence shows a slow rise component. This behavior is quite similar to the result reported for the PMMA film doped with pyrene. (23) A delay in the excimer formation process was interpreted as the time taken for the two molecules in the ground state dimer to form the excimer geometry. Dynamic data of the ablated area observed at 375 no (monomer fluorescence) and 500 nm (exciner fluorescence) are shown in Figure 5. When the laser fluence increased, the monomer fluorescence decay became slower. The slow rise of the excimer fluorescence disappeared and the decay became faster. 

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

The decay of monomer emission is thus a sum of two exponentials. In contrast, the time evolution of the excimer emission is a difference of two exponentials, the pre-exponential factors being of opposite signs. The time constants are the same in the expressions of iM(t) and iE(t) (/ , and fl2 are the eigenvalues of the system). The negative term in iE (t) represents the increase in intensity corresponding to excimer formation the fluorescence intensity indeed starts from zero because excimers do not absorb light and can only be formed from the monomer (Figure 4.8A). 

In this case, the fluorescence decay of the monomer is a single exponential (Figure 4.8B). The relevant decay time (1/X) is equal to the rise time of the excimer fluorescence6 . 

Fig. 4.8. Fluorescence decays of the monomer and the excimer. (a) Dissociation of the excimer within excited-state lifetime, (b) No dissociation of the excimer.

The excimer decays giving two ground state aromatic sites and emission of fluorescence. 

Invariably, the energy of the light emitted is less than that originally taken on. By studying the amount and energy of the fluorescence radiation decay rates, depolarization effects, excimer stability, and structure can be determined. 

Fig. 21 Fluorescence decay profile of excimer (480 nm) for no DTAB, curve A (premaximum), and 0.001 M DTAB, curve B (postmaximum), corresponding to pH 6.4 in Fig. 19...

In the time-resolved quenching method, the decay kinetics of the monomer and the excimer emission are monitored in the presence of a micelHzed medium. If the micellar system is viewed as a group of individual micelles with probe occupancies 0, 1, 2, 3, etc., the probabihty of micelles with n probes, Pn, may be related to n, the average number of probes per micelle by Poissonian statistics through relation. 

Fig. 46 Pyrene monomer and excimer decay profiles in SDS micellar solutions [SDS] = 8.2 X 10 kmolm , [NaCl] = 10 kmolm , CMC = 1.5 x 10" kmolm", pyrene levels are indicated as the ratio of micellized SDS to added pyrene emission monitored at 383 nm for monomer and 480 nm for excimer. (A) Monomer emission for SDS/Py = 2160, (B) monomer emission for SDS/Py 108 (C) excimer emission for SDS/Py = 108...

At higher concentrations in solution, the photodimerization of tS has been studied by means of picosecond electronic absorption spectroscopy. The 5i state of tS in benzene at 22°C is quenched with a diffusion-controlled rate constant of 2.03 X lO M s to give a new reactive intermediate exhibiting an absorption maximum at 480 nm. This new species decays unimolecularly with a rate constant of (2.40 0.37) X 10 s. It has tentatively been assigned to either the excimer or a biradicaloid species located at the pericyclic minimum. 

D. Decay Characteristics of Molecular and Excimer (Exciplex) Fluorescence 178... 

Evidence for photoassociation in the triplet manifold is at present inconclusive. Although Hoytink et al.20 have reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene, concentration and temperature-dependent studies of the emission characteristics must be extended in order to distinguish photoassociation of the triplet state from intersystem crossing of the singlet excimer and possible triple-triplet annihilation. Certainly the decay constant of the molecular triplet state in fluid media is relatively insensitive to solute concentration21 although this... 

It should be noted that excimer dissociation (process MD) is responsible for a nonexponential decay of molecular fluorescence which affords a criterion of photoassociation under conditions (A mdt 1) where neither self-... 

In the absence of the reverse absorption the radiative transition probability fquantum yield of fluorescence qmC) and the decay constant l/r (C)= 2 [Pg.200]

Lifetimes t°(C are available from analyses of fluorescence decay curves as described in Section II.D where, to a good approximation, 1/t (C is given as the experimental parameter Ax describing terminal decay for a system exhibiting excimer (exciplex) fluorescence only. 

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