Second irradiation effect

Figure 4. The effect of storage and a second irradiation (5 min) on brightness at 457 nm after irradiation of a peroxide-bleached spruce CTMP.

The second important effect is that irradiation absorption generates active states of the photoadsorption centers with trapped electrons and holes. By definition (Serpone and Emeline, 2002) the photoadsorption center is a surface site which reaches an active state after photoexcitation and then it is able to form photoadsorbed species by chemical interaction with substrate (molecules, or atoms, or ions) at solid/fluid interface. In turn, the active state of a surface photoadsorption center is an electronically excited surface center, i.e. surface defect with trapped photogenerated charge carrier that interacts with atoms, molecules or ions at the solid/gas or solidfiquid interfaces with formation of chemisorbed species. ... 

In this part we present an example for the pressure dependence of hole-size distribution. In our experiments we could not vary the temperature but keep it constant at room temperature. Details of experiments are described by Goworek [2007]. Figure 11.7a displays the mean, T3 (= (T3)), and standard deviation, <73, of o-Ps lifetime distribution and the o-Ps intensity h of PIB at 296 K as a function of pressure, P [Kilburn et al., 2006]. All of these parameters exhibit an exponential-like decrease with increasing pressure, h shows a small hysteresis which can be attributed to positron irradiation effects. T3 decreases from 2.0 ns at P = 0.1 MPa to 0.83 ns at 1.3 GPa and from 0.45 ns to about 0.05 ns. The low value of T3 =0.83 ns is possibly the lowest o-Ps lifetime observed until now for polymers. A lifetime of 0.5 ns is the theoretical limit of the pickoff annihilation for disappearing hole sizes [see Eq. (11.3)]. In polytetratluoroethylene (PTFE) a second, medium o-Ps lifetime of about 1 ns has been resolved and attributed to o-Ps annihilation in the densely packed polymer crystals [Dlubek et al., 2005d]. 

Changes in the photoinitiator concentration generally have moderate effects upon properties. Blends of PVEE and isobornyl acrylate (70/30) containing 2 and 3 percent benzophenone afforded values of 4.6 pli (cohesive failure) with 0.43 hour shear, respectively, using six seconds irradiation. Initiator concentration usually has its greatest effect on the time of exposure needed to achieve the maximum property values, but does not change the maximum obtainable for that formulation. 

We have tested the use of the 313 nm Hg line to effect the HOTachy— HOPre conversion as an alternative wavelength in the second irradiation step. This wavelength is absorbed more strongly and more selectively by the Tachychromophore (Figure 27.8). Essentially identical Tachy— Pre quantum yields... 

Diels-Alder reactions of vinylpyrazoles 45 and 46 only occur with highly reactive dienophiles under severe conditions (8-10 atm, 120-140 °C, several days). MW irradiation in solvent-free conditions also has a beneficial effect [40b] on the reaction time (Scheme 4.11). The indazole 48, present in large amounts in the cycloaddition of 45 with dimethylacetylenedicarboxylate, is the result of an ene reaction of primary Diels-Alder adduct with a second molecule of dienophile followed by two [l,3]-sigmatropic hydrogen shifts [42]. The MW-assisted cycloaddition of 46 with the poorly reactive dienophile ethylphenyl-propiolate (Scheme 4.11) is significant under the classical thermal reaction conditions (140 °C, 6d) only polymerization or decomposition products were detected. 

A quantitative comparison of DNA band intensity at adding of different AR homologues into the buffer after 300 seconds of irradiation on the transilluminator has allowed to obtain a more detailed information about the structural integrity of DNA, depending on the AR concentration (Table 2). It was found that Cg-AR protected DNA greater than 1.5-fold in the range of lO- M and SxlO- M, with a maximum effect (163.5 + 15.2%) at concentrations of Id 3M. On this background, Ci-AR and C3-AR demonstrated poor photoprotective activity and Cs-AR showed a similar protective effect only at concentration of Id M. 

Radioactive molecules can sometimes be formed by the processes normally involved in the separation. Foremost among these are (1) exchange in the solvent prior to chromatography and (2) thermal reaction during sublimation. Both lead to false results the second is easily eliminated, the first is often not. As an illustration of the effect of sublimation, it was found (61) that the yield of W(CO)g separated by vacuum sublimation at 50° C was 63.8%. The same irradiation conditions yielded 51.8% when the samples were first chromatographed and then sublimed. Evidence for similar effects was found in (PhH)2Cr (75), RuCpj (29) and others. Simple dissolution to break up the lattice, followed by evaporation and sublimation is usually considered satisfactory to eliminate thermal recombination of initially correlated pairs of reactants. 

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