An. Kuznetsov

Pressure effects on relaxation in a polymer glass: A persistent spectral hole burning study

The influence of hydrostatic pressure in the range of some kilobars on low-temperature (T < 20 K) relaxation in a polymer (polystyrene) glass after optical excitation of a probe chromophore in it is studied using two different kinds of spectral hole burning experiments—under isothermal-isobaric and in temperature cycling conditions. In the first case, the temperature dependence of the hole width reflects the dynamics of interaction of the electronic transition in a probe molecule with soft localized vibrational modes and with two-level systems, whereas, in the second case, the observed residual hole broadening after the temperature cycle arises from activated (overbarrier) transitions in almost symmetric double-well soft potentials. It is shown that both these processes are essentially suppressed by the applied hydrostatic pressure (the hole width in the first case and its increment in the second case are both reduced about twofold at 5 kbar). An extension of the soft potential model for glasses is proposed explaining in a coherent manner both effects. Its essential points are the presence in the potential of an extra term linear in pressure and the soft coordinate and an assumption about asymmetric distribution of the cubic anharmonicity parameter ξ in the potential.

Pressure effects on the spectra of dye molecules in incommensurate and commensurate phases of biphenyl

Abstract Low-pressure tuning of spectral holes, burned in the spectra of chlorin molecules doped into polycrystalline biphenyl, was studied for the incommensurate phase III of biphenyl at T =2 K and P=0.1 to 2.5 MPa. A blue pressure shift of holes burned in the outermost red line of an inhomogeneous spectral triplet was found, in contrast to the red pressure shifts of the other two lines. Extrapolation of these shifts to higher pressures shows the convergence of that triplet at a pressure above 200 MPa. Such behaviour was confirmed by high-pressure measurements of two-dimensional (2D) excitation–emission spectra, from which the inhomogeneous distribution function (IDF) was extracted. At 5 K the low-pressure triplet shape of IDF converges to a high-pressure singlet at 170 MPa, close to the critical pressure for the incommensurate–commensurate transition. The results support a view that the optical spectra reflect interaction of the impurity molecule with the incommensurate modulation wave in biphenyl host matrix disappearing at transition to the commensurate phase.

Ion beam induced excess vacancies in Si and SiGe and related Cu gettering

Implantation-induced excess vacancies and the related Cu gettering was studied in Si and in the solid solution Si0.93Ge0.07. The excess vacancy and interstitial generation during normal and incline ...

High pressure narrowing of zero-phonon lines in polymer glasses at different temperatures

Abstract Pressure-induced narrowing of zero-phonon lines, stronger at higher temperatures, established for chlorin molecules in a glassy polystyrene by using the spectral hole burning technique, is explained theoretically by pressure dependence of pseudolocal vibrations and two-level systems.

Molecular probing of low-temperature incommensurate phases

Two-dimensional (2D) excitation–emission spectra of biphenyl doped with free-base chlorin are measured at 5 K under various pressures up to 350 MPa. Besides the features related to zero-phonon lines and their phonon sidebands, a broad spectral band amounting to 80% of the total intensity at 5 K is revealed in the 2D spectra. The inhomogeneous distribution function obtained shows drastic changes with increasing pressure—the triplet structure observable at normal pressure in the incommensurate phase ICIII of biphenyl converges to a singlet in the high-pressure commensurate phase CI. These observations are assumed to reflect a relaxation specific to incommensurate phases after optical excitation of the probe molecules and their interaction with the incommensurate modulation wave.

Pressure-induced transformations of low-energy excitations in glasses

We show that the external pressure reduces the number of localized low-energy excitations (soft localized modes and two-level systems) in glasses. We also explain the experimentally observed narrowing of spectral holes in a dye-doped polymer glass (polystyrene) under hydrostatic pressure of some kilobars at liquid helium temperatures, as resulting from pressure-induced decrease of the density of states for these excitations.

On a model of F centres in alkali halide crystals

Abstract On the basis of analysis of available experimental data it is shown that a model of the F centre in alkali halide crystals (AHC) as a self-trapped electron (STEL) can more naturally explain some of these data than the commonly accepted de Boer model (an electron bound at an anion vacancy). It is supposed that the high binding energy of STEL in AHC may be caused by a strong electron-electron correlation.