Excitation yield

The physicochemical parameters commonly determined in mechanistic CL research are the rate constants, the activation parameters and the CL emission quantum yields (cl). as well as the electronic excitation yields (< > ), which can be singlet (4>s) or triplet... 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

Although the mechanism of the photo-induced generation of mono- and bimetallic metal clusters, except for the photographic application (Section 20.6), has been studied with considerably less detail than for the radiolytic route, some stable clusters, mostly of noble metals (Ag, Au, Pt, Pd, Rh), have also been prepared by UV excitation of metal ion solutions [129-141]. Generally, halides and pseudo-halides counter anions are known to release, when excited, solvated electrons, which reduce the metal ions up to the zerovalent state. Oxalate excitation yields the strong reducing carbonyl radical COO [30]. Photosensitizers are likewise often added [142]. Metal clusters are photo-induced as well at the surface of photo-excited semiconductors in contact with metal ions [143,144]. 

Further, one must pay attention to Eq. (5.10) for /mm - In isotropic collisions, when we have Tmm> — ram=m-M i and in the absence of an external field, when we have jkjJmm1 — 0) linearly polarized excitation yields /mm i = -f-M i-M- And since /mm i and f-M i-M enter into the sum (5.41) with equal coefficients, the aforesaid implies the absence of transversal orientation f v More precisely, the contention concerning the antisymmetry of the respective density matrix elements /mm i = —f-M i-M follows from the explicit form of the cyclic components of the polarization vector (see Appendix A), and from the symmetry properties of the Clebsch-Gordan coefficients (see Appendix C). 

The 130 keV State. The decay of the 130 keV state has been studied extensively, and several inconsistencies are being resolved. The results of different measurements of the mean life and decay mode of the 130 keV state are discussed by Fink and Benczer-Koller (8). The half-life of the state has been measured electronically, and the transition matrix element for excitation has been derived from Coulomb excitation data (12). The combination of the Coulomb excitation yield, the internal conversion coefficient (8) a = 1.76 =t= 0.19, and the branching ratio (8) PCo = 0.060 zb 0.008 for the crossover decay to ground, yields a half-life ti/2 = (0.414 0.014) ns in excellent agreement with a recent (15) Mossbauer determination of the line width, r = (4.4 zb 0.4) mm/sec, equivalent to t1/2 = (0.49 0.05) ns. Wilenzick et al. (15) do not indicate the thickness of the Pt absorber used. 

Most of these excitation yields have been determined by energy-transfer chemiluminescence using 9,10-diphenylanthracene (JJPA) and 9,10-dibromo-anthracene DBA) as fluorescers. 

As already pointed out on several occasions, the unique property of dioxetanes is to generate electronically excited states on thermolysis, which then manifest themselves by light emission (Eq. 28). The total yield of excited states (Eq. 33), that is, the sum of the singlet excitation yield (0 ), triplet excitation yield (0 ), and the spin-state selectivity (Eq. 34), that is, the ratio of the triplet and singlet excitation yields, are excitation parameters that characterize a particular dioxetane. 

Total Excitation Yield s 0 Spin-State Selectivity = 0 /0 ... 

Unlike activation parameters, the determination of which well-defined experimental kinetic methods exist, the state of the art for the determination of the excitation parameters leaves much room for improvement. However, a great deal of progress has been made in recent years. For the sake of simplicity and clarity, the methods for the determination of the excitation yields are classified into photophysical and photochemical techniques. This is warranted in view of the distinct experimental methodologies involved. 

Once the standardized and calibrated direct chemiluminescence quantum yield (0 ) has been acquired experimentally, the singlet excitation yield (0 ) can be calculated for the chemienergized process from Eq. 35. However, as already stated, this requires that the fluorescence quantum yield (0 ) be known under the same experimental conditions at which 0 was determined. This is not always the case... 

By means of steady-state kinetics, the relationship in Eq. 37 for the DPA-enhanced chemiluminescence quantum yield (0dpa) is derived in terms of the singlet excitation yield (0 ), the efficiency of singlet-singlet energy transfer (0 x), and the DPA fluorescence quantum yield (0dpa)- The 0 parameter can be readily assessed once the remaining terms are known. 

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