Vol 50, No 4 (2016)

Kinetic equations related to the model of random chain scission (RCS) in the initial oligomer and intermediates (radicals and biradicals) have been solved, and the time dependence of concentrations, the average chain length, and its variance have been found. Despite of the difference in the sequence of formation between the intermediates (sequential reduction of chain length in the model of successive detachment of terminal monomer units and the preferential buildup of short chains in the RCS model), the changes in the average chain length in the two models are similar; i.e., they coincide with an appropriate change of the time scale. In the absence of bimolecular recombination, both the models are ergodic and nonmixing, a property that is due to the equidistant nature of the eigenvalue spectrum. Recombination leads to mixing and transition to equilibrium depolymerization. The change of the mode is determined by the ratio between the bond rupture, recombination, and evaporation rates.

In this article, we investigate thermoluminescence properties of Nd³⁺ doped silicon dioxide optical fiber. The samples were exposed to 10 MV X-ray using a linear accelerator. The optical fiber was read out using Harshaw 3500 TLD reader. Nd-doped optical fiber displays a linear TL response for the absorbed dose. The sensitivity of Nd-doped optical fiber is 82.87 nC. mg^–1 Gy^–1, which is more sensitive to the other types of optical fiber. Nd⁺³-doped optical fiber displays clear single glow peak around 180°C. The peak shape method analysis reveals that the peaks obey general order kinetics. The activation energy of Nd-doped optical fiber is found to be nearly 0.5 eV less than TLD-100 (1.6 eV). Z_eff of neodymium-doped silicon dioxide optical fiber is 13.48 that is near to the human bone. All of these TL characteristics indicate that Nd107-doped optical fiber as a potential TL dosimeter, for measuring photon irradiation.

It has been shown that the RTL curve of n-tetracosane consists of several curves with dissimilar shapes. These curves have been separated. Their behavior depending on radiation dose, time, and bleaching wavelength has been investigated. Mechanisms for the formation of individual curves have been suggested.

The following three regimes of the radiation-induced production of liquid hydrocarbon fuel have been compared: (1) the direct cracking of molten C₁₇–C₁₂₀ synthetic wax, (2) direct gas-phase synthesis from methane, and (3) the indirect initiation of the fragmentation of wax diluted with methane. It has been found that in contrast to the direct cracking, regime (3) afforded lighter products containing more saturated and isomeric components. In this case, the degree of methane binding is higher by a factor of almost 3 than that in regime (1).

Using the method of microsecond flash photolysis with UV and visible light, the spectral and kinetic characteristics of intermediate products of photolysis in toluene and methanol solutions of a new biphotochromic compound have been studied, in which two photochromic moieties, spironaphthoxazine and azomethine, are linked to each other in such a way that there is conjugation between π-electronic systems of the moieties in the ground state of the molecule. Two intermediate products have detected, whose relative efficiency of formation substantially depends on the solvent and the wavelength of excitation light.

Spectral, luminescent, and photophysical properties of the BF₂ chelates with dichloroand dibromotetraphenyl-ms-azadipyrromethene (derivatives of tetraphenyl-aza-BODIPY) have been studied experimentally and theoretically by quantum chemistry methods. The positions of fluorescence bands, quantum yields, and lifetimes were measured experimentally. The rate constants of intramolecular photophysical processes have been estimated, and the quantum yields of fluorescence and phosphorescence and the lifetimes of excited states have been calculated. Complete energy schemes of electronically excited states and photophysical processes in the molecules of the compounds under study have been built on the basis of calculation results. The decrease in the fluorescence quantum yields upon excitation into the second absorption band and the absence of the phosphorescence of the chelates have been explained.

On the basis of the universal Kennard–Stepanov–van Roosbroeck–Shockley relation, a general approach is proposed for separation of the contributions of homogeneous and inhomogeneous broadenings in the absorption and luminescence spectra of colloidal quantum dots. The applicability of the developed method is demonstrated on published experimental data.

The effect of supercritical CO₂ (sc-CO₂) on the size distribution of free volume holes (nanopores) in a polyhexafluoropropylene (PHFP) polymer matrix has been studied. Residual (after CO₂ release) swelling improves gas transport properties of the material. Relaxation of these properties over time has been compared with changes in permeability and nanoporosity. For PHFP samples with different histories, the data on nanoporosity have been obtained using positron annihilation lifetime spectroscopy (micropores) and the lowtemperature gas sorption technique (mesopores and part of micropores). Matching of the data is intended to reveal the role of pores of different sizes in permeability of membrane materials to different gases and determine the specifics of application of the positron annihilation technique to studying sc-CO₂-modified objects.

A new synthesis method based on the coagulation of metal nanoparticles, introduced by laser ablation into superfluid helium, inside of quantized vortices has been used for the fabrication of nanoweb consisting of interconnected palladium wires of a 4 nm diameter. It has been found that at temperatures above 523 K, the Pd nanoweb effectively catalyzes the oxidation of CO with molecular oxygen. Temperature cycling leads to a shift of Pd nanoweb activity to lower temperatures. The catalytic action of the Pd nanoweb has been compared to that of Pd nanoparticles with a diameter of about 2 nm prepared by laser electrodispersion.

The yield of acid residues generated in water at pH 3–13 by pulsed plasma radiation of spark discharge with a power of 0.59 W has been studied. The power per pulse of electrical discharge has been optimized to obtain the maximum chemical effect in water. The results are compared with similar data obtained by treatment with 900-W gliding discharge. It has been found that the energy efficiency of pulsed plasma radiation is 100 times that of gliding discharge plasma. The results make it possible to clarify the mechanism of the gliding discharge-induced formation of peroxynitrite.

Chemical consequences of electron capture in the ⁵⁷Co nucleus in quenched eutectic melts of salts (nitrates, thiocyanates, and carbamides) have been investigated using nuclear gamma resonance spectroscopy. The posteffects of the nuclear transformation are interpreted in terms of the scavenger competition model.