Effect of preliminary electron-beam irradiation on hole transport in molecularly doped polycarbonate

The effect of preliminary electron-beam irradiation on hole transport in a molecularly doped polymer was studied with the use of the time-of-flight technique in the radiation-induced mode. Specimens that exhibit a plateau on their time-of-flight curves were selected for the study, since they suggest the occurrence of quasi-equilibrium transport in the system according to the conventional point of view. In the extremely small signal mode, current transients in the case of bulk irradiation have a form corresponding to dispersive, rather than Gaussian, transport, although hole movement is observed in the presence of charged sites (trapped electrons). On passing to the moderately large signal mode (preirradiation to a dose of up to 5 Gy), the current transients undergo noticeable changes, which might be mistakenly interpreted as evidence for the influence of charged sites on hole transport in accordance with the predictions of the dipolar glass theory. In actuality, these changes are due to the effect of a space charge field and the hole mobility remains almost unchanged in this case. The appearance of the plateau on the current transients is an artifact of the procedure, and the hole transport is dispersive.     

On the Involvement of Geminate Pairs in Radiation-Induced Conductivity of Polystyrene

Basic parameters of the generalized Rose–Fowler–Weisberg theory were experimentally determined for polystyrene. The obtained data were used to comprehensively analyze the evolution of geminate pairs in this polymer. It was shown that the generation of the prompt component of radiation-induced conductivity occurs by the trapping mechanism with two charge carrier states involved, the quasi-free and trapped. This generation under equilibrium transport conditions (diffusivity and mobility of carriers are constant) reportedly ensured by the geminate recombination itself was not observed in a real experiment. With allowance for dispersive transport of holes (the major carriers), the lifetime of geminate pairs were found to be as long as a few hundred of milliseconds at 300 K. The published data on the formation and decay kinetics in doped polystyrene upon its pulse radiolysis or photolysis were thoroughly analyzed. It was shown that, at ultrashort times before localization of generated carriers, the contribution of electrons to the observed conductivity prevails, although the basic contribution is made by holes in later times.     

Reflex discharges at low pressure

An investigation is made of reflex discharges with cold and hot cathodes at low pressures at which the free path of the electrons exceeds the dimensions of the discharge region. It was found that in the reflex discharge with hot cathode radial deformation of the potential is produced even at weak magnetic fields. The relationship between the anomalous motion of electrons emitted by the cathode and the experimentally determined anomalies of the static and dynamic characteristics of the suppressed reflex discharge was determined. The changes in the linear dimensions of the discharge region were also investigated for all the given forms of discharge.     

Radiation-Induced Injection Currents in Polymer Layers Exposed to Radiation on Their Outer Side

Radiation-induced currents in polymer layers exposed to radiation on their outer side were theoretically analyzed under conditions when the role of secondary emission processes in establishing steady-state charging regimes is very insignificant (the potential of the open surface does not exceed one tenth of electron beam energy). The cases of both electron and hole conductivity of polymers were considered. The determining role of the frequency factor for the events of thermal detrapping of charge carriers in the choice of a model for describing the phenomenon in question was revealed. Previously published data on the issue are discussed and partly criticized.     

Electron transport of holes in molecularly doped polycarbonate and its radiation-induced conductivity

The salient features of charge transport in a typical molecularly doped polymer (polycarbonate + 30 wt % DEH hydrazone) were studied by time-of-flight and nonsteady-state radiation-induced conductivity measurements. It was shows that the mobility of holes (major carriers) is due to dispersive transport in the temperature range 296–353 K covering the glass transition temperature at an observation time of up to a few seconds. The appearance of a plateau on the current transient, presumably manifesting the establishing of quasiequilibrium (Gaussian) transport, is the artifact of the time-of-flight technique when the charge carrier generation takes place at the sample surface. All of the obtained results can be satisfactorily rationalized in terms of the Rose-Fowler-Weisberg model with a uniform set of parameters of the model. Such an approach is compatible with the basic concepts of the radiation chemistry of condensed phase (the Onsager theory and the Langevin recombination mechanism), structural features of a disordered medium (transport zone, structural traps), and rotational diffusion of small molecules or their molecular groups in vitrified polymers.     

Temperature anomalies in the nonlinear magnetoacoustic properties of manganese-zinc spinel single crystals

The absorption of a magnetoelastic wave and the nonlinear resonance interaction of counterpropagating magnetoelastic waves in a manganese-zinc spinel single crystal have been investigated near the conjectured spin-reorientational transition. The convolution signal vanishes near T _tr=291 K; this is explained by the relaxational dynamics of the magnetic mode near the transition. The increase in the strength of the field corresponding to the maximum of the convolution with decreasing temperature is explained by a change in the demagnetizing field. Fiz. Tverd. Tela (St. Petersburg) 39, 652–655 (April 1997)     

Spectra of 3-keto-2, 3-dihydrothionaphthene and its derivatives

A number of thioindogenides are synthesized by condensing 1,8-naphthoxypenthiophene with aromatic aldehydes, and their phototropic properties are investigated. It is shown that illumination of solutions of these compounds leads to shift of _max toward the short wave region, and decrease in optical density.For Part VII see [1].     

Analysis of the Reversible Character of Radiation-Induced Conductivity in Polymers

The results of numerical simulation of the rise and decay of transient current in a virgin specimen of a model polymer upon continuous irradiation and in the irradiated specimen after its holding for a certain time are presented. The dependence of the annealing effect on the rest time was examined in detail. It was shown that long times are required for significant annealing, which strongly increase with a decrease in the disorder parameter (for example, from 4 × 10³ s at = 0.5 to 10⁷ s at = 0.3). An analysis of irradiation in the intermittent mode showed that this technique does not provide any new information as compared with the classical continuous irradiation (contrary to the opinion existing in the literature) but predictably complicates the interpretation of experimental data; however, it may be successfully used to measure the time-average current.