Carlo Maria Carbonaro

Light assisted dimer to monomer transformation in heavily doped rhodamine 6G-Porous silica hybrids.

The spectral properties of postdoping prepared type I Rhodamine 6G-silica hybrids were investigated in the case of intentionally large doping (5 x 10(-2) mol/L). Beside expected concentration quenching effects, steady state and time resolved optical spectroscopy measurements displayed the presence of different kind of aggregates, both fluorescent and nonfluorescent. As the irradiation dose on the sample increases, the emission features change: the overall emission increases, the peak of the emission is blue-shifted, and the emission decay time also increases. The modifications of the spectral properties under light excitation are interpreted in terms of dimer to monomer light assisted transformation, and a thermodynamic model based on a monomolecular kinetics is presented.

Ab initio study of oxygen vacancies in α-quartz

Abstract Extrinsic levels, formation energies, and relaxation geometries are calculated ab initio for oxygen vacancies in α-quartz SiO 2 . The vacancy is found to be thermodynamically stable in the charge states Q = +3, Q = 0, Q = −2, and Q = −3. The charged states are stabilized by large and asymmetric distortions near the vacancy site. Concurrently, Franck-Condon shifts for absorption and recombination related to these states are found to be strongly asymmetric. In undoped quartz, the ground state of the vacancy is the neutral charge state, while for moderate p -type and n -type doping, the +3 and −3 states are favored, respectively, over a wide Fermi level window. Optical transitions related to the vacancy are predicted at around 3 eV and 6.5 eV (absorption) and 2.5–3.0 eV (emission), depending on the charge state of the ground state.

A kinetics model for Tb3+ recombinations in low doped Tb:Lu1.8Y0.2SiO5 crystals

The detailed characterization of the terbium related optical properties of low doped oxyorthosilicates of lutetium and yttrium is reported. The introduction of terbium ions generates an ultraviolet absorption band peaked at about 242 nm and line shaped emissions in the 350–600 nm range. The transitions are related to the D53 and D54 levels and the analysis of the decay time measurements allows to individuate a cross relaxation mechanism among Terbium ions. We propose a three level kinetic model which is able to reproduce the experimental data allowing to discriminate among the radiative and nonradiative contributions to the transitions in the case of low content of Tb ions (nominal content 10 ppm). The reported study addresses two important goals, providing, from one side, a detailed characterization of possible inorganic phosphors, and from the other side, since traces of unwanted elements were identified in numerous commercial samples of cerium doped oxyorthosilicates, it can contribute to increase the ...

OH-dependence of ultraviolet emission in porous silica

Abstract Photoluminescence and Raman measurements have been performed on sol–gel synthesized porous silica monoliths. Porous silica and polyethilenglycole/silica hybrids thermally treated at different temperatures have been analyzed. By exciting at 5.6 eV a structured photoluminescence emission centered at 3.7 eV is found in the whole set of samples. Hydration treatment of the pore surfaces increases the H-bonding interaction between silanols, inhibits the isolated silanols vibrations and changes the photoluminescence spectrum increasing the emission peaked at 3.7 eV.

Ultraviolet excitation fine tuning of luminescence bands of oxygen-deficient centers in silica

The photoluminescence (PL) activity of oxygen deficient centers in commercial silica samples has been reconstructed by exciting in the spectral range of their optical absorption band around 5 eV. The fine tuning of the excitation was provided by a tunable pulsed optical parametric oscillator seeded by a Nd:yttrium–aluminum–garnet laser. The experimental results, analyzed in the spectral moment framework, indicate the presence of different centers whose spectral features are modulated by structural disorder typical of the glassy matrix. The photoluminescence excitation spectra at different emission energies evidence the composite structure of the luminescence bands around 3.1 and 4.2 eV, usually ascribed to triplet and singlet transitions of the same oxygen deficient center. Besides the well-known emission bands centered at 3.15 and 4.20 eV, the contribution of a new PL band below 3 eV is reported. Time resolved photoluminescence measurements of the low energy band give an estimate of its spectral characte...

Optical and structural characterization of cerium doped LYSO sol-gel polycrystal films: potential application as scintillator panel for X-ray imaging†

The optical and structural properties of sol–gel prepared polycrystalline films of Cerium doped Lutetium and Yttrium oxyorthosilicates (Ce:LYSO) are investigated by means of optical spectroscopy, XRD diffraction and Raman spectroscopy. The sol–gel samples are compared to commercially available Ce-doped LYSO single crystals, and a detailed study of the emission properties from the different cerium sites is performed. The main result is that the polycrystalline films do show very good radioluminescence properties for possible applications as X-ray scintillating panels. The structural characterization indicates that the sol–gel polycrystals have similar phase composition to commercial monocrystals; concerning the excitation and emission properties, the same radiative recombination channels are observed in the polycrystals and commercial monocrystals with a slightly faster decay time in the first case. In addition, low temperature measurements (8 K) indicate the presence of a non-radiative cross-relaxation mechanism among cerium ions in the sol–gel grown samples in the two possible crystallographic sites. The results indicate the possibility to develop sol–gel synthesized polycrystalline thin films as scintillating materials in the X-ray energy range.

Ultraviolet excitation of photoluminescence of porous silica under vacuum conditions

Abstract The optical properties of porous sol–gel silica are investigated by measuring the absorbance spectrum in the vacuum ultraviolet energy range and the excited photoluminescence. The emission spectrum shows a luminescence with two maxima at about 3.5 eV and

Vacuum ultraviolet absorption spectrum of photorefractive Sn-doped silica fiber preforms

Vacuum ultraviolet (VUV) absorption data have been obtained up to 8.3 eV on Sn-doped silica fibre preforms. Measurements have been carried out before and after exposure to 248 nm radiation from KrF excimer laser. The absorption spectrum is composed of three structures peaking at about 4.9, 5.8 and 7 eV, with the absorption edge at about 8.2 eV. The main effect of 5 eV irradiation is the decrease of the spectral components at 4.9 and 7 eV, whereas a small increase of absorption intensity is only observed just below the band at 4.9 eV. According to the observed negative absorption changes in the whole region of point-defect bands one would expect a negative refractive index change, contrary to the positive change previously reported in optical fibres of the same composition. Structural modifications accompanying the defect photoconversion process are suggested to be responsible for positive refractive index changes.

On the formation of aggregates in silica – rhodamine 6G type II hybrids

The spectroscopic features of dye aggregates in organic–inorganic Rhodamine 6G–silica hybrids are investigated in sol–gel prepared type II bulk and thick film samples in the 10−4–10−3 mol l−1 concentration range. No aggregates are observed in film samples irrespective of the dye concentration and the spectroscopic features are ascribed to fluorescent monomers. On the contrary, by means of excitation and emission fluorescence measurements and the analysis of fluorescence decay kinetics the formation of fluorescent aggregates is reported in bulk samples and the geometry of the formed aggregates at the silica surface is discussed on the single exciton theory framework. Beside the distribution of a single emitting specie observed in bulk samples and individuated as oblique J dimers, the analysis of the excitation spectra indicate also the presence of “dark” monomer units. An energy transfer mechanism is hypothesized to explain the interaction of the monomer–dimer pair.

Vacuum ultraviolet absorption of silica samples

Abstract Optical absorption analysis in the ultraviolet and vacuum ultraviolet region from 4.0 to 9.0 eV has been performed on different silica materials. We analysed natural and synthetic silica samples, both as grown and irradiated with different irradiations, neutrons, γ, and ultraviolet photons. Regardless of irradiation we always observed in the spectrum of irradiated samples the growth of the band at 5.8 eV, a band attributed to the E′ paramagnetic center. Comparing this absorption under different irradiation, we suggest the B2 and E absorption bands are not correlated and therefore cannot be assigned to the same defect.