Initiation, quantum yield

The initial election—hole separation distance, and the quantum yield, ( )q, are derived by fitting with the Onsager model. When the initial quantum yield,... 

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

The initial quantum yield for dimer formation (i.e., decrease in absorbance) in oligo T (see Glossary for definition of oligo ) is about 25 times that for Tp, but decreases to about the same as that for Tp with increasing radiation dose.7... 

Fig. 50. Dependence of the thermaliz-ation distance r and initial quantum yield d>0 for polycarbonate film on tri-phenylamine concentration [287]...

In eq. 5, t is the measured inhibition period, and [in] is the concentration of inhibitor present, which must be overcome before initiation can occur. Molecular weights (both Mp and Mw) can be determined as a function of percent conversion, and their values related to the degree of polymerization and the kinetic chain length, and finally to an initiation quantum yield. 

Fig. 2.23. Initial quantum yield

In our study, for all applied intensities of the irradiating light in the range 0.2-10 8 Einstein-s 1-cm 2 < Io < 1.6-10 8 Einstein-s 1-cm 2, the experimental-ly obtained initial reaction rate W0 was found to be proportional to the intensity I0 of irradiating light (Fig. 2.24), i.e., the initial quantum yield of the reaction cpo is independent of this intensity. 

A comparison of the experimentally obtained dependence of the initial quantum yield adsorption isotherm over an aqueous suspension reveals a qualitative similarity in the form of indicated curves. 

A characteristic increase of 90 after the horizontal section is apparently more pronounced when the potassium oxalate K2C2O4 is used as the electron donor instead of the sulfide ions (Fig. 2.25). A qualitative similarity of the adsorption isotherms and the MO concentration dependence on the initial quantum yield indicates that the adsorbed dye molecules take part in the reaction. Note that all kinetic curves attain the same value of the stationary quantum yield ratio depends on the nature of polymeric surfactant used for stabilization of CdS colloid. With PAA, this ratio equals ca. 0.5, and 0.6 with PVS. 

Expression (2.24) gives at least a qualitative description of the experimentally observed dependence of the initial quantum yield cpo of reaction on the concentration of MO acceptor. Indeed, the number (concentration) of dye ions MOad adsorbed at the surface of a colloidal particle before the light illumination, is a function of the MO concentration in a the solution and follows the dye adsorption isotherm at the same surface. Since MOad increases with increasing MO concentration, at surpassing of a certain concentration [MO], the ratio kj" K MO can appear to be much less that unity. If the Dad value is fixed, the second term of sum (24) becomes constant, and hence... 

We reiterate our belief that there are still other classes of radical reactions which may produce electron polarization and which are awaiting to be discovered. For example, a possible mechanism that would produce polarized radicals is one in which the three triplet sublevels of a photoexcited molecule react at different rates. This concept was suggested by El-Sayed (51,53) in connection with some solid-state photochemical systems. Also, the observations reported by Gupta and Hammond (66) on the small change in the initial quantum yields in the benzophenone-sensi-tized isomerization of stibenes and piperylene in the presence of an external magnetic field have not been completely explained (14). 

The initial quantum yields for the oligomerization and the polymerization of DSP, P2VB, and p-PDA Me are summarized in Table 2. 

The initial quantum yield of ICl decomposition was found to be 2. This value decreased with the addition of large amounts of non-reactive gas (CO2 or N2). These observations are interpreted according to the mechanism... 

Kinetics in the irradiated system HI-NO have been studied by Holmes and Sundaram . They used 3130-3660 A radiation and a reaction cell temperature of 25 or 45 °C. Uranyl oxalate actinometry was employed. The photolysis of HI in this wavelength region produces hydrogen and iodine atoms which in turn react with either HI or NO. Holmes and Sundaram found that at 25 °C additions of NO significantly reduced the initial quantum yield of Hj. As the NO/HI ratio increased, the quantum yield fell to a limiting value. Additions of nitrogen to pure HI had no effect on the quantum yield. At 45 °C the reaction products were the same but the actinometry was irreproducible due to formation of ammonium iodide on the cell windows which reduced incident light intensities. 

Irradiation of triarylmethylazides (2537 A) in hexane solution yielded benzo-phenone anils, as in the thermolysis . The initial quantum yield is high, about Q g273,23i then rapidly decreases owing to accumulation of the highly absorbing triphenylmethyl radical it was shown by labeling techniques, that a second primary process is operative, viz. 

The initial quantum yield for this process was determined as ca. 3.8 [1427]. At 10 "C, slight traces of methanal, H CO, were detected, but there is little evidence to sustain the claim [269] of its intermediacy in the gas phase reaction. 

Other examples of such photosensitizers include carbonyl compounds such as ketones, both aliphatic and aromatic. Aromatic ketones are, however, more useful in commercial practice, since their absorptions occur at longer wavelength and their efficiency of initiation (quantum yield) is higher. 

Figure 14 Initial quantum yield of ct tt photoisomerization of 27 by sensitization with 9-fluorenone as a function of [ct]. (Reprinted from Ref. 123 copyright 1998 American Chemical Society.)...

PHOTOLYSIS OF POLYETHYLENE TEREPHTHALATE IN VACUO AT 313 nm (INITIAL QUANTUM YIELDS x 104)... 

The efficiency of PIS is described by two quantum yields the initiation quantum yield (f), which represents the number of starting polymer chains per photon absorbed... 

The following example helps to understand how the kinetics affects the practical efficiency expressed by the rate of polymerization Rp. The dissociation quantum yield in the case of a cleavage process in the absence of oxygen is given by (10.71) and the corresponding initiation quantum yield (f>i connected with Rp is defined by (10.72)... 

FIGURE 17.2 The initial quantum yield for oxygen uptake of HDPE as a function of the amount of LMW-HALS-1 for formulation with 0.15%wt/wt BFS-1 (1), S-l (2), BFS-2 (3), and without a sulfur containing stabilizer (4) [98]. 

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