[en] Absorption, photoluminescence emission, and excitation measurements of 'as grown' and neutron irradiated amorphous silica characterized by different impurity and OH content, confirm the composite nature of the ≅ 5.1 eV (B₂) absorption band and make it possible to distinguish between structural defects and impurity-related centers. A detailed analysis of the published results on the subject shows that none of the up date existing models fully accounts for the complex of the experimental observations. A new scheme is proposed in which all the available data find a consistent collocation. It is suggested that two structurally different centers, with optical absorptions in the B₂ region, can be simultaneously present in amorphous SiO₂; they exist in two variants: bare defect (also produced by neutron irradiation) or impurity-perturbed center. Some proposals for a deeper understanding of absorption and photoluminescence in glassy SiO₂ are finally advanced. (author)

[en] Photoluminescence spectra at 100 K, the temperature dependence of the luminescence in the 85 to 250 K range, and photoluminescence excitation spectra at 85 K are measured in CdGa_2xIn_2(1-x)Se₄. The recombination processes typical of AB₂X₄ semiconductors are present in CdGa_2xIn_2(1-x)Se₄, too. This confirms that the photoelectronic properties of the AB₂X₄ systems follow a common trend. Finally, the first experimental determination of the CdGa_2xIn_2(1-x)Se₄ energy gap is reported. (author)

[en] The luminescence of CdIn₂S₄ single crystals induced by transient optical excitation is presented and discussed; time resolved spectra and luminescence decay curves are measured in the 10 to 10⁵ ns time interval using different excitation energies. The features of the luminescence bands depend on the energy of the exciting photons and on the delay after the end of the excitation; the recombination process in CdIn₂S₄ displays characteristics typical of heavily doped semiconductors. The energy level scheme proposed for CdIn₂S₄, and the model for the excitation and recombination processes in this semiconductor find further support. (author)

[en] The photoluminescence of CdIn₂S₄ single crystals grown using the Bridgman technique is analysed in detail; the effect of different treatments (thermal treatments in various atmospheres, electro-diffusions, and γ-irradiation) on the shape of the emission bands is studied. From this analysis one of the two emission bands which characterize the CdIn₂S₄ luminescence can be attributed to a centre formed by a sulphur vacancy; the other band is due to a centre probably connected to some kind of intrinsic disorder characteristic of CdIn₂S₄ (exchange Cd reversible In for instance) and not to a stoichiometric defect. (author)

[en] A detailed analysis of the excitation and recombination processes of the charge carriers in CdIn₂S₄ crystals is discussed. The luminescence spectra and the luminescence excitation spectra at temperatures ranging from liquid helium temperature to room temperature are presented; the effect of a secondary low energy radiation on luminescence is also discussed. Models are proposed for the excitation and recombination processes and a scheme of the levels localized in the gap of CdIn₂S₄. The two emission bands observed are attributed to an interimpurity recombination and to an electron transition between a donor level and the valence band, respectively; the presence of a non radiative recombination process is also proposed. (author)

[en] Experimental results on the 1.9 and 2.2 eV luminescence bands in neutron irradiated amorphous silica are reported. All the four types in which silicas are usually classified (natural and synthetic, both dry and wet) are considered. Photoluminescence spectra in the 80 to 650 K temperature range, photoluminescence excitation spectra at 80 and 300 K, and spectrally resolved thermally stimulated luminescence are measured. The comparison of the intensity of the 1.9 eV band in the different types of silicas allows us to support one of the models recently proposed for the structure of the emitting center. A first systematic description of the main properties of the 2.2 eV emission band is given. (orig.)

[en] Axion-like particles are an important part of the spectrum of anomalous gauge theories involving modified mechanisms of cancellation of the gauge anomalies. Among these are intersecting brane models, which are characterized by the presence of one physical axion. We overview a recent study of their super-symmetric construction and some Lhc studies of the production rates for a gauged axion.

[en] Optical absorption measurements in the 3-9 eV energy range and electron paramagnetic resonance (EPR) have been performed at 300 K and at 80 K on Sawyer PQ quartz. Both as-received and neutron-irradiated samples (neutron fluences up to 3 x 10¹⁸ neutrons cm^-2) have been studied. The absorption spectra have been analysed in terms of a sum of elementary Gaussian components. The effect of the neutron irradiation is to induce the presence of absorption bands at 7.6 eV (E band), at 7.1 eV (D band) and at 6 eV (E' region). The most intense peak of all the spectra is the E band (7.6 eV); its shape is complex and the existence of a double structure can be suggested. The D band is evidenced for the first time in neutron-irradiated synthetic quartz; our measurements show that this structure is correlated with the 6.0 eV band. The 'E'' region is complex; in fact, at the highest neutron fluence the optical absorption spectrum reveals the existence of four structures, at 4.7, 5.2, 5.6 and 6.0 eV, where the 4.7 eV band has the same characteristics as the D_o band, which is present in amorphous silica. The comparison between the results of EPR measurements and the analysis of the complex structure of this absorption indicates that the correlation between optical absorption bands in this region and the EPR E' -centre signals suggested previously must be re-examined. (Author)

[en] There is a wide set of literature reports that suggest that over-coordinated oxygen or self-interstitials are, directly or indirectly, the chemical bridge between thermal donors, oxygen precipitates and dislocations, capable of supporting a common origin of their emission features in the 0.7-0.9 eV range. Finding the experimental proof of these suggestions was the aim of this present work, which required both appropriate preparation of samples and their careful optical, electrical and microscopical characterization. We were able to show not only that the photoluminescence emissions from oxide precipitates could be correlated to their density and to the presence of closed dislocation rings around them, but also that the precursors of dislocations are optically active as well. For samples thermally annealed in the range of thermal donors, we were able to show that their optical activity seems to be correlated to a transition from a shallow donor level of thermal donors to a deep level of a C_iO₂ complex. (author)

[en] A preliminary analysis is given of the optical and photoelectronic properties of CdGaInS₄. The analysis of optical absorption, photoconductivity, and photoluminescence allows to deduce an energy level scheme and a model for the photoelectronic processes in CdGaInS₄ that are analogous to those that are proved to be typical of other AB₂X₄ semiconductors. The comparison of the results with the published properties of CdGaInS₄ shows that this pseudoternary alloy is characterized by a localized level density lower than that generally found in layered AB₂X₄ systems. (author)