[en] From the data of a transmission experiment we have extracted the total neutron cross-section of a sample of gaseous hydrogen (T=75.03 K, p=84.8 bar, n=8.42 nm^-3) with a thermodynamic equilibrium ortho-para content (48% ortho, 52% para). The experiment was carried out on the PEARL instrument operating at the ISIS pulsed neutron source. After an accurate data reduction, the neutron spectra have been analyzed in the framework of the Modified Young and Koppel (MYK) theory, which is a successful extension to interacting fluids of the original Young and Koppel model valid for a dilute gas of hydrogen molecules. The total cross-section calculated with MYK theory, whose unique unknown parameter - the mean kinetic energy of the molecular centre of mass - was obtained through an independent path integral Monte Carlo simulation, shows a satisfactory agreement with the experimental results

[en] The angular and temperature dependences of the spin-lattice relaxation (SLR) rate of Vsub(k)-centers in LiF doped with Mg or Ag have been investigated. In the temperature interval 4.2 to 100 K the results can be fitted by the formula tau^-1 = A(theta)T + Besup(-Δ/T). A mechanism for the SLR is considered, assuming the modulation of the hyperfine interaction by phonon-induced transitions between the ground and excited states of the resonant molecular vibrations of the Vsub(k)-center. This mechanism is found to explain the value, the temperature dependence and the isotropy of tau^-1 in the interval T = 20 to 100 K. The one-phonon SLR mechanisms of the Vsub(k)-center in the T < 10 K region are discussed. (author)

[en] In orthorhombic BaBr₂ : Ce³⁺ two kinds of luminescence bands at room temperature have been attributed to charge-compensated Ce³⁺ centres. One type was associated with potassium (or some other monovalent cation) on a neighbouring Ba site and another one associated with an unidentified defect. A third kind of emission, observed only as low temperature photoluminescence (PL), is ascribed to isolated Ce³⁺ ions. The charge-compensated Ce³⁺ complexes are active both in PL and photostimulated luminescence (PSL) following X-ray irradiation. The PSL is nearly as efficient as in the case of the commercially used X-ray storage phosphor BaFBr:Eu²⁺. The X-ray induced electrons are trapped in F-type centres whose band position is characteristic for the nearby Ce complex. As shown by the fingerprint character of the PSL itself, the hole partner in the recombination is also associated or identical with the same Ce complex

[en] The incorporation site of Cu in the scintillator and thermoluminescent dosimeter material lithium tetraborate Li₂B₄O₇:Cu has been shown to depend on the charge state of the dopant. As confirmed by a refined analysis, the EPR spectra of an unpaired hole can be ascribed to Cu²⁺ situated near the Li lattice site with C₁ symmetry, in contrast to the higher (C₂) overall symmetry reported earlier for Cu⁺ in its relaxed excited state, the latter suggesting interstitial incorporation for the monovalent dopant. Consequences for charge transfer processes involving copper ions are discussed.

[en] The influence of Cu, Cr, Ni and Er cation impurities on the vibrational structure of LiNbO₃ crystals was investigated by means of laser spectroscopy using a Kr⁺ laser of 647.1 nm and an Ar⁺ laser of 514.5 nm at 78 K. Raman spectra were analysed and no further mode could be found in the spectra till 2000 cm^-1. In Ni- and Er-doped LiNbO₃ there are little differences in the crystal symmetry compared with pure crystals (about 15 %). In Cu- and Cr-doped crystals differences of about 50 % exist. A depolarization of radiation was detected especially in pure and Cr- and Cu-doped crystals

No abstract available

[en] Based on a model of coupled processes with differently time-dependent decay kinetics we present a critical review on photoluminescence (PL) and transient absorption (TA) experiments in undoped and Mg or Fe-doped LiNbO₃, together with a comprehensive interpretation of visible radiative and parallel non-radiative decay processes on timescales ranging from 50 ns up to minutes. Analogies and peculiarities of the kinetics of mobile self-trapped and pinned excitons are investigated and compared with those of hopping polarons in the same system. Exciton hopping with an activation energy of ≈0.18  eV is shown to govern the lifetime and quenching of the short PL component above 100 K. Strong interaction between excitons and dipolar pinning defects explains the exorbitant lifetimes and large depinning energies characterizing delayed TA components in doped LiNbO₃, while restricted hopping of the pinned excitons is proposed to play a role in strongly delayed PL in LiNbO₃:Mg exhibiting a narrowed emission band due to locally reduced electron–phonon coupling. Atomistic models of pinned excitons are proposed corresponding to charge-compensated dipolar defects predicted by theories of dopant incorporation in LiNbO₃ and are systematically assigned to absorption bands observed near the UV edge. Excitation in these bands is shown to lead directly to pinned exciton states confirming also the previously proposed two-step exciton-decay scenario in LiNbO₃:Fe. Weak intrinsic sub-80 ns luminescence in congruent LiNbO₃ is explained as an opposite effect of enhanced electron–phonon coupling for excitons pinned on Nb_Li antisite defects. The comparison of the different observed stretching behaviors in the paradigmatic system LiNbO₃ provides an intuitive picture of the underlying physical processes. The findings are relevant not only for holographic and non-linear optical applications of LiNbO₃ but are of general interest also for the treatment of stretched exponential or other time-dependent kinetics in complex condensed systems ranging from nanocrystals and polymers to liquids and biophysical systems. (topical review)

[en] Typical sources of error in the UV spectroscopy of DNA near and below 200 nm are pointed out. In the case of native DNA a procedure for extending the wavelength range and for analysing the spectra is proposed. As illustration, spectra for DNAs of various origin recorded in the 164-350 nm range at room temperature and at 110 K are presented. (Auth.)

[en] We report on the use of thermoluminescence and radioluminescence to monitor first order phase transitions. The luminescence signals not only offer information on the mechanisms of the relaxation processes, including changes in charge trapping energies and emission spectra, but also provide unique, sharply defined, temperature signals to act as temperature calibration standards for thermoluminescence. The wider implications of the method are mentioned, including the inverse problem of detection of phase transitions. (author)

[en] The Z-centre hypothesis in LiF:Mg and LiF:Mg, Ti systems is re-investigated. On the basis of available optical absorption measurements, summarizing the already emerged contradictions of this hypothesis, and by comparing the characteristic properties of the Z centres in LiF with those in other alkali halides it is established that the Z centre identifications in LiF can not be considered as proved. (author)