F-center accumulation in filar KCl crystals irradiated with X-rays

The article outlines research on the kinetics of F-center accumulation in filar KCl crystals of various diameters and various origins. The mechanism of dislocation-group formation and the crystal surface cannot account for the radiation-induced coloration of alkali-halide crystals at room temperature. Differences in the properties of filar crystals grown by various techniques were observed for the first time.     

On the temperature dependence of the F-coloring efficiency in KCl

A simple model is presented to account for the major features of the temperature dependence of the primary F-coloring efficiency in KCl. It assumes that irradiation creates Frenkel pairs in three basic configurations followed by some correlated diffusion, leading either to annihilation of the pair or formation of a “surviving” primary pair. Steady irradiation as well as fast pulse data are analyzed in terms of the model.     

Picosecond ground state recovery of the F-center in RbCl,KCl and KBr

Abstract

Optical pump-probe measurements on the F-center in RbCl, KCl and KBr show a ps ground state recovery, when a substantial number of F-aggregates is present. The observed de-excitation process becomes faster for a larger concentration of aggregate centers and is related to energy transfer from the excited state of the F-center to aggregate centers. This energy transfer occurs towards F₂- and N-centers in KCl. Our results are in accordance with measurements of the quantum efficiency of the F-center luminescence and with transient absorption measurements.     

Theoretical investigation of the angular-momentum dependence of the mean fission lifetime of excited nuclei

Physics of Atomic Nuclei - Mean fission lifetimes of nuclei excited to energies of 80 to 400 MeV were recently measured at the GANIL accelerator by the crystal-blocking technique. Those experiments...     

The frequency and temperature dependence of the transverse optic mode self-energy in KCl

Measurements of the frequency dependence of the real and imaginary parts of the anharmonic self-energy of the q ≈ 0 transverse optic mode in KCl at 105 and 300 K are presented and compared to the predictions of theoretical calculations.     

Unusual temperature dependence of the positron lifetime in a polymer of intrinsic microporosity

The performance of polymeric membranes for gas separation is mainly determined by the free volume. Polymers of intrinsic microporosity are interesting due to the high abundance of accessible free volume. We performed measurements of the temperature dependence of the positron lifetime, generally accepted for investigation of free volume, in two polymers of intrinsic microporosity (PIM‐1 and PIM‐7) in the range from 143 to 523 K. The mean value of the free volume calculated from the ortho‐positronium lifetime is in the range of typical values for high free volume polymers. However, the temperature dependence of the local free volume is non‐monotonous in contrast to the macroscopic thermal expansion. The explanation is linked to the spirocenters in the polymer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the temperature dependence of the visible emission in thallium doped KCl and KBr crystals

The visible emission of KBr : Tl and KCl : Tl crystals was measured in the temperature range 10–300 K, on excitation in the low-energy tails of the corresponding A-bands. The peculiar temperature dependence of the visible emission is explained in terms of absorption changes due to thermal broadening of the A₁-side band itself and due to the growing overlap of A₁-band with the A-band on warming. A quantitative analysis is given which enables the computation of some parameters of the A₁-side band.     

Negative photoplastic effect due to the relaxed excited state of the F-center in alkali halides

Abstract

To make clear the mechanism of the photoplastic effect (PPE) in alkali halides containing F-centers, on the basis of the photochemical reaction of the F-center, temperature dependence of the PPE in KC1, and KBr and NaCl was studied from 95 K up to RT. The characteristic critical points θ _ts in colored KC1 and KBr were determined to be 110 and 190 K, respectively. Below θ _t the sign of the PPE in both crystals was negative, i.e., decrement of the flow stress during light illumination was observed. Moreover, the θ _ts were in good agreement with the temperature where the photoconductivities show remarkable increase in the course of the measurements. The negative PPE is explained in terms of the rotation of the principal strain axis of the relaxed state of the F-center having ?100? tetragonarity as to relax the stress around the edge dislocation coming closely.     

Dislocation motion in photomechanical effects and the lifetime of photochemical products of the F-center in alkali halides

A simple theory describing photomechanical effects in colored alkali halides was proposed based on statistical dislocation dynamics. From the present theory together with empirical data on photomechanical measurements, the lifetime of the F^--center in KCl is found to be about 2 s at around RT and the relevant activation energy is 0.29 eV.     

The effect of temperature on the pulsed conductivity of a KCl crystal excited by picosecond electron pulses

This paper discusses the temperature dependence of the pulsed conductivity of a KCl crystal in the interval 12–300K when it is excited by an electron beam (0.2 MeV, 50 ps, 300A/cm²) with a time resolution of 150 ps. It is shown that the electron lifetime is τ<100 ps in the entire interval under consideration, while the conductivity increases with temperature. The experimental results make it possible to obtain the temperature dependences of the effective electron-hole recombination cross section S∼T ^3.5 and the separation probability of genetic pairs. Fiz. Tverd. Tela (St. Petersburg) 41, 429–430 (March 1999)     

Studies on nonmetals during irradiation: IV — The effect of strain applied during irradition on the gamma-ray-induced F-center coloring of KCl at room temperature

KCl F-center coloring curves measured at room temperature on unstrained crystals contain one linear and three saturating exponential components. After deformation beyond 0.25 per cent, the coloring is markedly increased, and is described by a linear term and two exponential components. The post-strain coloring curves can be reconstructed from the measured parameters with strain the only independent variable. These observations indicate that the strain-related coloring depends on dislocation content.     

Power-law temperature dependence of thermally excited transport in one-dimensional systems from Monte Carlo simulation

We investigate the temperature dependence of electric conductance in one-dimensional (1D) systems with thermally excited electron transport under various bias voltages by using Monte Carlo simulation based on the variable-rang hopping (VRH) formula. We find that the temperature dependence of the transport can show a power law behavior as a result of summation over a large number of electron traveling paths although the hopping intensity in every step in the VRH formula is exponentially dependent on the temperature. This can well explain the temperature dependence of conductance measured in various experiments on 1D systems. Without taking the interaction between electrons into account, we can also merge most of our data onto one “universal curve” suggested from the Luttinger Liquid theory. This indicates that the phonon assisted hoppings in disordered 1D systems play an important role at finite temperatures and can provide a simple and efficient explanation for the experimentally observed behavior.     

Nucleon-content effect on the fission lifetime of excited nuclei

It was shown previously that the fission lifetime of a nucleus excited to about 100 MeV depends strongly and nonmonotonically on the initial value of its angular momentum L ₀. This result was obtained on the basis of a refined version of the combined dynamical and statistical model. The present study is devoted to a theoretical analysis of the dependence of the fission time on the nucleonic composition of the nucleus involved. The respective calculations were performed within the same model. The dependence of the average fission time 〈t _f 〉 on the initial fissility parameter (Z ²/A)₀ appears to be of a resonance type and is similar to its dependence on L ₀. This dependence of the average fission time on (Z ²/A)₀ stems both from statistical calculations and from a dynamical simulation of the fission mode with allowance for friction. The conditions under which the average fission time reaches a maximum are specified. The dependence of the average fission time on (Z ²/A)₀ remains nonmonotonic in the fusion-fission reaction as well, in which case the distribution of compound nuclei with respect to the initial angular momentum is broad.     

Radiative lifetime of resonantly excited excitons in quantum dots

Il Nuovo Cimento D - We report on the intrinsic radiative lifetime of excitons in InP quantum dots embedded in an In0.48Ga0.52 P matrix. This lifetime is about 400 ps as measured by time-resolved...