Iari-Gabriel Marino

Green pigments of the Pompeian artists’ palette

Green colored samples on wall paintings and green powder from a pigment pot found in Pompeii area are investigated by micro-Raman, FT-IR and, for one sample, SEM-EDX. To obtain the green color, green earths and malachite were used, together with mixture of Egyptian blue and yellow ochre. The mineralogical identification of the green earths has been attempted through the comparison of the vibrational features, discriminating between celadonite and glauconite spectra. Traces of a modern synthetic pigment containing copper phthalocyanine were found in a fresco fragment.

Micro-Raman spectroscopy as a routine tool for garnet analysis.

A rapid system to obtain molar compositions of minerals belonging to the garnet group by means of Raman spectroscopy is illustrated here. A series of standard garnets, whose composition was determined by means of Wavelength Dispersive System (WDS) electron microprobe measurements, was used to correlate the wavenumbers of the different Raman peaks with chemical composition. A simple software routine was then developed in order to obtain garnet molar composition starting from the Raman spectrum, based on the assumption that in a solid solution belonging to the garnet family the Raman wavenumbers are linear combinations of end member wavenumbers, weighted by their molar fraction. The choice of the Raman bands used for the calculations and their behaviour are also discussed. The method, called MIRAGEM (Micro-Raman Garnets Evaluation Method), was then tested on a second series of garnets with satisfactory results.

Spectroscopic study of the degradation products in the holy water fonts in Santa Maria della Steccata Church in Parma (Italy)

Two holy water fonts (dated at the beginning of the XVII century) in the Santa Maria della Steccata Church in Parma (Italy) have recently been restored. Before the intervention, a detailed investigation on their degradation products was carried out to understand the mechanisms of alteration of the materials and to suggest appropriate restoration procedures. The analyses were performed by means of powder X-ray diffraction (XRD), micro-Fourier transform infrared (FTIR) and micro-Raman spectroscopies. Calcite, iron oxides, silicates and sodium chloride (from salted holy water) were found in the red coloured calcarenite. On and under the lead coverings, different lead oxides (mainly massicot), other lead salts (lead basic carbonate, cerussite, plumbonacrite Pb10O(OH)6(CO3)6 and lead-chlorine compounds as laurionite PbOHCl and phosgenite Pb2CO3Cl2) were identified by Raman spectroscopy and XRD. Haematite alpha-Fe2O3, goethite alpha-FeO(OH) and lepidocrocite gamma-FeO(OH) were found on and around the iron hinges. Lead compounds and sodium chloride, through crystallization and solubilization cycles, were responsible for the stones degradation, whereas the iron corrosion materials on the hinges produced mechanical stress and cracks in the stone. Various suggestions have been given on how to restore these fonts and to remove the causes of damage.

Polarization holographic gratings in hybrid solgel films doped with Disperse Red 1.

Polarization holographic gratings in sp configuration are written at 488 nm in photorefractive organic-inorganic films based on SiO2. The films, prepared by a solgel technique, contain Disperse Red 1, carbazole units, and 2,4,7-trinitro-9-fluorenone. The gratings are characterized by their diffraction efficiency for a 632.8-nm probe. The polarization gratings act as a half-wave plate, and the diffraction efficiency is independent of the polarization direction of the probe.

Modeling and experimental study of photoinduced anisotropy in hybrid solgel films

The mechanism of photoinduced anisotropy through photoisomerization has been studied by measuring photoinduced dichroism and birefringence in solgel silica-based glasses containing an azo dye (Disperse Red 1) and a plasticizer (carbazole). The equations commonly adopted to describe the molecular orientation (Sekkats theory) have been solved numerically and analytically within two different approximations. The results have been applied to photoinduced birefringence measured with expanded beams, and the dominant mechanisms driving the azo-dye molecular orientation in the investigated materials during both illumination and relaxation have been inferred. The experiments of photoinduced dichroism, performed with nonexpanded beams, suggest that the distribution of times describing the kinetics of the anisotropy formation, usually attributed to inhomogeneities in the chromophore environment, may be ascribed instead to the nonuniform profile of the beams.

Micro‐Raman Determination of the Composition of Ugrandite Garnets

A fast method to obtain the molar composition of garnets starting from their Raman spectra was recently proposed. The method, called MIRAGEM—MIcro‐RAman Garnets Evaluation Method is based on the assumption that in a garnet solid‐solution the Raman wavenumbers are linear combinations of end‐member wavenumbers, weighted by their molar fraction. In this work, the application of the MIRAGEM method to garnets belonging to the ugrandite family is discussed in detail, showing very good correspondence with electron‐microscope‐microprobe results in the grossular‐andradite series. Unexpected two‐mode behavior of a Raman peak in garnets with large amount of uvarovite is found.

Photoinduced optical retardation in mesostructured dye-doped films investigated by an imaging pump-probe technique

The photoinduced optical retardation in mesostructured polymeric materials containing photo-orientable dyes has been investigated as a function of time and pump irradiance by an imaging pump-probe technique. The mesostructured samples show higher maximum retardation and lower residual anisotropy than amorphous ones. Several hypotheses are made to explain both results, taking into account the influence of the mesostructures on the chromophores. The retardation-irradiance curves measured in different samples are fitted against current available models.

Hybrid sol-gel films for holographic applications

Films based on SiO2 and containing Disperse Red 1 (DR1), carbazole units and 2,4,7-trinitro-9-fluorenone (TNF) have been prepared by a sol-gel technique. By photoinduced birefringence and photorefractive effect, holographic gratings have been produced using 488.0 mn and 632.8 nm light, respectively. Pure polarization gratings show diffraction efficiencies higher than those obtained by light intensity modulation. Various processes concerning the DR1 molecular orientation such as angular hole burning (AHB), angular redistribution (AR), Cis molecules decay and rotational diffusion are discussed to explain the temporal behaviour of the diffraction efficiencies. A poling field was not necessary to obtain photorefractive gratings, which show Asymmetric Energy Exchange (AEE) in two-beam coupling (2BC) experiments.

Polarization gratings in sol-gel thin films investigated through the moving grating technique

Intensity and pure polarization holographic gratings in silica-based sol-gel thin films, containing carbazole group and Disperse Red 1, are obtained by photoisomerization at 488 nm and are investigated by means of the moving grating technique (MGT). General equations relating the modulated MGT signal to amplitudes and phase shifts of the index and absorption gratings are derived for thin films in the case of different polarization configurations. It is shown that the technique, commonly used to characterize photorefractive gratings, is a powerful tool to investigate the orientation mechanisms responsible for the holographic polarization gratings formation.

Holographic gratings in hybrid sol-gel films

Organic-inorganic films based on SiO2, containing Disperse Red 1 (DR1), carbazole units and 2,4,7-trinitro-9-fluorenone (TNF), have been prepared by a sol-gel technique. Diffraction gratings have been produced through different effects: photoinduced birefringence and photorefractivity, using 488.0 nm and 632.8 nm light, respectively. Pure polarization holographic birefringence gratings have been investigated and diffraction efficiencies higher than those obtained by light intensity modulation have been measured. The study of the temporal behavior of the diffraction efficiencies makes possible the identification of different processes involved in the DR1 molecular orientation: angular hole burning (AHB), angular redistribution (AR), Cis molecules gain has been determined by two-beam coupling (2BC) measurements. The effect of the polarization of the writing beams on the grating and of a circularly polarized photoisomerizing radiation during grating erasure has been interpreted in terms of an orientation contribution to the grating formation.